Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hi all
I have a Session Rockette 30watt amp of mid-80s vintage. It's a good little amp, but it's started to get a hum/buzz, too noticeable to record with.

I am thinking that it's the electrolytic caps - is this a likely culprit? If so - is it good idea to replace all the electrolytic caps - there's only 12. This could be cheaper than going to an amp tech, and I am very experienced with a soldering iron so it wouldn't be a problem.

If 'no' - what is more likely the cause of the hum?

If 'yes' - is there a fav brand of caps to use - is it true that Elna Silmic II are the ones to go for? I have sourced them on ebay, though they are not cheap - am I going overboard here, or is there a cheaper brand of cap that will be just as good? For the power supply end I have found some Panasonic FC and FM pairs which are the right spec (note I tried to get all 12 from Panasonic, but there's a few which aren't available in their FC, FM ranges).

And just one more question: at the very input end of the amp circuit, it specs a 1uf 63v electrolytic, but I can't seem to get that as a Panasonic or Elna - so could I use a 2.2uf 63v (or a 1uf 50v) of either of those brands instead, or should I just get another one I found on the web which is this spec:

MCGPR63V105M5X11 Multicomp CAPACITOR, 1UF 63V
Capacitance: 1F
Capacitance Tolerance: 20%
Diameter: 5mm
Lead Spacing: 2mm
Operating Temperature Range: -40C to +85C
Voltage Rating: 63VDC
AC Ripple Current: 12mA
External Length / Height: 11mm
Lead Diameter: 0.5mm
Leakage Current: 16.86A
Operating Lifetime: 2000h
Operating Temperature Max: 85C
Operating Temperature Min: -40C
Package / Case: Radial
Ripple Current Frequency: 120Hz
Ripple Current Temperature: 85C
Terminal Type: Wire Ended

I have attached the schematic for this amp, which I found on the Session website. Note that at the input it has that 1uf 63v electr cap - but could that have been a 2.2uf 63v or a 1uf 50v?

Thanks I hope that's not too many questions at once.

Merry Xmas,
Jim

Replace ALL the caps,,, Yikes!!! like what for?  :loco

That is about the hardest, longest, most expensive way to go and you have no idea if that will fix it. The shoot anything that moves approach is not the way. xP

Get the circuit board in good light and just quietly peruse the real estate inside, First noting the Mains voltage wires, fuse, switch etc and keep a mental note of those mains wires and never touch them.
Now look for any bulging caps mainly the electrolytics but flip it over and With very good light and a magnifying glass check the solder joints of the big electros, you are looking for hair line cracks around the solder pads.
Re flow any suspect pads and try again, if you can't find any obvious faults then take some pictures of both sides of the boards and post them that will help us to help you.

Caps; a capacitor needs to be *Above* the working voltage of the circuit,,so if you have a 50 volt supply rail the filter caps will probably be 63VDC. Always add at least 10%~20% over the working voltage.

Re the brand name game;
For guitar amplifiers as long as they are the right value and voltage they will sound the same in a blind test.
Yes Yes I know some big name guitar player said blah bla bla about Bright Orange caps improving the tone mojo.

If His ears can hear it,, then it can be measured,,, but sadly most of this has been tested to death and never shown anything meaningful.
There are rare circumstances where a certain type of cap will improve something but you will find that will only help if you are building a space station.

BTW, Leo Fender purchased all the seconds from the factory to save a few bucks, yet some of his amps are legendary,,,go figure.
Phil.

Hi Jungle-Jim and welcome.


There are sundry bits of zombie mythology that just seem to keep coming back no matter how many times they get shot down, and your post contains two of the most common (which ain't your fault);

1) "whatever is wrong with your amp, changing all the caps will fix it"; and

2) "randomly changing components is a reasonable way to service an amp".

Caps certainly fail, but not nearly as often as the "experts" on some music forums seem to think.  The most common cause of amp problems is dirt, typically in connectors and associated switching contacts, followed by broken solder joints and the like.

Real techs (those of us who actually earn a living repairing amps) don't use the "blunderbuss" or "scattergun" technique of randomly replacing components because it is wasteful of time and components, but mainly because it is ineffective - bench time is valuable and you can run up a tidy sum in fitting new components and still have the fault :(, at which point you have to do what should have been done in the first place - diagnosis of the fault, then fixing the cause.


(http://i43.photobucket.com/albums/e353/tomukka/Soittokamat/SessionRockette30.jpg) (http://s43.photobucket.com/user/tomukka/media/Soittokamat/SessionRockette30.jpg.html)

(http://i180.photobucket.com/albums/x120/mamouna_1/Rockette%2030/Rockette30_9.jpg) (http://s180.photobucket.com/user/mamouna_1/media/Rockette%2030/Rockette30_9.jpg.html)

(http://i180.photobucket.com/albums/x120/mamouna_1/Rockette%2030/Rockette30_2.jpg) (http://s180.photobucket.com/user/mamouna_1/media/Rockette%2030/Rockette30_2.jpg.html)


First up we feed a signal into Fx Return (e.g. just plug your guitar in there) and see if we still have the fault.  If so then we look to the power amp section, if not we look to the preamp.  That way we cut the whole amp in half and see which half is the source of the problem.

A very big thanks to Session UK for putting up the circuits of their amps, and yours seems to be this one (includes pre and power supply);

http://www.award-session.com/pdfs/manuals/sg30-sch.pdf (http://www.award-session.com/pdfs/manuals/sg30-sch.pdf)


To answer your specific questions; C1 1uF/63V can be replaced by a 50V unit, but I seriously doubt that is the cause of your trouble. {what phatt said is generally true however this particular cap is an input signal coupling cap and is unlikely to be exposed to more than a few volts.}

If any caps need to be replaced they will most likely be the 2200uF/35V pair in the power supply, but do the Fx Return test before rushing in with your soldering iron.


Generally speaking there isn't much practical difference between caps of a reputable brand, such as Elna or Panasonic, and there is certainly no need to pay through the nose for "boo-teek" caps at several times the price.  I have been using Elna caps for around 50 years and found them perfectly satisfactory if properly specified (e.g. not over-volted etc).

Conduct the Fx Return test and post results.

Also, do you still get the buzz with all the volume controls at zero?

Hi phatt and Roly

Thanks for your sound advice to get the ball rolling...

Yep - Roly - that is the same amp - a Rockette 30. For several reasons, I want to keep this amp - it was made not far from here in the south-east of England, and is a relic from the days when things were made in small production facilities here (now it all comes from China). Apparently the guy who designed the Session amps is still around, and even though the company was bought-out then went out of business, they still have a factory where they rebuild and retro-modify these amps. Publishing the schematics is a good move - it helps people to keep the amps alive. I'd rather fix this than have a shiny new thing from China.

Anyway - here's the key things it's doing...
* When you switch it on with nothing in the jacks, there's a metallic, jagged hum, which is constant, and may be a 50hz hum. Note to answer your Q Roly - this hum remains when all knobs are turned to zero.
* When you put an instrument lead into the FX Return jack, the hum gets just a bit quieter. When you play an instrument through this jack, the tone is clean and good, it's just that there's a hum in the background.
* When you switch the amp off, it usually makes two sounds - first is a clicking or popping to accompany the switch, and after a short delay, another sound a bit like a wooden gate slamming shut.
* With Channel A, the channel sounds OK without overdrive, but if you wind the Overdrive knob up from zero, a white noise-hiss kicks in, on top of the metallic hum.
* With Channel B, the volume knob sounds poor (there is no ODrive knob), and white noise/hiss increases badly with volume (the Ch B 'volume' knob has a similar hiss to the 'overdrive knob' of Ch A).
* I noticed that when you switch the amp off with the instrument lead in the FX Return, it may have the first click/pop of the switch going, but if you take the instrument lead out within about 5 seconds, the second sound accompanies this moment. If you wait longer, you can take the lead out without this sound happening then.
* The background metallic hum is consistent, and the Ch A & B hisses are the same every time, but the sounds that the amp makes when you switch it off and on are not always consistent - sometimes they are louder or quieter.

I hope there's something in there I've said which sounds like hallmarks of common problems you've seen before.

Thanks
Jim

1) don't replace parts at random, just because you read it somewhere
2) brand means zilch, just buy standard generic commercial products, whatever a reputed supplier carries (Mouser/Digikey/Farnell/etc.)
3) test what's suggested above: get a spare plug, short it (plan B is to get a regular guitar cable and wrap the free plug end with aluminum paper or similar, so you short tip to body) and in sert the other end in the FX return jack.
That kills all of the preamp, including Reverb, and shorts power amp input.

If nasty buzz stops, it comes from an earlier point (preamp/reverb/poorly grounded pots and jacks/etc.)

If it stays loud and annoying, it's in the power amp or supply area.

Big suspects are main filter caps, although we can not discard a poor (rusty/corroded/loose)  ground connection.

I have part number labelled components, to the avoid vague descriptions .
If thare are silkscreened IDs on the PCBs, please correct the ones I assigned; if not use mine.

Measure and post:
+HT
+LT
-LT
-HT

At each of them, DC and AC voltage.

Warning: if using a cheap multimeter (the ones which have only 2 AC scales, typically 200VAC and 750VAC), it might give you impossibly high AC readings, such as 50 or 60 VAC .

Also measure VAC supplied by the transformer, you can measure at F1 and F2 .

Hi all
Following instructions from JM Fahey, these are the results...

- Shorting the FX Return Jack:
* The metallic buzz remained unaffected.
* Even with all the knobs at zero, there is a fair bit of white noise from the amp when it's sitting idle, and shorting the FX Return takes 90% of this white noise out.

Conclusion? there's two problems - the metallic hum which is continuous no-matter what you do, and a definite level of white noise/hiss from the pre-amp.

Other info you requested:
(Using a little Circuitmate DMM)
Voltage from transformer at F1 and F2: 53.7VAC

You asked for AC and DC voltages: (to do this I was seeing what voltage was on either side of the two 680R 3W resistors)
+HT  / 78.1VAC / 35.7VDC
+LT  / 35.5VAC  / 16.4VDC
-LT  /  35.2VAC / -16.3VDC
-HT / 77.7VAC  / -35.5VDC

Are those VAC readings in error? I have another multimeter, a Holdpeak Automotive Multimeter, and that didn't give a result at all for the  AC measurements.

Am I right to conclude that perhaps the power supply caps are causing the metallic buzz, but that the white noise is coming from something to do with the jacks and knobs - because there is a lot of 1/4" jacks on that amp. If so what's some basic cleaning and testing I can do for all those?

Thanks, and merry Xmas.
Jim

Thanks.
I'm chopping your answers to what I find significant to this problem.

Quote from: Jungle-Jim on December 23, 2014, 06:51:58 PM
- Shorting the FX Return Jack:
* The metallic buzz remained unaffected.
* Even with all the knobs at zero, there is a fair bit of white noise from the amp when it's sitting idle, and shorting the FX Return takes 90% of this white noise out.

Conclusion? there's two problems - the metallic hum which is continuous no-matter what you do, and a definite level of white noise/hiss from the pre-amp.

OK, the main problem is still the buzz.
The white noise may be addressed later.
That said, those amps were not considered noisy in a band environment, what makes me think that you are testing them in a quite silent place,maybe an appartment or bedroom.
Is that so?
Because that multiplies perceived noises by 20X , yet later in a garage/rehearsal studio/pub/stage they are not noticeable at all.

QuoteOther info you requested:
(Using a little Circuitmate DMM) <-- ouch!! ;)
Voltage from transformer at F1 and F2: 53.7VAC <--OK if measured from F1 to F2, that's some 27VAC to ground each, which is reasonable

You asked for AC and DC voltages: (to do this I was seeing what voltage was on either side of the two 680R 3W resistors)
+HT  / 78.1VAC / 35.7VDC
+LT  / 35.5VAC  / 16.4VDC
-LT  /  35.2VAC / -16.3VDC
-HT / 77.7VAC  / -35.5VDC

Are those VAC readings in error? I have another multimeter, a Holdpeak Automotive Multimeter, and that didn't give a result at all for the  AC measurements.

They are wrong, but I had warned you:

QuoteWarning: if using a cheap multimeter (the ones which have only 2 AC scales, typically 200VAC and 750VAC), it might give you impossibly high AC readings, such as 50 or 60 VAC .

Obviously the holdpeak is better.
Does it have a sensitive AC scale?
Something like 200mV AC
If so, we can use it to measure noise.
Post the exact model to google its manual.
All 4 DC voltages look fine.

QuoteAm I right to conclude that perhaps the power supply caps are causing the metallic buzz,

Big filter caps seem to be working .
I'd expect more of a deep "hum" than a higher pitched "buzz" , which makes me suspect more grounding than anything else.
We'll see .

Quotebut that the white noise is coming from something to do with the jacks and knobs - because there is a lot of 1/4" jacks on that amp. If so what's some basic cleaning and testing I can do for all those?

Jacks and knobs do not create white noise, that's an active electronics job :(
In this amplifier, think Op Amps .
But please don't rush to replace them, let's go step by step.
For now, Merry Xmas too.

And rememberv to post multimeter specs or model.
Also some MP3 or YT showing amp noise might help.
Play something at a comfortable level (so we have a reference) and then put all guitar controls to 0, touching anything else.
Uso no pedals, just go straight from guitar to amp.

If you wish, play a little bit distorted, then a little bit clean, with and without reverb.
That should cover most (all?)  possibilities.

To JM Fahey
Many thanks again for good advice.

I haven't made an audio recording yet - I can do that soon - but you asked what model the DMM was - it's a HoldPeak HP-760J - it was fairly cheap. I don't know why it didn't do those VAC readings - I tried it in the 200v and 750v ranges - and I'm sure the 20v range as well - blank screen. It did the VDC readings fine. It does however have a 200mV AC range. Unless I was supposed to take the red connector and put it in one of the other jacks - I just left it in the same jack it usually sits in.

What I plan to do is to unscrew the amp board off its mounting so I can see underneath - having a good look for bad solder joints or anything that doesn't look right. Also - I guess checking for earth problems I should run around the circuit board with the DMM checking all the 'ground' points for earth, and also checking the power plug as well as all the jacks and pots.

Also - there are four 8-pin ICs - I haven't got the amp unit open now, but I assume they are probably housing pairs of op-amps? What else could be they be? Anyway they are pressed into bases - perhaps I could carefully remove and reseat them (I have worked with ICs before this won't be a problem).

Using the DMM to test noise in the op-amps - I would be really interested to find out about how to do this - so if you do post info about this it will be greatly appreciated, and I will definitely not waste your time by not following up all your instructions.

Thanks again,
Jim

Meters can run into problems when trying to measure "mixed" voltages such as AC ripple on a DC supply.  You would think that a DMM would only respond to AC on the AC ranges but they also respond to DC and will give wildly high readings.

To measure the AC component on a DC rail (ripple voltage) most DMM's will require a small DC blocking cap in series with the probe - 0.1uF to 0.47uf poly, whatever you have to hand, not critical.


Your eyes are your best service instrument - and paying attention.


IC's; 1) supporting the board underneath, press each chip firmly down.  They will normally make a small click as they sit down 0.5 - 1mm; 2) gently lever each end of the IC up a couple of mm with a screwdriver, then reseat as above; 3) fully remove IC and replace.

If 1) doesn't do it, then 2) often does, progressing to 3) is not normally required.


DMM's and noise: yours is a good quality DMM but like most it has a limited AC frequency range and poor signal sensitivity, so it won't be suitable for wide band noise measurements.

A quiet preamp into a computer-based instrument would seem more hopeful.

Just checked the Holdpeak.

Looks like a cheap but somewhat above average (in that class)  meter.

It should measure AC without big trouble, there's one way to tell:
Get a (working ;) )  9V battery.
Set meter to 20V DC scale,
black probe on "Com T-  C-"
red probe on "V ohm Hz ... etc "
Measure 9V battery, it should somewhere around 9V

then set meter to 20VAC , it should measure close to 0 or maybe some initial value (going from 0 to 9VDC is AC) and in a couple seconds drift down to 0.
If so, it can be trusted measuring AC mixed with DC

If not, and it either displays some 20/25VAC it is not suitable.

Mind you, if it tries to display a value above nominal 20VAC (actually displayed as 19.99) it will show "overflow" , maybe by blanking the screen, or showing "+1---" (just the leading digit but blanking the following ones)
Unfortunately it seems to be doing this :(

But set to the 200VAC scale, that 9V battery should show some 20/25V "AC" .
Test it and confirm, so we can go on measuring knowing what to trust.

As of the noise measuring, it's basically not much more than measuring VAC at the output of the amp (that's why we need a sensitive AC scale here, usually 200 mVAC is fine, although I who use it often have an old and trusty Leader millivoltmeter with scales down to 1mVAC, full scale, and flat to 100kHz).
But of course in my case it's justified by somewhat regular use.

And the test is basically measuring VAC at the amp output, say you have 25mV noise (which is quite annoying in a silent room) and then, after replacing filter caps it's still heard, but now , say, 8 mVAC , which shows us a measurable improvement ........ or not ....  same as before ... in which case we know the problem is somewhere else.

Trying to attach some numbers to problems is much better than plain labels, such as annoying / bearable / etc.

Hope you all had a great xmas,

With this Session Rockette 30 amp:
Today I replaced the 4 power supply electrolytic caps with Panasonic units I got off the web. I also went around the circuit board with a soldering iron resoldering any joint which looked dull or not right (don't worry - I used to work as an electronics assembler - I am a very good, neat solderer). I also re-seated the 4 DIP8 IC's which are all Motorola TL072CP K8733.

Anyway - after replacing the Caps, re-soldering and reseating the IC's - I can tell you that the hum/buzz is down to an acceptable level. With the jack in the FX Return, there is a tiny bit of hum, it's not dead-silent, but I'm sure the replacement caps have solved this problem.

However, the pre-amp hiss/white noise continues, more-or-less exactly as it was.

So I would be interested to see if there's any tests I can do regarding this pre-amp hiss? I was reading on another web forum, and people were talking about Session Rockette 30s and one person was saying their's was very quiet (not hiss/hum) - so these amps can be quiet. (Another person reported a hiss/white noise like mine.)

Also, using my DMM I checked earth/ground at many places around the circuit - I couldn't find an earth/ground point which was above the 0.1ohm the DMM was giving.

Regarding my Holdpeak DMM:
I did the 9v battery test - and yes, on the 20VAC range the DMM registered a voltage at first, which dropped to zero fairly quickly. Does that mean it works OK for op-amp testing purposes?

Thanks again,
Jim

Quote from: Jungle-JimI'm sure the replacement caps have solved this problem.

Yep, in this case it looks like the main filter caps were down on spec.   :dbtu:  ( :lmao:)

Hum
These supplies are very simple, and while normally adequate for stage use, could be improved on for session work at the expense of slightly reduce maximum power by adding extra filtering and or regulation.  The only practical way you are going to get close to zero hum is to have a regulator in each of the main supplies. 

Hiss
If there is no hiss with the main amp input shorted (at Fx Return, no audible hiss generated in the power stages), then we can assume that it its accumulating through the preamps op-amps.  {the dominant noise source at these levels are the active components, transistors/op-amps.}

The TL07n series aren't too bad, but things can be taken too far, then there are quieter ones.

The very first front end ones are the really critical ones, IC1(a,b) and if you were to replace any TL072 with, say LM833's, then the very front end one(s) would be the first choice, but I'd be inclined to do them all with LM833's, or better ('tho drive capacity might be more important for IC4(a) and 2(b).


We test amps, preamps or power amps, for residual hum and noise with their respective inputs shorted.  This means that any signal appearing at the output is internally generated (or coupled) and not coming in the input.  Hiss mainly comes from the transistors in the op-amps, but op-amps are very hardened against hum or noise getting in from their supplies.  Hum in preamps is either due to earth loops, or single-ended stages such as transistor buffers that have no supply rejection (however preamp supplies are typically regulated, at least with zener diodes as here.


Noises off
Mains-borne fridge pops, drill whine, etc.  One thing I'd do to a session amp is fit a mains line filter scrounged out of a computer power supply or monitor.

(http://images.altronics.com.au/prod/p/P8352A.jpg)
http://www.altronics.com.au/p/p8352a-c14-male-socket-chassis-mount-emi-filter-6a-iec/ (http://www.altronics.com.au/p/p8352a-c14-male-socket-chassis-mount-emi-filter-6a-iec/)

Thanks Roly

OK - with the op-amps - because they are in DIP8 sockets, and relatively cheap, and I don't have the skills to test them individually for noise, it makes sense to just buy some and spend 5 minutes swapping them over.

In which case which ones should I get? I am not worried about losing a bit of volume - it'd be mainly for practising and recording, and if there's a gig it'll just go through the desk. So low noise would be a priority. However maintaining the same amp tone is also important.

For pin-compatible low-noise replacements for TL072, you mentioned LM833N - is this the one to use? Googling the topic and reading a few articles I see people saying things which might be BS, but one person was saying that TLE2072 were low noise, and had a similar audio characteristics.

But one other question about this pre-amp noise: I can't believe that an amp would be released on the market with the hiss this one's got, so have the components deteriorated?

Voltage Regulation
What would be involved in putting more voltage regulation into this amp? Are there voltage regulator ICs available? The two power supply levels are + & - 16VDC AND 35VDC. I'll also see if I've got a mains filter from a computer - I've got plenty of those around.

Thanks
Jim

There is aging, product spread, and the input stage of anything can have an "interesting" life that it might find tiring, a FET input op-amp such as the TL07n-series in particular.

These are all "low noise" these days, so you have to actually dig into the datasheets to get the noise numbers.
Here's a quick noise reality check.

Whatca got;
TL072 - Vn = 18nV/rootHz Typ at f = 1 kHz
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz


("The typical noise voltage is 18 nanovolts per square-root of the bandwidth, at 1kHz")

{For reference, the LM747, which is similar to a LM741, has a noise of 2uV/rootHz, about 100 times greater.
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz {4000 z's} zzzzzzzzzzzzzzzzzzzzz> (2000nV)
}


TLE2072 - 17nV/rootHz (and the "experts" lose again)
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz


LM4558 - 12nV/rootHz (cork sniffers favorite)
zzzzzzzzzzzzzzzzzzzzzzz


OPA2134 - 8nV/rootHz (Hi-Fi corksniffers)
zzzzzzzzzzzzzzz


My personal favorite is;

LM833 - 4.5nV/rootHz (about $3 a dual, IMO the best noise and bandwidth value for money)
zzzzzzzz


{Then we really go up market (but singles not pin-compatable)...

NE5534N - 4.0nV/rootHz
zzzzzzz


NE5534AN - 3.5nV/rootHz (Texas Instruments)}
zzzzzz



There is a chance this hiss could be due to something else, particularly if it's "rough", variable, or like scrunching paper, that's more likely to be an electro in the signal path or board contamination (sugardrink, beer, wine, coffee, rodent urine), but the input op-amps are my first obvious target, cheap and easy try.  If it's something else the upgrade won't will do some good anyway.  And no, you won't hear any tonal difference, just less hiss.


This amp has been built down to a price.  The minimal chassis provides little screening for the circuitry on the board, and has no protection at all against mains-born noise, so I think that's a must for a recording amp.

You can give it the post design tweeking it needs to shut it up and make it insensitive to external fields, etc., some screening/goundplane, get the power cabling dressed away from the signal wiring and circuits, (is the input socket of the shorting type, are its solder joints solid or wrenched and cracked?)


Voltage regulation

For the preamp a pair of three-pin regulators of the 78vv and 79vv families will perform better than the current zener regulators.

The main amp supplies are a bit more difficult, but similar, perhaps again using three-pin regulators as the essential element, but with current booster transistors so they can handle the current - but tractable at this power level.

A half-way since you have a bit of spare space between the amp board and the reverb line, would be to add some extra large value electro can caps, and feed them from the existing supply via a couple of power resistors, mebby a few ohms, 5W, making a second L-section filter for each main supply; might be easier if supply ripple turns out to be your problem, and you can then add regulators to this supply later if needs be, but it might be enough alone.


{ed: braino)

Thanks Roly
That's good advice about those op-amps - I bought some LM833N dirt cheap on ebay - it'll take 5 minutes to put them in. Will post about the results when they arrive.

Back to the other topic of improving noise levels - following on from your last post, two q's come to mind:
* Shielding the amp from interference, and keeping the power leads away from the signal leads. I guess I could look at putting a shielding over it - what would be a good way to do this - what about having a plastic cover over the amp (inside the cabinet of course), which has al-foil stuck to it which is earthed? And then there's the EMI filter plug.

* Improving the power supply: the pre-amp spec is + and - 16vdc - yet 78/79** controllers only do 15vdc (12, 18.., not 16). I don't know if that's ok. Could you possibly do a little sketch or drawing giving me an idea of an improved power supply I could give the pre-amp? The power amp is + & - 35vdc - perhaps leave that one for the time being, pre-amp is probably more sensitive to supply.

Thanks very much for your help - I have learned a considerable amount already during this - much appreciation to Roly and JM Fahey who have both given a lot of great advice. Thanks guys.

Jim

Hi JungleJim,
                If the circuit has design flaws no amount of filtering or sexy opamps will rid you of the dreaded hiss. As this rig has a hot channel then likely to be an opamp with a couple of diodes, usually a high value pot is added which gives the OD effect. If this section has no buffer then the hiss can be horrendous.

If my hunch is right then the problem is resistor noise which has little to do with type of opamp.
In the circuit below; if R2 and R3 are large,,,,, expect hiss/fizz.
Meantime Google "boltzmann's noise" or "Johnsons noise".
Roly might like to add comment on that subject. 8)
Phil.

{Just to explain my first remark above; any component that is connected to or near a socket is somewhat at risk.  Guitar inputs are typically 1 megohm impedance and are therefore somewhat more susceptible to damage from excessive voltage being applied.

ESD - ElectroStatic Discharge is one, particularly in dry air conditioned atmospheres and synthetic carpet (sound like the pub where you play?).  These conditions can lead to people building up very high voltage electrostatic charge of many thousands of volts.  Despite the very high voltages the storage element (e.g. you) has a very small storage capacitance, so the current and therefore power is small, but with bad static conditions the power level can rise high enough to cause subtle damage to components, semiconductor junctions in particular.

By "subtle" I mean the formation of a small crack in the chip lattice that doesn't cause instant failure, but with thermal cycling may get larger and spread, causing odd noises and finally chip failure.  A study done here of chip failures in telephone exchanges showed a significant peak around three days after a passing thunderstorm.}


I expect that simply changing that front end dual to an 833 should give you a noticeable drop in noise/hiss.  The sockets make this A/B test easy.  Just be extra careful when initially aligning the chip pins into the socket - it's dead easy to get one wrong and grinch it - and make sure it's the right way around before switch on.   


Yeah, the '833.  I'm retired at 65 and I've been playing with electronics, mainly audio, since my pre-teens, and in all that time I have never seen anything like the LM833.  I have some in the preamp of my homebrew Twin-50 keyboard stage amp, and they are as close to "a perfect audio op-amp" as I've ever seen.  They are effectively audio silent, distortion-free, and have an extremely wide bandwidth (which was actually a problem), and are excellent op-amps with high input impedance and good output drive capability.  Best of all they cost next to nothing and it's worth the postage overhead to get the ten-up discount and have a few spare kicking around for those "midnight ideas".

A possible exception is in fuzz boxes where the crappy audio characteristics of the LM741 might be significant, but otherwise an LM833 will improve any circuit where it will fit.


Shielding

It should be obvious that all the stray capacitances from a circuit board inside a metal box connected to its own ground, will be to its own ground, and there will be no external signal capacitive coupling or pickup.

What may not be so obvious is that a ground plane mounted close to the circuit board can get most of the same effect by being, by far, the dominant stray capacitance.  All the previous stray coupling of external signals still take place, but the dominant stray capacitance from every point on the board is now its own ground plane, and this may be several orders of magnitude greater than the stray capacitances, so these unwanted signals will be reduced accordingly.

The first element is something conductive; ali roof flashing, kitchen alfoil and glue is good, and something to insulate it from accidental contact with the circuit, a filing card tray, self-stick films of all sorts.  The metal must be connected back to the circuit central ground point, and presto - quietness.  Take these elements and look around you at the various thin conductive things like kitchen alfoil and heavy foil baking trays, and insulating things like cling-film, document protectors, plus handicraft/glues etc.  Graphite paint also works (see modem insides).  It's mainly a matter of exploring what you've got in the home office, or can easily get from the local office supply joint/supermarket.


Improving the power supply

That preamp will have a considerable tolerance to operating voltages.  I would expect it to work just fine from +/-12V to +/-18V, and somewhat better towards the top end (op-amp specs tend to improve with supply voltage, and you obviously also get more signal headroom to overload/clip). 


(http://image.slidesharecdn.com/icvoltageregulators-141013083606-conversion-gate01/95/ic-voltage-regulators-11-638.jpg?cb=1413207506)

NOTE CAREFULLY!  THE PINOUTS ARE DIFFERENT!
(http://www.talkingelectronics.com/pay/BEC-3/T0-220_regulators.gif)

See attach.

---

Ultimate noise

Fx pedals and guitar front ends are a doddle when you come to look at quality microphone preamps, say for capacitor-capsule mics.

For a standard "600-ohm" balanced line input the actual load resistor(s) is about 1k3 ohms.

All resistors (or any component having any resistance) generates a tiny amount of electrical noise due to thermal agitation, and the more resistance it has, and the hotter it is, the more electrical noise it produces.

http://en.wikipedia.org/wiki/Noise_figure (http://en.wikipedia.org/wiki/Noise_figure)

When you pass any current through the resistor you generate (yet) another source of noise.  In fact there are several different characteristic noises and sources, some mechanisms generate low end rumble, others high hiss, and others can be wide band.

With a mike like this you are looking for a noise floor perhaps -120dB below full output, but "full output" may be only a few millivolts, so you are starting to get down to the theoretical minimum noise of all your components (unless you want to suspend them in liquid nitrogen).  Thankfully guitar doesn't need ultimate noise.


Bench instrumentation is particularly demanding because you want it to remain good and faithful down to at least one-tenth of what you will be measuring with it.


{ed: typos}

The attach is a capacitance multipler rather than a voltage regulator as such, since your target is really hum, AC on the DC lines.


Low-Pass Filter time constant;
R1 * C1 = 100 * 470*10^-6 = 0.047s,
1/t = f = 1/0.047 = 21.277Hz -3dB hinge fc (i.e. below 50/60Hz power freq).


Capacitance multiplier;

C1 * hFE = 470 * 20 = 9400uF min at Imax, more at lower currents, e.g.

470 * 70 = 32,900uF

The MJ3055/2955 suggest themselves for ease of heatsinking, but just about any power pair to hand, such as the TIP-series, would do equally or better.

Season with gain or capacitance to taste.


LTSpice sim
There will be a drop of about 7 volts across the transistor(s) for an average current of 1 amp, so some heatsinking will be required for 2 times 7 watts, perhaps a scrounged CPU cooler?

The supply rails are reduced to +/-22V.

After doing a lot of sums I concluded that it will still be "quite loud enough" for studio work.

Thanks Roly
I've spent a bit of time today going over your posts to make sure I understand what you're saying...

Firstly - the easy ones - I was searching for an EMI filter amongst computer gear, and can I use one of those inline units you get on video leads - like this? http://www.ebay.co.uk/itm/6-Pcs-Clip-On-EMI-RFI-Noise-Ferrite-Core-Filter-for-5mm-Cable-/331361952793?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4d26b62419

I will look at making a grounded metallic cover around the amp - but one issue is encasing the heatsinks from the main amp chips. I doubt that's a good idea.

But onto your circuit designs...
Firstly - thanks - this is really interesting, and it forces me onto a learning curve and that's great because it's stuff I always wanted to know.

Out of the two circuits you suggested, did you decided after posting the regulator circuit that you actually thought the capacitance multiplier was a better idea? Which would you suggest I try? I can build either, that's within my capabilities.

With the capacitance multiplier, where would it be inserted (presumably immediately after the power supply?) And is it just for the pre-amp circuit? And is there a negative voltage supply version? (and what's the size for the large caps?)

With the voltage regulator circuit - are you sure 30vdc is ok for the power amp? Why does the designer give it 36v, and why the pre-amp at 16v? These are unusual amounts.

Also - a couple of Q's about that design (excuse them if they're a bit L plate):
* Resistors R1a and R1b in series - both 1 ohm 5w - is that a deliberate doubling up, or because you can't get a 2 ohm R?
* Do the 4 LEDs have a diode function here, or are they cosmetic? They would look pretty but be overkill here.
* I am a bit confused about the fact that your new design starts with a +/-36vdc supply. Does that mean that it's to attached to the output of the original supply (in other words going through the two large caps), or do you mean attaching it straight after the rectifier? In fact what I'm actually confused about here is: across the fuse, prior to the rectifier, it's 53vac, or 27 to ground each side. What would that be straight after the rectifier, as it meets the two big caps  - a crude +/-27vdc? In which case how does it end up 36vdc because of the large caps? I'm missing some basic knowledge here.

As you can see I am doing my best to keep up with this, so please bear with me when I have to ask stupid questions.

Thanks again,
Jim

Apart from practicing electronics in its many forms I have also spent a fair chunk of my like teaching it, and 'tho it may be an unpopular idea these days, learning anything takes some strain, it has to be worked at, chewed over, until that wonderful light bulb moment when a trainee who has been looking so puzzled suddenly says "oh ... I get it!"


The ferrite EMC filter is similar but a bit different and won't be as effective as a proper line filter, but better than nothing.

This is a repackage I did of an open line filter from a computer monitor but they are also found inside 'puter PSU's, and TV's and other AV gear when they are often built on a corner of the main board that can just be hacksawed off and remounted.  Buy, build or wangle, lots of options (but this is a good idea, not totally essential).

http://www.ozvalveamps.org/mainsfilter.htm#4amp (http://www.ozvalveamps.org/mainsfilter.htm#4amp)

Quote from: Jungle-JimOut of the two circuits you suggested, did you decided after posting the regulator circuit that you actually thought the capacitance multiplier was a better idea? Which would you suggest I try? I can build either, that's within my capabilities.

Remember, I'm only stumbling along just ahead of you saying "I think it's over this way".

1.  You want to convert the preamp power supply from zeners to 3-pin regs.  There is nothing wrong with doing this, some fun to be had and things to be learned, and you should end up with a superior power supply for the preamp (and therefore some confidence it is problem-free allowing you to concentrate on the shortcomings of the preamp alone).  In technical terms there will be some marginal improvements but I don't expect it to make any audible difference.


2.  Adding the small series resistor and another 2200uF to the main rails should produce some audible improvement in the power amp residual hum, but it's very hard to quantify.

Adding the capacitance multiplier (alone or in conjunction with the improved filtering) should again produce a marked improvement in residual hum levels, albeit for some loss of maximum power output.  In the LTSpice screenshot above you can see that the output hum level (red) is very greatly reduced on the primary supply hum level (green), and this is under full load, worst case.

The next step would be a full-on regulator, but it's also a matter of what is sufficient to the task, and if the capacitance multiplier alone reduces power amp hum to a negligible level under studio conditions, then we can neglect it (unless you want to go full regulation as an educative exercise  ).

---

You can stop wherever you like, go back, go forward, try something sideways like a big battery bank - when you pick up a screwdriver or soldering iron you are making several major statements; you are going to take control, you aren't going to be a consumer victim, you are going to build your own stuff making heavy use of recycle and personal ingenuity, you are authorised to change things until you are happy with them, break stuff to see how it works and what its limits are.

---

Quote from: Jungle-JimI will look at making a grounded metallic cover around the amp

...which is why I got into ground planes - they do most of the job without disrupting air flow.  In your case I'd look at some sort of metallic lining on the inside back surface, under the chassis and reverb tank.  Since hum is an issue I'd be looking for a plated steel L-plate to act as a shield between the power transformer and everything else.

These are all shot-in-the-dark ideas because the real way to find out what's needed is to take measurements and try some simple experiments.  Where is the hum getting into the signal path?  Preamp?  Power amp?  Ground loop or common earth path due to poor (PCB) layout?  A common problem is that the high current AC path from the transformer, rectifier, and main smoothing caps, is somewhere shared with a sensitive signal return path (normally somewhere near the main filter caps themselves).  Sometimes cutting tracks and re-routing them using insulated heavy wire to get the PCB layout around the PSU right, is required.  Power and signal ground path problems, where power supply noise gets coupled into the amp signal path, are actually very common even in commercial builds because it is an important topic that gets too little consideration - "meh, it's only ground".  No, it's actually the other half of your power supply flow and layout.

Quote from: Jungle-JimWith the voltage regulator circuit - are you sure 30vdc is ok for the power amp? Why does the designer give it 36v, and why the pre-amp at 16v? These are unusual amounts.

No.  With the 36V you measured 35V caps aren't good practice, 50V or 64V would be more the go, but at least with a margin of 20% over the highest switch-on, off-load voltage they will ever see.

If you fit 3-pin regulators the preamp rails should be ironing-board flat, a "blameless" power supply as Doug Self (http://www.douglas-self.com/) would say.  It may not be perfect under a microscope, but for our needs it may as well be, particularly as op-amps are built to have a very high power supply signal rejection, typically -120dB.



The negative supply version of the capacitance multiplier is simply the inversion, the cap reversed (+ve to ground) and the PNP MJ2955 used in place of the NPN MJ3055 of the +ve version (see attached).

Power amps are pretty tolerant to their supplies (which is why they are normally so basic, as here).  As the supply rails drop the power amp circuitry does a pretty good job of struggling on, but if anything may need to be reset for best sound at reduce supply it will be the idle bias current, but that may prove trivial too - see what it sounds like before you go solving a problem you may not have.


R1a and R1b are two one ohm 5 watt.  The "right" component would be 2.2r/10W but 2x 1r/5W gives us the same result with the waste heat spread over more area, the possibility of trying only one, or the two in parallel if the sag turns out to be excessive (trading supply sag for hum reduction here).  And if they turn out to be left overs later, are more generally useful than a single 2.2r/10W.  Getting the job in hand done is only part of the considerations of a designer; will this component still be available when the amp breaks down?  How hard/expensive will it be to get/replace/substitute?  Here I am also factoring in that you are a tinkerer and will have uses for floating parts somewhere downstream.

Quote from: Jungle-JimDo the 4 LEDs have a diode function here, or are they cosmetic? They would look pretty but be overkill here.

Okay, a bit of glitz, but they do have a value, particularly if your amp is playing up and you are trying to fix it in the gloom of backstage half and hour before you go on.  An extreme for built-in instant diagnostics was a broadcast compressor/limiter I worked on that had tell-tail LED's a glimmin' and a flashin' all over the board, like a New York ant city after dark.



Why the supply voltages?

The preamp rails in op-amp preamps are a pretty arbitrary choice (which may have more to do with having thousands of 16V zeners in the store that need to be turned over).  Component supplies may also level their stocks by making a special offer on something similar but different, e.g. you order 35V electros for a 30V supply and they offer you 64V at the same price because they are overstocked, and you accept  for the extra safety margin.  All sorts of non-technical reasons.

Some op-amps are comfortable on +/-4.5V (split 9V battery), most need +/-12V, and most have a maximum of +/-18V where they typically give their best, and where you also get maximum headroom to clip/overload.  While op-amp supplies are often amongst the best, cleanest and most stable, they are really overkill for op-amps which are the most tolerant of rough or "moody" supplies.


The rails in the power amp are driven by the need for a given peak voltage across the nominal speaker impedance to develop the required watts.

Starting assumptions;
R = 8 ohm speaker load (treat as 8 ohm resistor)
P = 30 watts

From these we can calculate the voltage for that power in that load (and the current), and find the peak voltage required, the minimum possible supply voltage for the power amp.

P = E²/R

P R = E²

E = sqroot(P R)

(30 * 8.0)^0.5 = 15.5V_RMS

Epk = root(2) + V_RMS

1.414 * 15.5 = 21.9Vpk

So at least +/-22V rails are needed for 30 watts into an 8 ohm speaker.

The devices in the output amp are not ideal, so some voltage will be lost across those, so the actual rail voltages have to be increased to compensate or the amp won't make rated power when the output stage is clipping.  This will normally only be a few volts due to the drops across the output transistor (due to its own Veb and the Veb of the driving transistor, the saturation voltage of the Voltage Amplifier Stage before that) and the drop across the obligatory 0.22r/5W emitter resistor, roughly around 4-5 volts total.  So the supply rails might be nominally 28V on load rising to 32V off load.

Quote from: Jungle-Jim53vac, or 27 to ground each side

Ah!  Somewhere above I thought I picked up that you had measured 36VDC.  +/-27VDC is much more reasonable on 35V caps.  Cool.  Sorry.


53/2 = 26.5 * 1.414 = 37.5Vpk
37.5 - (2 * 0.6) = 36.3V off load (corrected for rectifier diode drops)

26.5 * 1.1 = 29.2V on-load estimate


The overview remains the same, just scale the voltages down a bit.   :dbtu:

This might help with rectification.
Phil.

I just re-read most of this and correct me if I've missed something but back at "Reply 10" Jim noted that things had improved after replacing caps and other stuff but still the hiss remained.
If this is all that is wrong then surely the problem is more likely to be circuit design not shielding.
Though by the look of the pics some extra shielding would be helpful. :cheesy:

I'd hate to see Jim throw hours of work into this only to find the hiss still remains. :grr

Worth note;
A lot of bedroom guitar players tend to turn the gain way up and master volume low and often complain about the annoying background hiss/fizz.
But in a live gig you would never hear the background noise until the band stopped playing.
Of course if it's really bad then it needs looking into.
Phil.

Yeah, we're just discussing this while the op-amps are cooking (but there was some residual hum, and its a recording amp, so as a soundie I'd like him to present with a nice studio-friendly amp).

As you say about the minimalist "chassis".  But not too hard to fix on the kitchen table.   :dbtu:  It's more tedious than tricky or expensive, a bit of alfoil and some sticky schoolbook covering.

And as for the hiss, "whatever it takes baby" as Prime Minister Credlin would say.   :trouble

My money is on IC1.    8|

Roly
Thanks again - this is great. I am following this with great interest and making sure I get what you're saying. And phatt - Roly isn't getting me to do work that needn't be done. Nobody is wasting anybody's time here. As Roly says -

"...when you pick up a screwdriver or soldering iron you are making several major statements; you are going to take control, you aren't going to be a consumer victim, you are going to build your own stuff making heavy use of recycle and personal ingenuity, you are authorised to change things until you are happy with them, break stuff to see how it works and what its limits are."

That pretty well sums up my attitude to most things: learning to fix the things you use is a way to take control, and save money of course. I have the same approach to my car - I keep an old Volvo 240 on the road (here in UK) - and it's old-school enough that I can fix anything, and I know its systems. People with new cars can't even find the dipstick.

The work on this Session amp is in many ways going beyond simply trying to fix the amp itself: it's a good choice of amp for this purpose - it's simple, and the schematics were published. These amps were well regarded - their reputed strength being a good tone, and being loud for their wattage - so they are probably best suited to live work, maximum bang for the bucks and not too many frills. So it's a good project to look at the power supply and noise issues.

It's new year but the LM833N's will arrive in the post soon, so that'll tell us something once they're in about noise levels. However, the project has turned into an exercise in improving this amp, and possibly making it better than when it was new.

Back to responding to your last post Roly:


I get this bit:

E = sqroot(P R)

(30 * 8.0)^0.5 = 15.5VRMS

Then you write...

Epk = root(2) + VRMS

1.414 * 15.5 = 21.9Vpk

(L Plates out)
- are you multiplying the Vrms by 1.414 because the power rating was AC, and it needs to be recalculated into DC?
- and what does 'Epk = root(2) + Vrms' refer to? because it doesn't appear to correspond to the equation on the following line.

Also, thanks very much for doing the revised schmetics for the modified circuits - they look great. I will look inside the box and work out where to insert an additional circuit board, or whether there's room to hack the existing board to insert new parts.

But a question is - you were saying that the board needs at least 21.9V to give out 30W, but yet your revised power circuit has +/-21Vdc for the power amp. At the moment the board is getting +/-35Vdc - that's quite a drop down to 21.


Onto the next topic - the EMI filter pre-power supply:
I have computers PSU's lying around. Here's one I disassembled, with a photo of the choke/filter section. Can I cut that off, and insert it at its AC power inputs, and outputting into the amp transformer from the two points where the 4 rectifying diodes begin? And I notice that this part of the circuit includes a big 2-lead thing which looks like a transformer but is in fact some sort of inductor - so I guess that has to go in as well. Am I on the right track here?



And lastly - 'ground plane'.
Thinking about some sort of grounded cover for the circuit, you have mentioned 'ground planes' - by this do you mean at least giving it a grounded metallic shield for one side, in this case beneath the circuit board, which could soak up most of the interference? If so I could try that first.

Thanks again, and have a great new year (it's probably already happened in Australia).

Thanks
Jim

E = sqroot(P R)

...gives us the RMS (or DC equivalent) voltage for the power in the load.  This is presumed to be a sine wave, and the peak value of a sine wave, Epk, is;

Epk = root(2) + V_RMS

root(2) is 1.414

Therefore the peak voltage (minimum supply voltage) for the given power in the load is;

1.414 * V_RMS

Quote from: Jungle-JimAt the moment the board is getting +/-35Vdc

Now I'm confused.  This is what I initially understood you to say, then you revised it down to 27V (just above), now we are back to 36V - I don't get it.  What are the supply rails under idle conditions?

The first two stabs, the resistors, and the capacitance multiplier are not voltage regulators so the output voltages will wander up and down with the supply as the load varies with playing; it only subtracts a few volts from the incoming supply for its own headroom.  If the incoming is 36V then you will get around 32V out, if it's 27V in then around 22V out.  The drop can be reduced but this also reduces your hum immunity.

Quote from: Jungle-JimI will look inside the box and work out where to insert an additional circuit board

Actually the parts are minimal, it's the heatsink for the regulators and capacitance multiplier transistors you will have to find room/mounting for - most of the mods mount on this.  Mount a heasink (e.g. CPU cooler) between the PCB and the reverb line, and mount a small bit of board on that to carry the few components that need it.


I think that is a low voltage transformer that provides the standby and USB power when the 'puter is "off".

The line filter section is attached.  If you are not clear about the mains side of 'puter PSU's then for safety sake I suggest that you hunt around for one of the enclosed IEC type (pic posted above) with spade terminals on the end.

Quote from: Jungle-Jimby this do you mean at least giving it a grounded metallic shield for one side, in this case beneath the circuit board, which could soak up most of the interference? If so I could try that first.

I do.  This is pretty easy to busk up out of minimal stuff such as alfoil and glue, or if you want to get fancy a sheet of thin ali from the hardware store.  I've lined many sensitive things with alfoil, built some radio station mic preamps in sardine cans, chopped up other cans to make solderable tinplate shields on boards, and it tends to be high profit for the effort.

Thanks Roly,
I'll reply to this now, and go out later to celebrate NYE.

Quote from: RolyEpk = root(2) + V_RMS

Do you mean Epk = root(2) X V_RMS? I think I get what root(2) is now - thanks.

Quote from: RolyNow I'm confused.  This is what I initially understood you to say, then you revised it down to 27V (just above), now we are back to 36V - I don't get it.  What are the supply rails under idle conditions?

Sorry for the confusion.
I just measured again to be sure:
Coming out of the transformer, across the fuses, before the rectifier - it's 26.9VAC each side.
After the rectifier and big caps, in other words at the end of the power supply stage, it's + and - 37.5VDC for the power amp supplies (and after the big resistors etc the pre-amp supplies are +/-16.3VDC). And I don't know if it's significant, but those readings are with the speaker disconnected (because it's hard to get in there with the speaker connected).

Regarding the Noise Filter - earlier I did a graphic of this part of the circuit board, based on a reversed photo of the back of the circuit board, to clarify what's there, and I am assuming this is the noise filter. It's the first bit of the PSU to get power, and it's before the rectifying diodes. What else can it be? One curve-ball is that it's got that large inductor - is it assumed that this should also go back on? The unit you showed was tiny compared with all those components, let alone that large inductor. Am I on the right track here? Mains stuff isn't my forte, but I'm sure I can handle cutting this part off and re-using it. That power supply hasn't been used for months or longer. Shall I short all the electro caps on it?

Thanks again,
J

Disconnected speaker is okay for idle measurement.


I'd cut it fairly square, much as my cropped pic above, between the rectifier and the torroid, up past the mains connections, then across, leave some room for drilling mounting holes.  Forget the large inductor.

Mount it in a plastic box.

Profit.

{incidentally that four pin flat pack sitting over a slot in the board just near the mains connection is an opto-coupler.}

(http://www.learnabout-electronics.org/ac_theory/images/fig1-1.gif)
We hardly ever use the "average" value, it's mainly RMS or peak.
N.B. - 0.707 = 1 / sqroot(2) = 1 / 1.414


Yeah, you can short the two big caps next to the rectifier diodes, but if you've been handling the board and haven't been bitten, they're discharged.

Hi all - and Roly when you see this.
Hope you all had a good new year.

There is an update on this Rockette-30 amp...

Firstly - I cut the noise filter out of the computer PSU, cleaned it up and mounted inside the cabinet on a plastic base with plastic spacers and screws. I can make a plastic lid for it, but it's secure. Did it make a difference - I really can't say - before and after comparisons are only from memory. At the least I am glad that it went in fine, the amp still works, and I learned a bit and got some confidence from attempting this.

Then I put a ground plane behind the circuit board - again - that was a bit of fun, and when I was out getting some takeaway Indian food, I got them to give me some ally food containers, and used those which was a good bit of improvising - flattened and glued down with contact cement. Again - it didn't solve all the noise problems, but it can't have harmed it.

Then today the LM883N's arrived, they went in, and they really didn't solve the noise/hiss issues, in fact I can't even say whether they improved it at all. 

What next? The hum is not shocking, but is still worse than the little Yamaha JX20 I use as a practise amp for my keyboard, which is very quiet. And the hiss is still hardly there with the instrument in the FX Return, but appears when you wind the pots up a bit.

I will look at Roly's schematics for the power supply modifications now, and chase up ordering the bits.

Thanks
J

I don't know if I specifically mentioned that your new Indian ground plane ( :dbtu:) has to be securely connected to the amplifier common/earth.  I didn't expect the filter or ground plane to make an obvious difference, that's more a realisation one day that you haven't heard fridge pops or drill whine from your amp in a while (normally triggered by somebody else's doing it and yours not).

Like your Line Filter, but I would like to see an insulating cover, less exposed mains the better.

The LM833's not improving the hiss is not what I expected at all.

I would try to localise the source (if this is even possible) using the following steps;

(since they are happily in sockets) pull all the preamps IC's;

Shorting plug into Fx Return and measure the residual noise with a millivoltmeter*;

Remove short and confirm the noise level stays the same;

(power off)
Insert IC2 (the output end of the preamp);
(power on)
Measure residual noise at the amp output;
(pwr off)
Insert IC1
(etc)
Then 3 then 4 (reverb section).

Your hiss will either suddenly reappear at one of these steps (good) or it will just get gradually louder as the IC's go back (accumulated noise, not good).

I still think that IC1 is your prime suspect, but I'm starting to think of second order causes, and the only one I can think of ATM is that one of the protection diodes on the inputs has taken a belting at some point and gone leaky (noise generator).


{* don't got no millivoltmeter?  Beg, borrow, or steal something like a cassette deck with VU meters on the front, anything with VU meters on the front.  It won't be calibrated but it should still give you a good relative idea of level.  Fx Send to Rec In.}

Hi Roly
Thanks for being patient here...

I took these readings from the speaker cable, with the speaker removed, with the DMM at the 200 mV AC range:

The 4 LM883N are IC1-4.

With FX Return grounded, and no ICs - 0.4mV
With FX Return grounded, all 4 ICs - 0.4mV

The following are with no shorted cable in FX Return...
No IC's         8.6mV
IC 1 only      8.6mV
IC 2 only      8.6mV
IC 1 & 2       8.6mV
IC 3 only      0.7-0.8mV
IC 3 only - using a TL072  0.7-0.8mV
IC 2 & 3       1.6mV
IC 3 & 4 - with Reverb at 0 -   0.6-0.7mV
IC 3 & 4 - with Reverb at 10 - 0.9-1.0mV
IC 3 & 4 - with Reverb at 10, while tapping the reverb box - fluctuating up to 10mV
IC 1, 3 & 4   0.7-0.8mV
IC 1,2,3 & 4  1.0-1.1mv

With the speaker connected - with all ICs and FX Return not grounded - 1.0-1.1mV (appears to be the same if the speaker was connected or not.)

Is this useful, or did I need to test those ICs in and out, with the speaker connected?

Also, playing a guitar through the amp today, at a rehearsal volume, putting it through its paces, can I describe the sound: amongst the hiss, which I still think is too much, I noticed that when you hit a note or chord, that trailing behind the note, slightly delayed, is a slightly mushy clump of hissy distortion. Obviously if you wind the overdrive up, this is even more noticeable, as is the noise. Is this a sign of anything?

I am inclined to try to solve this pre-amp noise first, rather than modify the power supply - because unless I can solve this, and know the amp can sound good, tackling the remaining underlying PS hum is a secondary issue which I know I can deal with later.

(Oh and yes the ground plane is earthed).

Thanks
J

Quote from: Roly(power off)
Insert IC2 (the output end of the preamp);
(power on)
Measure residual noise at the amp output;
(pwr off)
Insert IC1
(etc)
Then 3 then 4 (reverb section).

I should have make it clear that the order of trying the IC's, 2, 1, 3, 4, was specific.  There is no point in trying IC1 alone since there is no pathway to the output if IC2 isn't in place.  The idea being to progressively add stages from the power amp back towards the preamp input.

Quote from: Jungle-Jim
With FX Return grounded, and no ICs - 0.4mV

No IC's         8.6mV

IC 1,2,3 & 4  1.0-1.1mv

This is a bit confounding.  With the power amp input shorted you have about half a millivolt of residual (which looks quite reasonable), but even with all the IC's in this rises to only about a millivolt.  I would think that this is also reasonable, and am surprised that you can even hear it - a millivolt into a speaker should be inaudible.

Just a check on terminology to be certain we are on the same page; "hiss" to a tech is a high pitched sound like steam escaping, this is what you are hearing?

I should have asked before, but specifically with the clean channel, is the hiss effected by any of the channel controls, volume, treble and bass?  I'd try to ignore the overdrive and reverb paths for the moment, try and get a handle on the simplest case, the clean channel.

{there is a side point here about needing a DC blocking cap in series with your meter when measuring AC where there might also be DC (which confuses the meter), but this normally results in the meter over-reading, so we can at least take your readings as "no worse than".}

Roly
Listening to the amp, without taking it apart to do voltage readings, and not plugging in an instrument because it's too late at night...

My definition here of hiss - it's probably closer to white noise or a snow blizzard rather than steam escaping, but it's not unlike that. And it's more of a constant, smooth stream rather than a coarse or jagged noise. And when all knobs are on zero, the hiss is more-or-less not there. It's only when you start bringing the knobs up from zero...

With the switch on Channel B, the clean channel, it does the same whether it's got a shorted jack (in the input 2 jack in this case) or nothing: from about 30% upwards (on the vol knob) the hiss becomes noticeable, very much so upwards of 60%. This channel on its own is fairly low-volume, so you'd have the vol over 50% to get anything out it, by which stage the hiss is undoubtedly present. The bass and treble knobs don't seem to introduce any new hisses of their own, but particularly the treble knob obviously emphasises the hiss in the channel.

The hiss in Ch B is not completely appalling on its own, but the usual way to play this amp is to combine both channels to get any volume or tone out of it (it's a funny set up with the two channels combining or being separable) so that means having both channels up and running. The thing is as soon as you've got a bit of volume on both channels, and a bit on the overdrive knob - to even just get to 'bedroom' volume - you've got a very noticeable hiss - you wouldn't want to record with this.

With Channel A on its own via the switch, with jack in input 1, again, on the vol knob there's not a lot of hiss introduced (but then there's virtually no instrument volume either unless you introduce the overdrive knob), but as soon as you give the overdrive knob anything, the hiss starts, and is noticeable over about 10%, annoying upwards of that. Also - stop press - I've just noticed that the overdrive knob is a bit crackly if you wind it up full - I hadn't noticed that before.

Also - I said in the previous post about another thing it does - I'll re-paste it as I described it:
"...playing a guitar through the amp today, at a rehearsal volume ...amongst the hiss... I noticed that when you hit a note or chord, that trailing behind the note, slightly delayed, is a mushy clump of hissy distortion, like an echo. Obviously if you wind the volumes up, this is even more noticeable, as is the noise."

Another observation I made was that the Ch B has a nicer tone - warm - while it's really hard to get a nice tone out of Ch A - sure it's overdriven, but it's always a bit scratchy, no depth about it.

It's clearly in the pre-amp, because playing the guitar through the FX Return is admittedly a fairly dry sound, but still it's a decent tone without hiss.

What about other possible causes then - grounding? shielding? poor power supply, input diodes, dirty pots? Sorry if this is turning into a wild goose chase.

Cheers again, much appreciated for looking at this,
J

Hi Jim,
I'd swap the chip Roly mentioned then if that does not help consider tracing the circuit,, not hard as it's single side board and the issue will be in the preamp section so you only need to work out that part.

The symptoms you describe are found in many small combo amps, some are as you say unusable at any high volume setting.

Also be prepaired to find build mistakes, It is not uncommon to find a 100k resistor on a board with 10k printed clearly on pcb.  :duh
It can be that simple but insanely hard to find if you do not have a good idea of what should be in place.

If it helps,,,I wus once a knob with a soldering iron and long before internet I drew up many circuits by simply copying the PCB back to a schematic.
Working from the component side hold it up to strong light and you will be able to follow the copper tracks on the other side with ease.
You will waste quite a few pages but you will get there and then you will have a real schematic and learn heaps in the process. :tu:

Phil.

Quote from: Jungle-JimWhat about other possible causes then - grounding? shielding? poor power supply, input diodes, dirty pots? Sorry if this is turning into a wild goose chase.

Nah, it's just getting interesting.    8|


...um... it's a bit of a shot in the dark, but hiss, odd lingering distortion ... I'm starting to wonder ... could there be a supersonic instability issue emergent here?  Aging/broken 100nF supply bypass caps perhaps? {and it hasn't been done right, each rail to ground when it should be bypassed rail-to-rail.}


Good advice from Phil, teaching yourself how to trace a PCB becomes mightily useful on the service bench.


On the circuit we've got;
Clean Ch
x10 (HF rolloff) -> tonestack -> x10 (HF rolloff) -> volume (top coupled) -> x2 (HF rolloff) -> power amp

This does not compute; my Twin-50 LM833 preamp has almost this configuration and is utterly silent (so I'm puzzled...)


The values of the HF rolloff caps isn't given on the circuit, but I'm beginning to wonder about off the circuit, if this amp has been hacked/modded at some point perhaps to get extra treble, and that the HF rolloff caps have been removed ... or something.

So are there any signs of vacant holes, desoldered pads?

I'd also try a test with the protection diodes at the inputs disconnected (lift one leg of each).

Quote from: RolyNah, it's just getting interesting.

I'm glad about that Roly - and again thanks so much for your help.

Quote from: Roly...um... it's a bit of a shot in the dark, but hiss, odd lingering distortion ... I'm starting to wonder ... could there be a supersonic instability issue emergent here?  Aging/broken 100nF supply bypass caps perhaps? {and it hasn't been done right, each rail to ground when it should be bypassed rail-to-rail.}

Is it worth a shot to replace those 2 100nF (0.1uF) 250v MKT caps? (for reference they are orange MKT metallised polyester film caps)
These amps were designed by Stewart Ward - who presumably is something of a SS amp guru - why would he get that wrong?

Quote from: RolyGood advice from Phil, teaching yourself how to trace a PCB becomes mightily useful on the service bench.

OK - this is a good point. Clearly there is a limit to how much someone can diagnose a circuit like an SS amp without learning to follow a circuit in a meaningful way and knowing what readings to take from components. If so - can somebody teach themselves to do this? Should I get an oscilloscope? And probably a signal generator? I have done quite a bit of backyard electronics over the years - fixing things by eye or hunch or basic knowledge - but my knowledge falls short fairly quickly. However I seem to be able to follow this SS amp circuit using the schematics - it's just that I can't really troubleshoot.

So anyway back to the amp...

Quote from: RolyI'd also try a test with the protection diodes at the inputs disconnected (lift one leg of each).

OK - I've located those 4 IN4148 diodes on schematic and board. That's doable - but test them one by one? and get microvolt readings or just listening by ear for changes? I'll see if I've got any spares of those - they're so common.

Quote from: RolyThe values of the HF rolloff caps isn't given on the circuit, but I'm beginning to wonder about off the circuit, if this amp has been hacked/modded at some point perhaps to get extra treble, and that the HF rolloff caps have been removed ... or something.

Well done - good bit of detective work there - I don't know your reasoning, but you're onto something here - does this help...
Looking at the schematic, I could see 3 caps which didn't have values. They were across pins 1&2, 6&7 of IC1, and pins 6&7 of IC2. All those caps are parallel with a 1M res. Guess what, looking at the actual circuitboard: While IC2 pin6&7 has a 1M res and a 22pF green ceramic cap, on IC1, pins 1&2, 6&7, the 1M res's are there alright, but where the caps were supposed to be is unused pads - there's never been caps on them. What does that mean?

Quote from: RolySo are there any signs of vacant holes, desoldered pads?

Nothing that's been removed, leaving desoldered pads - but there's unused spots like those two caps from IC1 and also blank (never went in) are D9 & D10 - the pair of opposing parallel IN4148 diodes between earth and the Ch A/B switch.

Any of that useful info?
J

Quote from: Jungle-JimIs it worth a shot to replace those 2 100nF (0.1uF) 250v MKT caps? (for reference they are orange MKT metallised polyester film caps)
These amps were designed by Stewart Ward - who presumably is something of a SS amp guru - why would he get that wrong?

You could, but I personally wouldn't bother unless they show signs of damage; these are normally pretty damn reliable class of components.

I'm a friggin' Guru too (in fact lotsa Gurus on this site  8| ), and I make mistakes, and have bad days, and misunderstand, and forget...  but I wasn't meaning to imply that there was some instability as an initial design fault, but rather from a aging/damaged/failed component, crazy mod, joint, PCB trace...

Quote from: Jungle-Jimcan somebody teach themselves to do this? Should I get an oscilloscope? And probably a signal generator?

Yes, yes*, buy, or more interesting, build.  {you can make various test signal recordings in Audacity (freeware) and play them back on an MP3 player, or similar with cassettes.  A really basic spot frequency test oscillator can be built in a sardine can around a single transistor, and there are a wide range of circuits ranging in complexity/quality up from there.}

*There are a number of free PC 'scopes and many unloved desktop machines that they will run on.  Between these and Audacity there is a lot you can wangle.


The front of my bench setup has evolved to be the CRO and the homebrew Noise and Distortion/audio millivoltmeter sitting on top.

Normally the bench signal probe is connected to the input of the mVmeter and all range setting is done with its range switch.  It has a normalised output which is distributed to the CRO channel A and the bench frequency counter behind (CRO-B Ch also has a probe and is under direct control).  The input range of the mVmeter is 1mVAC to 200VAC in 10dB steps.  (This NaD/mVmeter also has high purity sine spots available for 100Hz, 1kHz, and 10kHz distortion measurements).  The normal bench signal source is a modified ZA-1202 kit from (old) Dick Smith using an LM13600 OTA, a good design obviously bought in.

Quote from: Jungle-Jimit's just that I can't really troubleshoot.

Da kneebode connected to da thigh bone,
and the thigh bone connected to the ...

Up the river from New Orleans, step by step methodical.  Get a very firm grip on Ohms Law, the relationship  between voltage, resistance and current, and circuit theory.

Your best test instrument are your eyes.  Observation.  Be suspicious.

Designers (like me) are lazy, we reuse and recycle everything we know works.  Look in most valve guitar amps and you will find a 12AX7 first stage with a 100k anode load, 1k5 and 25uF in the cathode - it's a Lego^(tm) brick, and most of the gear we deal with is, it's highly generic.

Solid state preamps tend to be a lot more "imaginative", but mainly we are still dealing with the same set of Lego^(tm) bricks, just that there has been a whole lot of channel switching and control introduced, but that's pretty generic in itself too.

Work out your flows of power and signals, power flows downhill from the supply to the most insignificant LED (always check the supply(s) first), and signals from inputs through components and devices, switching, to the power stage and speaker.

A lot of faultfinding is binary division, splitting in half and finding which half the fault is still in, then splitting that in half, and so on until you are at the stage, then the component.  Which is cause, and which is effect?

---

Input protection diodes - lift one end of each of the four diodes, and see if the hiss stops/reduces, is one of these diode generating the hiss? {reasoning: there is no series resistance to protect these protection diodes themselves, so it's possible some excessive voltage has been applied, say from another amp, or electrostatic discharge, that has caused one or more to go leaky and thus noisy?}

Quote from: Jungle-JimI don't know your reasoning

Funny noises, tinfoil in the piano, "squrglies" (that sometimes also appear in digital systems), here makes me think that a possible cause is ultrasonic instability, either constant, or parasitic (depending on a signal to drive it over the edge).

Possible causes are;
loss of supply line bypassing, allowing unwanted coupling between stages
loss of a deliberate stability rolloff.  All the caps I mentioned (and some others) are used to reduce or limit the upper frequency bandpass, locally on each stage.  These are mostly small caps, a few tens to a few hundred pF, most likely tiny disk ceramics with obscure markings, maybe just a short number.

These are in parallel with the feedback resistors of each stage, so as the frequency rises so their capacitive reactance,
Xc = 1/2 Pi f C,
(or "AC resistance") falls to equal the resistor it is in parallel with, and at that frequency (the hinge, corner, or f_c) the response starts dropping at -6dB per octave (-20dB per decade).

Had some prune decided to create a "treble monster" by cutting out all those naughty frequency limiting caps - then I can play like Hendrix?  Wow!  Well... maybe not, but in old gear anything is possible (including Pythons  :o ).

---

Okay, here's what I would do now to try and get a handle on it.

1. Select channel B, Ch A volume, overdrive, filter to zero.

Starting with the Fx Return shorted, speaker connected, and AC millivoltmeter clipped across the speaker.
Confirm you are still getting your 0.4mV reference noise level


2. Remove short from Fx. (oops!)
Insert IC2, Ch B volume at zero.
This will give you the noise contribution of the IC2b stage only, the preamp output buffer (which has a gain of about x2).  Record.


3. Center the Treble and Bass controls, short the wiper of the treble pot to ground with cliplead o.n.o, Ch B volume to max.
This will give you the cascaded noise contributions of IC2a and b (~x10 x2).  Record.


4. Remove the short from the treble pot and place it across the 1M resistor connected to pin 3 of IC1a.  Insert IC1.  Set treble and bass to 12 o'clock/50%.
This will now include the noise contribution of IC1a (~x10 x10 x2).  Record.


5. Remove the short on the 1M resistor, and record open input noise level.



{ed to correct #1}

Roly, you omitted at which step he should remove the short from FX return.  I'm guessing at the end of step #1 ?

Roly
Thanks for that last post - there's several different things you're talking about which are all of great interest.

If I get time tonight I'll do those tests - but - in case you see this before I post my results - you didn't respond to what seems like a significant find here, as I described in the previous post:

You're talking about caps being missing - well - there's supposed to be caps (in parallel with 1M res's) coming off pins 1&2, 6&7 of IC1 of the pre-amp - but - well these caps aren't there, and never were - the pads are unused. What does that mean? There are also some diodes also missing - see the last bit of my previous post for more about this.

I'll send this now and go back to digesting all the stuff in your previous post.

Thanks a lot Roly - you're very generous and patient. There was no insinuation that you weren't 'guru' status - far from it - your ability to predict that those caps were missing was amazing. I'd buy you a beer but I'm on the other side of the planet!

Cheers
J

Quote from: g1Roly, you omitted at which step he should remove the short from FX return.  I'm guessing at the end of step #1 ?

Yep.   :-[   Corrected, thx.

---

Quote from: Jungle-JimYou're talking about caps being missing - well - there's supposed to be caps (in parallel with 1M res's) coming off pins 1&2, 6&7 of IC1 of the pre-amp - but - well these caps aren't there, and never were - the pads are unused. What does that mean? There are also some diodes also missing - see the last bit of my previous post for more about this.

Well let's put the missing diodes on the back burner; they are limiters for the signal coming from the Overdrive channel, which we currently have no interest in.  Later Mon.


The missing caps means that this preamp is wide open, no "dominant First Pole", i.e. a frequency selective network(s) that starts to roll the high frequency response just about our highest needs (20kHz for "Hi-Fi", maybe 10kHz for guitar).

The original TL07n series is no bandwidth slouch, but you have now fitted LM833's and these have a bandwidth out to 3.5MHz!  Yes, radio frequencies.  So these are particularly likely to find stray capacitance feedback paths and oscillate.  (the first incarnation of my Twin-50 LM833 preamp happily oscillated at 455kHz until I forcefully cut the bandwidth back to 31kHz.

A bit of doodling in LTSpice produces the values for an overall 10kHz top cutoff.

At IC1(a), a really tiny 15pF.

At IC2(a), 1nF.

At IC2(b), another 15pF.

Hi Roly (and whoever else is following this)

I'm just writing a note to say - no I haven't disappeared, but progress slowed because I hit a problem which took me a while to figure out.

The problem started because I couldn't find one of the caps Roly referred to, and I think one of tests he suggested in the previous post didn't appear to correspond with the PCB of the amp. To clear this matter up, I began following all the traces from the pre-amp IC's, but ended up wasting a bit of time thinking I was too idiotic to read a schematic, because it wasn't matching up, and I kept getting weird readings.

Then I ended up realising that the schematic has this error: two of the op-amps are listed as being in the wrong IC: IC2(a) and IC3(b) are in fact the wrong way around. I have taken the schematic into Indesign (a graphics program) and correctly relabelled those two op-amps.

One question: are the two op-amps in each IC working in tandem? - presumably not or else they wouldn't be able to be swapped like this.

So when I get a minute (I've been a bit busy this week), I need to print out the revised schematic, and do all of Roly's tests, then probably order a few ceramic caps and take it from there. I really question why those caps were missing, and I am keen to put in caps of the right spec and see what happens.

I'll get back in a few days when I've had more progress. 

J

Life is what gets in the way when you are trying to do things.   ;)

Quote from: Jungle-Jimtwo of the op-amps are listed as being in the wrong IC: IC2(a) and IC3(b) are in fact the wrong way around.

Oh great, that helps a lot (but at least we have a circuit).  This sort of thing (the board overprint not matching the circuit) is a not an uncommon error, but as you have found it can have you running in circles until you work it out.

Quote from: Jungle-Jimare the two op-amps in each IC working in tandem?

With few exceptions the op-amps in duals and quads are quite independent of each other apart from sharing power supply pins.  You can ignore the fact that there are two or four in the package and treat them as if they were singles.

Quote from: Jungle-JimI really question why those caps were missing

This is the sort of thing that is modified during manufacture, a production line mod, based on experience with the amp as production proceeds.  Sometimes you will find unused component holes, other times you will find extra components tacked on to the copper side of the board.  The boards may only cost a dollar each but if you have ten thousand in the store then cutting or linking tracks, omitting components or tacking extra ones in is better than junking ten grands worth of boards for such minor changes.  We can only guess that the manufactured amps turned out to be a bit duller than the production prototype, and an easy way to brighten it up a bit was to reduce or omit these top end rolloff caps.  Now that you have wider bandwidth LM833's in there it looks like these caps are again needed to keep the preamp stable.

Roly

I have attached a revised version of the schematic. I have done a number of tracing tests to confirm that IC2(a) and IC3(b) need to be renamed one another, and info about the caps on several of the op-amps.

Today I was in the local Maplins electronics shop looking for some 15pF Ceramics, and they didn't have them so I bought a pack which had a wide selection of ceramic caps - I figured it would come in handy anyway - but nearest to 15pF, it's got 10pF or 22pF. You specified 15pF across both IC1(a) and IC1(b) - but which out of 10 or 22pF would you suggest trying instead, or do I need to specifically use 15pF and if so order them in? Also - IC3(a) specifies 22pF on the schematic, but there's nothing installed on the board. Should I just put a 22pF in there?

But before that, I am conscious that in a previous post you gave a set of tests for me to do - involving putting the DMM across the speaker at 200mVAC, taking the ICs out and doing a series of tests reintroducing them. Should I still do these tests, or can I just try the caps in and see how it sounds? The thing is I was now hoping to avoid taking the LM833N ICs out again because during the last tests they went in and out several times, and it was hard not to bend pin legs, and one LM833 actually went bad during the swapping, luckily I had a few extras.

So - do those tests again - or just put caps in see how it sounds? (And if I put the caps in, is there an order to try them in, or just stick all three in?)
Thanks again,
J

Nah, you can just whack in whatever you feel like and see how it flies.  I'd start with the higher option, 22pF, and if you think it's too muffled you can change it down in value.  Don't forget that you can obtain other values by placing caps (or resistors) in series or in parallel.  For example two 27pF in series will give you 27/2 = 13.5pF, two 33pF 33/2 = 16.5pF, and so on.

The important point is to try and get the amp clean and stable, no funny sound effects.  If you overdo it the amp will sound muffled and lacking in tops, but better to overdo it and ensure the amp is stable and not supersonically oscillating (which is what I think was going on), then you can try reducing the cap values if you need more tops, but suck it and see.   :dbtu:

IF?? this was in front of me I'd be changing resistor values around IC2b (mixer chip,Top right) as that in my umble experience will be the main source of the hiss. :-X
I've just finished testing a circuit I designed which was plagued with the dreaded hiss at high levels,, so tiss all fresh in my mind.

Can't hurt to change 4 resistors,, suck it and see. ;)

You have 4 resistors connected to pin 6 of IC2b these can all be scaled down x10,, so 560k becomes 56k and so forth,, 27k, 47k. 1Meg is then 100k.

(Roly can correct me if I'm wrong here) but that should give very close to the same output but at a much reduced impedance.
The only potential gotcha is that it will likely alter the tone which could actually be a good thing depending on your musical preference.

Re those caps; My test circuit is similar in that it has 2 stages before IC2b and the cap in my circuit is 220pF. (my fb res is 100k) Now if your music is all heavy metal then you may want more top end but if you want a bluesy sound you won't like heaps of top edge,, as Roly says,, try different values.

If I get time I'll test it out as my Breadboard is running hot from much testing in the last few weeks.  :lmao:
cheers,, Phil.

Just been fiddling with some ideas and here is what I found;
Channel B,
IC3b pin 6 has 2 R's and 1 Cap and I just tested this for real and it is a big noise issue. :grr

Raising the value of both R's x 100 will deliver same gain and noise is down a lot, So 100k to ground and 1Meg between pins 6&7. You will need to change the 1nF Cap to say 10pF for the same freq response. Left at 1nF you will have no treble.

Ran out of time to workout the A Ch and it's a little more complex,, but give me time. :)
Phil.

To Roly, and Phatt.
Firstly, updating this, responding to Roly's previous posts first...
I have put 22pF ceramic caps in those three spots which had missing caps. It seems to have calmed the hiss down a lot - but not completely. I will email Award-Session shortly and see if I can get any info about why those caps were missing - because it clearly needed them - if I'm lucky I might be able to contact the designer Stewart Ward.

But - the hiss not completely gone, so I am interested in continuing to work on this amp, and use Phatt's recent posts.

Also - next time I have the amp on the work table I will also think about replacing those input 4 1N4148 diodes - I have some at home bought for another project. (Roly had previously suggested that could be one possible problem). I could test each one individually, but it seems a lot of work for components worth pennys. It'd be just quicker to replace the four and see how that goes, can't go wrong.


Hi Phatt
Thanks for your posts.

I'll go over what you're saying to clarify your points...

Firstly - you've made two separate suggestions - and I'm taking it that the second one would be the one to try first?

Going over the two posts:
In your first post, you suggest scaling down x10 the resistors around IC2b. You point out that a possible consequence may be an altering of tone - I have to say that I don't want to change the tone. With this amp I am after blues and jazz sounds - not heavy metal. I only ever have it on the edge of overdriving, and there's a sweetspot on Input B/Ch B where I can pull a nice soulful blues sound (Ch A is better if I want to cut through with a rocky lead break).

I don't really want to experiment with dropping various resistors and caps in - though I guess with those resistors being much lower, I could probably get away with leaving the old resistors in and tacking the lower ones across the underside of the board in parallel - that would be a way to test different values without doing too much component replacement (and obviously I'd properly replace the resistors if I liked the change).

Looking now at the second post...
It sounds like you're more convinced about this modification: you are suggesting bringing IC3b into the same cap/resistor combination as IC1a, IC1b, IC2b and IC3a - the 10k res becomes 1M, but the cap goes down from 1nF to 10pF (the others are all on 22pF, though Roly preferred 15pF on IC1a and IC1b).

Roly - have you got an opinion on either of these modifications? I'm more inclined to try the second one first.

And Phatt, you're saying that the mods you are talking about refer to Ch B, and that you didn't get time to look at Ch A. Well as it stands, though the amp still has some hiss, Ch B is better than A. Ch A is more hissy, particularly once the overdrive knob is over 3-4.

I need to get the amp on the work table this week, and try those mods to IC3b and report back. I will also order the parts needed for Roly's power regulation circuitry.

Thanks
J

Hi Jim, Ok you like the sound/tone and obviously you don't want to loose that. 8)

Yes I'd try tweaking IC3b first just go up x10's so 1k becomes 10k and 10k becomes 100k.

**If the hiss goes way down then forget tweaking IC2b as you then know which part of the circuit is causing the hiss.**

The tricky part is the cap value, as I'm not good with maths I cheat and use simulations to get a close value and then back to the bench and test for real.  8)

Simulation reveals that with 10k and 100k around IC3b the cap value becomes 100pF and this gives identical output and tone response. Of course sims are not perfect so once you have a ball park number you poke a few different caps in the circuit until it sounds right to your ears.

Worth a mention here,
Be careful with the PCB as it's likely the only non replaceable part in the whole amplifier.
Yes you certainly can just parallel resistors for a lower value thereby not disturbing the pcb tracks which can be easy to damage.
When you do need to lift a C or R just lift one end and float it rendering it out of service until you establish the new value, one less chance of damage to board.

If you do the above and hiss is still an issue for Ch B Then maybe look at changing values around IC2b (the mixer section)

With IC2b you can leave the 22pF cap in place for now just lower those R values and see if the hiss goes down, worry about the cap later. my guess is that those large value series resistors are causing a fair amount of the hiss or at least needlessly adding to it.

Re IC3a (Channel A) that 22pF cap you note as missing will certainly help with hiss but I have a hunch there are deeper issues.

I've still got some metal work to finish my new Cruize Control project but I'll try to get some time this week to ponder the A channel.
Phil.

Quote from: Jungle-JimI could test each one individually, but it seems a lot of work for components worth pennys.

A reason for taking the long way around (and you only have to lift one end of each diode) is that you can potentially identify exactly what is causing your excessive hiss, and know for certain you have found it.

I didn't expect the roll-off caps to totally cure your hiss, some of it will still be in-band, but they will cut off bandwidth that your guitar doesn't need and is only amplifying hiss.

I have to say that I'm a bit surprised that you still have detectable hiss with LM833's at these low gain levels.  As Phil suggests, there may be something else we have overlooked.

Quote from: Jungle-JimRoly - have you got an opinion on either of these modifications?

No, this is Phil's dance, I'll sit this one out.   ;)

 :lmao: Thanks Roly but you better keep an eye on me cause my Tango looks more like a Tangle these days. ;)
Phil.

Thanks a lot Phatt and Roly
I will put the amp on the bench and do those experiments this week. I suppose I am a bit reticent to try tests which involve a lot of "solder, connect speaker, power up, power down, disconnect speaker, soldering, power up power down" etc, because the speaker cable is so short, and it's difficult to test the amp without almost fully reassembling it. Plus this circuit board seems weaker than most - one or two solder/desolders on these pads they seem to collapse (and no the iron is not too hot). I guess I could extend the speaker wire - that'd make life easier.

Roly - with the diodes - yes it'd be nice to know which one, but I have an image of the current hiss levels from this fkn amp in my head, and if replacing the 4 diodes made a change, then I might not know which one it was, but at least I'd be able to tell the problem was in that area.

But thanks to both of you - this is getting somewhere. I still think it's a nice little amp, worth persisting with.
J

Hi Jim,
The amp works with or without those protection diodes so just lift one end of all four and see if it is the issue as Roly has loads of experience along with a nose for problem solving so I'd certainly check it out first. 8|
Oh dear I feel for you if tracks are de-laminating,, Yes I've run into that problem on older PCBs.
You may have to rig up a better way to hold/mount the PCB with a temporary bracket until you get it all sorted out. I've screwed blocks of wood to pcbs to hold them up and out for many days trying to sort out problems like this. worth going to the trouble as constant removal of circuits will usually lead to a bigger disaster.

Meantime;
Here is a screen shot of Rockette30 Chb (clean) First only modifying values around IC3b.
The two boxes down the bottom show both the freq response as well as the signal are identical in every way after the changes. The green traces (modified values) are so much the same it completely covers the yellow traces of the original values.

(IC2 on this simulation is IC3b on rockette 30 schematic)
The great part of this little sim prg is that you can reload several plots to see how the changes effect the outcome. You can't see the yellow trace but the labels are shown in the corner of each box to verify they are indeed present, (IC2Norm, written in yellow top left of each plot box)
Sadly you don't know how much noise will happen until you test it out for real which as mentioned I've already done so I can say with some certainty that this will at least help. :-X

BTW While your little story has been running I've had a few hiss issues with my own circuit so it's been good fodder to make me rethink some of my circuit. :tu:
As of the last few days I've finally finished my circuit and indeed found the above mod solved one hiss problem the other was a layout problem which I won't go into here.

So I'm fairly confident that given mods to IC3b will work. If noise is still a problem then next logical
candidate is IC2b (mixer) but simulation shows that changing those values does change the response curve by a small amount but adding the *Missing cap* on IC1a brings it very close to the original response. (2nd screen shot mods 02)
This part *I have not tested for real* so I can't say if it solves ALL noise issues but should help a lot.

At this point IF the clean Chb is now noise free or at least greatly improved then it's a fair bet that ChA has similar problems, attack that one latter.

Note; Added or changed values are marked for you. :tu:
Phil.

(http://www.yogitobye.com/wp-content/uploads/2010/06/2008-03-19-big-nose.jpg)

Quote from: Jungle-Jimbecause the speaker cable is so short

Client to service tech:
"Don't just stand their mumbling 'short circuit' - lengthen it!"

Quote from: Jungle-JimI guess I could extend the speaker wire - that'd make life easier.

Use your head to save your ... um ... posterior.


If I seem a bit picky it may be because I spent a lot of time doing industrial and mine safety equipment, then bio-med, ICU gear and the like, and when people's lives are hanging on the end of your work you need to be really sure because there is always a real possibility you may have to answer for what you did in the witness box.

An audience is less likely to scrag you if things go pear-shaped on stage, but having had that happen to me on a couple of occasions you may wish they did.  It's not "safety of life" but it's still pretty damn important if anything seriously interrupts the flow of the show.  You may never take this amp in front of an audience or studio mike, but I like to try and maintain high standards and best practice, if only out of habit.

Stop press,, I just tested the whole CHb for real on the bench and it's not making much hiss/fizz. :loco

The 10k/100k mod I mentioned at IC3b does help a little but somehow I now think something else is going on.
Bare with me ,,If I recall correctly Jim mentioned that other owners have No issues with hiss.

WotIf??? there is wrong banding on resistors?  I've had that happen to me once.
You measure a 10k resistor only to find it actually reads 100k.

So before you dig a deep hole that you can't climb out off,, how about some checking?
Most obvious candidates, 560k series R at IC2b.
if it's 56k instead of 560k then the hiss would be horrendous.

Same goes for both input stages, that 100k R that goes to ground through a 10nF cap if that is 10k the gain would go way up and hiss would certainly be evident.
R's are made in batches so if one 100k is wrong it's a fair bet all are wrong.

Love the dog Roly,,but can he dance? ;)
Phil.   

Thanks Phatt and Roly
Tomorrow night definitely I will get the amp on the bench, and get some readings and results and we'll see where it goes from there.

Thanks guys, this is brilliant, the amp is improving and I am learning an enormous amount along the way.
J

Hi Phatt and Roly

I spent some time tonight on the Session amp...

Firstly - I disconnected one leg off each of the 4 input diodes - no change to the hiss - soldered them back.

Following that, I did nearly all the tests and changes as suggested by Phatt - and I will leave what I wrote below so you can see what I did, but now... after again being a bit flummoxed by the PCB and there being a few resistors which were different to spec, and me not being able to find components which should be there, I scratched my head, then did another significant tracing of the PCB, and discovered one more thing:

The schematic got another pair of op-amps swapped in the wrong IC. FFS. IC3a and IC2b - mislabelled each other. The guys who made this board must have had a bloody good laugh because this has certainly confused me. I thought I'd correctly identified all the op-amps the previous time I discovered the other pair had to be swapped. I was correct with them, but I guess I didn't dig far enough into the circuit to see the error with the second pair.

So I'll do another PDF of the schematic, and undo all the changes I've made to the board so far, and get back to you with the new revised schematic, and maybe Phatt, if you don't mind having a look at the revised schematic when I post it tomorrow, and applying your theories to the new layout. We might have to rethink this again.

Below is a record of what I did tonight, though I now know that some of it will be useless because apart from in IC1, I was reading some of the op-amps round the wrong way...

--------------------------
Now - looking at Phatt's suggestions...
Firstly the possibly incorrectly banded resistors...
1 Phatt: "Most obvious candidates, 560k series R at IC2b. if it's 56k instead of 560k then the hiss would be horrendous."

Me before discovering the IC swap: On IC2b - both the 560k and 270k R's on pin 6 of IC2 are 470k, the 1M is 1M. Is this significant that they used 470k for both those? Should I try swapping them?

Me, later after discovering the IC swap: F*ckin glad I didn't: I now know that IC2b and IC3a are swapped - these readings are consistent with what IC3a should be - so ignore what I wrote, other than it confirming the op-amp swap.

2 Phatt: "Same goes for both input stages, that 100k R that goes to ground through a 10nF cap if that is 10k the gain would go way up and hiss would certainly be evident."

Me: On IC1a - pin 2 has 100K and 1M - which is correct, pin 6, the same - no errors on either. (NB: These readings are fine, because IC1 is correct in the schematic).

So I went to IC3b (correctly renamed after the first schematic revision):

3 Phatt: "IC3b pin 6 has 2 R's and 1 Cap and I just tested this for real and it is a big noise issue.
Raising the value of both R's x 100 will deliver same gain and noise is down a lot, So 100k to ground and 1Meg between pins 6&7. You will need to change the 1nF Cap to say 10pF for the same freq response. Left at 1nF you will have no treble."

Me: IC3b - I substituted the 10k across pin 6&7 for a 1M, and the 1k to ground became 100k. The 1nF cap across 6&7 became a 22pF. There was no particular change in hiss. I can't test the amp at any volume right now because it's late and people are sleeping, but I can say the hiss characteristics are more or less identical.

Given I've changed the res's and caps on IC3b - and it didn't solve it - should I replace the original parts back in seeing as that wasn't the problem?

So - I'm not sure what's next - I was out of my depth ages ago, and just relying on you guys. This schematic being wrong *twice* has cost me a bit of time - though I guess it's been a lesson is having to trace a board and solve a puzzle.

Thanks
Jim

Oh yes trying to correlate actual pcb and schematic can be frustrating but you are doing well. :tu:

Can I suggest isolating the sections to narrow down the field, divide and conquer.
The mixer is the last stage before poweramp and it mixes 3 signals, those being the 2 channels and the reverb return path.
By lifting one end of those 3 resistors one at a time might give a clue as to which section is causing the most trouble.

The other point
By lifting one end of those mix resistors from ChA and Rev return (points D&R)
Leaves just the clean ChB in circuit and if hiss is still present then just focus on fixing that section first.

For ref, IC1A has a gain of about x10 as does IC3B while IC2B (Mix) has a gain of x2. Obviously the higher the gain the more chance of hiss but having had this circuit working with little hiss evident I can only assume a wrong value somewhere causing way to much gain in one or more of those early stages.

There is one other possible cause of hiss which I'm reluctant to mention simply because I'm still not sure as to why it happens but Roly might know more about this odd quirk which I stumbled upon while building my latest circuit.

Both Rockette inputs are the same and just maybe,, IF? those 100k going to ground via 10n caps are close to the input cap it could cause excess noise as that was my experience with my build.
It took me a week of hair tearing trying to find the source of excess hiss/fizz when I turned up the gain in my test circuit.

I then noted when touching that resistor/cap section on the neg leg there was a lot of interference. Sure enough it was right next to the input and just by moving that pathway to the opposite side of the test board it stopped the excess hash/hiss considerably.
But of course if that be an issue it then becomes a layout problem which is a lot harder to fix.
I was so glad I found this quirk Before I etched the pcb.

Without a clear picture of your PCB (both sides) it would be impossible to know if that is the cause.

Sorry i'm not much help here and one other thing bothers me which I've only just realized and that is Ch Switch (SW1). It would seem that when SW1 is set to ChA (ODrive on) signal actually would pass through both channels via that 1k and 4n7. Which does then explain those extra 2 diodes hanging off point C which are then engaged,, maybe to add more clip.

This would then allow all front panel knobs control over the sound.

So Q? Does that happen in use.

Meantime I'll have to sleep on it. :-\
Phil.

Quote from: Jungle-JimI disconnected one leg off each of the 4 input diodes - no change to the hiss

Smart.  Now you have eliminated them as the source of your hiss you don't need to change them.

Quote from: Jungle-JimThe schematic got another pair of op-amps swapped in the wrong IC. FFS.

"You never find just one roach in a kitchen".

Our assumptions always bring us undone - check everything.

Quote from: Jungle-Jimshould I replace the original parts back in seeing as that wasn't the problem?

It's not critical, but I would so I was dealing with a reasonably consistent situation.  The more things you change, the greater the chance of confusion (and it's bad enough here that the board doesn't match the circuit numbering).

With repairs I try to be a bit Kung-Foo - hit it once in exactly the right place.


Phil - stray coupling?  The CR order might make a difference if the C was connected to ground or the op-amp input.  You could try screening the cap by wrapping it in grounded alfoil.

{and no, he doesn't dance - and my printer doesn't play MP3's either.  You seen the dingo singing the ABC theme?  Cracks me up every time.  :lmao: }


{ed speelinge}

Hi Phil, Roly, and all riveted onlookers....

Here's a new version of the schematic with corrected op-amp labelling. Note IC2 and IC3 are now completely swapped around.

I will post another thread shortly with a few photos of either side of the PCB - if there's anything to be gleaned from these - eg component placement etc.

I have restored the original cap and res's to IC3b. None of these changes are impacting the hiss either way.

But there's one thing I have to report, and I don't know if it's significant: it's to do with the guitar signal that comes out according to different switch and input settings... bear with me... (I can't say when this problem started, but some time prior to me attempting to fix this amp)

I ran these tests:

Guitar in input 1
Switch on ChA: - Vol A/OD responds ok (while adding hiss), Vol B does nothing (but doesn't add hiss).
Switch on ChB: - same behaviour as above - Vol A/OD works ok (with hiss), Vol B does nothing (but doesn't add hiss).
Switch in middle position: - Vol A/OD responds ok (while adding hiss), but Vol B, when wound in, introduces hiss but no guitar sound. (this is the questionable one I want to highlight... surely the second channel is supposed to be audible on this setting)

Guitar in input 2
Switch on ChA: - Vol A/OD responds ok (while adding hiss), Vol B does nothing (but doesn't add hiss).
Switch on ChB: - Vol A/OD does nothing but adds hiss, Vol B responds normally (while adding hiss).
Switch in middle position: - Vol A/OD responds ok (adding usual hiss), Vol B responds ok (adding usual hiss). (if both channels are responsive when the guitar is in input 2, switch in middle, why is that not the case when the guitar is in input 1, switch middle??)

So - a question mark there: admittedly this amp has a slightly confusing set up with the channels, but surely this is not doing what it's supposed to?

Phil - I will detach legs off those resisters to isolate those pre-amp sections later tonight if I get time.

Jim

First up, I'm surprised that SW1 has three positions - on the drawing it is shown as only having two.

Whatever, it selects the active channel by shunting (shorting) the unwanted channel, so in the middle position both channels should be active.

Quote from: Jungle-Jim(this is the questionable one I want to highlight... surely the second channel is supposed to be audible on this setting)

I would have thought so from the circuit.

Quote from: Jungle-Jim(if both channels are responsive when the guitar is in input 2, switch in middle, why is that not the case when the guitar is in input 1, switch middle??)

And that is a damn good question.

The primary reason would be the shorting contact on the lower input socket that grounds the join of the 1k and 470k when nothing is in the lower socket.

Hi there

Quote from: PhattCan I suggest isolating the sections to narrow down the field, divide and conquer.
The mixer is the last stage before poweramp and it mixes 3 signals, those being the 2 channels and the reverb return path.
By lifting one end of those 3 resistors one at a time might give a clue as to which section is causing the most trouble.

The other point
By lifting one end of those mix resistors from ChA and Rev return (points D&R)
Leaves just the clean ChB in circuit and if hiss is still present then just focus on fixing that section first.

I have done this experiment - here's the findings...
There are the three tracks which come into IC3a - from the Clean, OD Channel and Reverb, each with a resistor. I desoldered one leg of these resistors to disconnect these in this sequence...

Just the Reverb circuit removed (so OD and Clean)....
No change - same hiss as usual.

Reverb and OD removed, so just Clean channel...
Quieter, but some some hiss still present.

Rev, OD and Clean all removed...
Quiet, same as if the Return jack was shorted, as you would expect.

Reverb only...
Not totally silent, but acceptable levels of hiss given that it doesn't get noisy till the knob is 6/10.

Reverb and OD channel only...
Significant noise from the OD channel, on the OD and Vol A knobs, once they are past 3/10.

Reverb and Clear only...
Quieter than 'Rev and OD only', but still there's enough hiss to be annoying past 4/10 on the Vol B knob. Not quiet enough to record with. Still more noise than my other little Yamaha JX20 amp at comparible volumes.

So as it stands I have left it with the Clean and Reverb channels still connected, the res on the OD channel still disconnected. The Reverb channel isn't a problem.

The thing is that the Clean channel is just on the wrong side of being acceptable for hiss - just a bit less might be ok, meanwhlie, the OD channel has a worser hiss problem again.

Where do I go from here?

Is there a clue in that switch setting doing something a bit unexpected? Maybe not. What about that other idea mentioned about shielding the caps at the input stages.
Thanks guys for looking at this.
Jim

  The center position of the switch is where it must be set when using an external footswitch.
See pg.5 of schematic.  They call it "off", but it looks to me that when set in centre, with no external footswitch, everything should work.

  To clear up a couple more design/operation issues:
input 1 is for ch.A use only
input 2 is for ch.A or B
So that should explain some of your questions about your tests
(http://www.award-session.com/pdfs/Award-Session%20History0026.pdf).

[On a side note, it irks me that they have called the clean channel "B".  This is so non-standard, I have to keep reminding myself B=clean here   :grr]

Now, 2 issues you mentioned previously.  You said clean channel was quite weak and required adding in from dist. channel to get level.
  This does not seem right and is possibly a defect that has been overlooked.  Using ch2 input, switch set to clean, how far up do you have to bring the volume to get decent level (without adding anything from dist. channel) ?
  The other thing you mentioned was a bit of distortion at the trailing edge of the note as it fades.  Maybe a cross-over distortion issue.

Thanks g1

To answer your question:
Yes the clean channel is not as loud as the OD channel - but that's more or less to be expected. The clean channel is reasonably loud, but you get a whole new level of noise (overdriven) if you crank the OD channel. I don't know if that's a problem per se.

Yes there *was* bad 'trailing edge' distortion on a guitar sound, but I think between changing all the pre-amp ICs to LM883N's and putting those 22pF caps across the op-amps which were missing, that problem was reduced by about half - in other words I can still hear some sort of glob of noise just following a note, but it's not quite as bad as it was.

Yes making input 2 (not 1) the main input jack for the instrument seems arse-about doesn't it.

Thanks for clearing up the design questions about this amp - it's always been a bit confusing. I have seen that article before.

This amp is flummoxing me - both pre-amp channels have some hiss/noise, and I just don't know how to track it down. And this hiss can't be 'just how this amp is' because if I bought a new amp and it had this sort of hiss I'd been returning it under warranty.

Jim

Quote from: Jungle-JimThere are the three tracks which come into IC3a - from the Clean, OD Channel and Reverb, each with a resistor. I desoldered one leg of these resistors to disconnect these in this sequence...

Good diagnostic thinking.   :dbtu:

Quote from: Jungle-JimIs there a clue in that switch setting doing something a bit unexpected? Maybe not. What about that other idea mentioned about shielding the caps at the input stages.

Nah, just a draftsman being lazy.  The cap shielding was addressed to Phil with his layout.


Now we have the feedback caps in place I think we can eliminate HF instability due to the LM833's having such a wide bandwidth.

Given that we have now sorted out the IC misnumbering it might be an idea to revisit the progressive re-insertion of the op-amps working from the power amp input back towards the guitar inputs, measuring the hiss at each step. 

I'm still confounded that you are getting hiss with LM833's in a moderate gain circuit.  I've found them effectively silent and I'm dubious that what you are hearing is being generated in the IC's.

Perhaps if you could upload a few seconds of sound sample so we can hear the character of the noise?

{This one certainly proves the point that it is the subtle faults that are hardest to fix.}

Thanks Roly
I'll do this test again tomorrow, then do the audio recording when I can over the next few days.

Quote from: RolyGiven that we have now sorted out the IC misnumbering it might be an idea to revisit the progressive re-insertion of the op-amps working from the power amp input back towards the guitar inputs, measuring the hiss at each step.

So this must be the new sequence?...
(power off)
Insert IC3 (the output end of the preamp);
(power on)
Measure residual noise at the amp output;
(pwr off)
Insert IC1
(etc)
Then 2 then 4 (reverb section).

Quote from: RolyPerhaps if you could upload a few seconds of sound sample so we can hear the character of the noise?

Will do in the next few days - will take a little bit of setting up of recording gear.

Roly - if these tests yield nothing conclusive - are we at the limits of what we can diagnose without an oscilloscope? Would this be 15 minutes work running a probe around the circuit? The last tests, isolating these sections of the circuit, showed that both channels are generating hiss, albeit the OD channel being the bigger culprit. 

Jim

Quote from: Jungle-Jimare we at the limits of what we can diagnose without an oscilloscope?

Well you can do the MacGyver with a length of wet string and an unbent paperclip, but a 'scope certainly would make life a lot easier since you are trying to look at a signal that is low-level AC which is much more 'scope territory than multimeter territory.  I don't know about "15 minutes" but the view of the problem would be less obscure.

There are freeware software CRO's that will turn your sound card into a sort-of oscilloscope that may be sufficient for this job (but I would put a 0.1uF cap in between the soundcard active Line In and your probe, just to give it a bit of protection - or drag out that old desktop 'puter you don't care about).

These are pretty hopeless for absolute measurements but just fine for relative measurements, and that is really what you are interested in.

As both channels seem to be acting the same I'd concentrate on the simple clean channel for the moment.

Quote from: Roly on January 27, 2015, 02:32:33 AM

Quote from: Jungle-Jimare we at the limits of what we can diagnose without an oscilloscope?

Well you can do the MacGyver with a length of wet string and an unbent paperclip, but a 'scope certainly would make life a lot easier since you are trying to look at a signal that is low-level AC which is much more 'scope territory than multimeter territory.

There are freeware software CRO's that will turn your sound card into a sort-of oscilloscope that may be sufficient for this job (but I would put a 0.1uF cap in between the soundcard active Line In and your probe, just to give it a bit of protection - or drag out that old desktop 'puter you don't care about).

Agree.

Use any software scope which uses the 1/8"PC soundcard line in as input, but put bthis between the real world and that fragile input:

(https://imageshack.com/a/img189/6097/pcscopeattenuator.gif)

http://www.zeitnitz.eu/scope_en

Quote from: Roly on January 27, 2015, 02:32:33 AM
As both channels seem to be acting the same I'd concentrate on the simple clean channel for the moment.

I'm with Roly, Disconnect ChA and Rev and work back through ChB until you find the issue.
With only Clean in circuit maybe flick that channel switch and see if that makes a difference as
somehow these 2 channels are interconnected after the first stage, that part along with those extra diodes on ChB has me stumped as to how that works. :loco
Phil.

Many thanks again Roly and Juan

I've learned so much in the past month or two, and the adventure continues...

I'll go back to the actual amp in the next post, this one's about the software oscilloscope...

OK this is brilliant - I certainly have computers and sound cards lying around. How's the Linux version - called xoscope? That'd save me having to get a Windows PC up and running - or is www.zeitnitz.eu the least pain-in-the-a** option?

With your buffer circuit, Juan, are all those resistors 1/4w? And what are the diodes - 4148 or something bigger? I noticed that the xoscope guy had a fancy buffer which cleverly fits into a 5 1/4" disk blank, becoming a front-mounted fixture. http://xoscope.sourceforge.net/hardware/hardware.html

This being yet another thing I have never done, but would like to attempt... so if I did get this 'scope going, what tests would I perform with the probe?

For instance - would I work backwards from the point the pre-amp goes into the power amp, watching the shape of the wave, then keep going til I find the points where the hiss begins?

With any lucky the next test I do, which will be Roly's suggestion of pulling the pre-amp op-amps out, and replacing them in order, that might tell me something. I guess because the hiss happens on both the Clean and OD channels suggests the problem starts early in the chain of op-amps.

Thanks again,
Jim

Just be aware; (If I'm reading this right) Even when using the clean ChB and nothing plugged into Hot ChA any noise generated in the hot circuit could still bleed through to mixer section because it's output to mixer is still engaged. (more common to ground that point with a switch)

There is a fair chance the issue is in those middle stages of Hot ChA and even when not in use the hiss might be bleeding through. There is also that funny cross link after the first 2 input stages controlled by the switch.
Sw1 does not kill the hot ChA when the switch is set to ChB

Having tested the Clean ChB on my bench with the schematic shown it's not insane noisy so I'm inclined to think something else is going on so if you isolate that and Still get noise ,, then we have to find another way to fish. :loco
Phil.

LINUX XSCOPE should work fine.
The one I linked was just one among many.

In fact it has a 15 day trial period or something, while I'm quite certain the Linux one is truly free.

As of the input buffer, that guy (like 99% today) comes from the Digital world, and 5V is as high a voltage as he'll ever meet, so he either buffers or preamplifies his signal (I guesss he expects an electret microphone driving it)  while we come from stone age tube electronics, with macho voltages 500V or worse, so we usually think attenuate!!!

The resistors can be 1/4W ones and the diodes any you have, from 1N914 or 1N4148 to 1N400x

The scope suggested is not sensitive enough to measure noise directly at every stage, but we don't actually need that.

You scope the speaker output, will clearly see noise and more important, what kind is is (real noise is called "grass" for good reason, you'll notice why) , while RF oscillation, which often sounds like noise, looks very different.

Then you will molest your amp, by tweaking controls, shorting to ground different key points, switching channels, etc. , until you see what kills or substantially lowers it.

Hi all

The following is the results of some tests Roly had asked me to do in previous posts...

Test A  - testing mV output with ICs inserted in the order of 3,1,2,4...

"(power off)
Insert IC3 (the output end of the preamp);
(power on)
Measure residual noise at the amp output;
(pwr off)
Insert IC1
(etc)
Then 2 then 4 (reverb section)."

I put the DMM across the speakers, set to 200mVAC... All these tests were done with the 270k Res connecting the OD channel one-leg-unhooked. So just Clean ChB. To save myself having to make a table here, I've attached a spreadsheet below giving the mV results of various permutations of ICs being removed. I hope it's self-explanatory.

Test 2
On Jan 5th, Roly gave me these tests to try...

Quote from: Roly
1. Select channel B, Ch A volume, overdrive, filter to zero.

Starting with the Fx Return shorted, speaker connected, and AC millivoltmeter clipped across the speaker.
Confirm you are still getting your 0.4mV reference noise level

This test came out as 0.7mV - I was getting 0.4mV before - don't know why the change. In this test the pre-amp hiss goes to almost nothing, revealing an underlying hum which I know would improve with a better power supply. I'll think about improving the power supply once I get rid of this f*cking hiss.

Quote from: Roly
2. Remove short from Fx. (oops!)
Insert IC2, Ch B volume at zero.
This will give you the noise contribution of the IC2b stage only, the preamp output buffer (which has a gain of about x2).  Record.

I'm inserting IC3 now, because that's what we used to call IC2, the final IC in the chain. Result: 0.7mV (same as if the Return was shorted - but it sounds very different)

Quote from: Roly
3. Center the Treble and Bass controls, short the wiper of the treble pot to ground with cliplead o.n.o, Ch B volume to max.
This will give you the cascaded noise contributions of IC2a and b (~x10 x2).  Record.

Again, IC2 is now IC3 - result - 91.8mV and a terrible moaning hiss. Take the ChB vol pot to zero and it's back to 0.7mV

Quote from: Roly
4. Remove the short from the treble pot and place it across the 1M resistor connected to pin 3 of IC1a.  Insert IC1.  Set treble and bass to 12 o'clock/50%.
This will now include the noise contribution of IC1a (~x10 x10 x2).  Record.

This test wasn't affected by the IC's being wrongly labelled. Result - with the ChB Vol at 100% this test with the outside leg of the 1M grounded, comes out as 2.1mV. If you take the short off the 1M it's back to 1.8mV (that's with vol 100%).

Quote from: Roly
5. Remove the short on the 1M resistor, and record open input noise level.

This will give us 0.7mV if the Chb vol is 0%, 1.8mV if the ChB vol is 100%, as per usual.

Are these tests and the spreadsheet showing anything?

Some observations:
* With no IC's at all, the output jumps to 8.1mV and there's a moaning hum.
* Once IC3 is in, there is this hiss floor as usual, even with all pots on 0.
* That once IC3 was in, and no matter if it was in on its own or the other ICs were added it gave the same readings - ChB vol at 0% = 0.7mV, ChB Vol at 100% = 1.8mm. And at 0% the hiss was at it's usual lowest level, and at 100% the hiss rose by the same amount in each instance.

Conclusion?
The hiss is with IC3, and it's there even with all pots at 0. If the problem's in IC3, the final IC in the chain, and IC1 doesn't lift the hiss floor, then that might explain why both ChA and ChB are affected by hiss - because IC1 and IC2 are not at fault, it's the last stage of the pre-amp.

If so - what tests can I start doing on IC3?

Phew this turning into a saga to rival War And Peace.

Thanks again for your patience.
Jim

Try sticking a 0.1uF (or what have you) directly across the IC power supply pins 4 and 8 on the copper side right under the IC3 chip, nice and close.

{Just a wild idea.}

Elsewise I'd say Phatt's suggestion of lowering the impedances around this stage may help.

Subtle problems can be tricky.

Quote from: Roly on February 01, 2015, 02:06:43 AM
Try sticking a 0.1uF (or what have you) directly across the IC power supply pins 4 and 8 on the copper side right under the IC3 chip, nice and close.

{Just a wild idea.}

Elsewise I'd say Phatt's suggestion of lowering the impedances around this stage may help.

Subtle problems can be tricky.

Thanks Roly
OK good idea I'll try the cap under IC3 pin 4 & 8. If this made a change it would certainly prompt an upgrade in the power supply.

But back to Phatt's suggestion about lowering the caps/res around what we now call IC3a, the very last op-amp before output. Bear in mind that I have already done the other experiment on IC3b (AKA the 2nd op-amp in the Clean channel) - where I increased the caps and res 100x - and it made no difference.

After all the confusion of the ICs being mislabelled, and before I start soldering the board again, could I just get you or Phatt to confirm again what this other mod was supposed to be?

Was it about dropping the res's 10x around IC3a? Currently V- or pin2 brings in the outputs of the three circuits - ChA, ChB and Reverb, via 270k, 560k and 470k res respectively. And across pin 1 (Vout) and pin 2 (V-) there's a 1M res, and I put in a 22pF ceramic on blank pads which had no cap (I really don't think this cap helped much if at all).

Thanks
Jim

Working on only the clean Ch B I've circled the two resistors to change in red.

With Reverb and ChA not in circuit this should make the clean quieter, If it does then I'm certain there is just simply to much gain especially after the tone pots as that is a hi z setup which can be a dog to keep quite. xP
I had a dog of a time with my PhAbbtone circuits until I learned about High Z and high gain are a bad combination.

This will just turn down the sensitivity of the IC that picks up the tone and then add the extra gain in the next stage, as can be seen the outcome is the same but should deliver less hiss. You share the gain around until you find the best compromise.
A buffer is good at isolating the high Z but more parts count. :-X

Phil.

Thanks again Phil

I'll try those resistor changes later this evening - will report back.
Much appreciated.
Jim

Hi all
Sorry - another week delay.

I've done the two tests set for me -
Roly - the 0.1uF across pin 4 & 8 of IC3. I noticed that there's + and - rails which links pins 4 and 8 of all the ICs, so the cap could have gone anywhere along that line, but I put the cap very close to IC3 incase that had an impact on noise. Result - no discernible change. I have left the cap on, figuring it can't harm it.

Secondly, I did Phatt's two resistor changes around IC3. A 1k res became 10k, and the 560k outputting the Ch B pre-amp became a 100k. Unfortunately no change. If anything, the mV readings at the speaker changed - they changed to 0.7mV - 2.0mV when the Ch B vol went from 0-10, where as before, and in most other configurations, this was typically 0.7mV - 1.5mV.

I have left the cap in, and I've left the replacement resistors in for the time being, it's a struggle to change components on this board without the pads disintegrating.

This time I have uploaded an audio file - unfortunately recorded with my phone, I simply can't get any recording gear and the amp in the same room conveniently. There is something you can hear there. There is a very slight flanging sound in this audio which isn't in the real thing.

Thanks again for your patience,
Jim

Quote from: Jungle-Jimthe 0.1uF across pin 4 & 8 of IC3. I noticed that there's + and - rails which links pins 4 and 8 of all the ICs, so the cap could have gone anywhere along that line, but I put the cap very close to IC3 incase that had an impact on noise. Result - no discernible change. I have left the cap on, figuring it can't harm it.

I was explicit without explaining why.  Just considering DC the cap could go anywhere between the IC and the power supply, but considering AC as you move from the IC pins back along the supply rails you start introducing the series inductance of the PCB traces, and this makes the cap less effective as an AC bypass.

So when I say "right on the IC pins, VHF-style (short leads)" I really mean it.  If the noise were due to a parasitic RF or VHF oscillation then the location of the bypass cap would be critical, but as you have put it on the IC pins and it has made no change I think we can now rule out RF instability as the cause.  Narrows down the field.   :tu:

After hearing the hiss,, a couple of observations.

Are you aware that most guitar circuits do have some background noise and by the sound of that small clip you are ahead of most.
If you are playing at low volumes a lot the hiss can become annoying.

Judging by the level of your voice and then the level of hiss which was only evident at the very last I'd say you have no real issue.  8)

Gee I had a chap bring me a Laney Valve Amp a couple of years back and the hot channel was so bad the hiss/buzz/hum was louder than anything less than a power chord. :duh
Frankly I don't think you can do much better than what you have.

Some options 4 U, Turn up the gain at the first stage and then back off the gain after tone section.
As there is a lot of bass cut in the first section this does tend to accentuate the top end and that can make for a fizzy amp.

A 50~100pF cap across that 1Meg FB resistor on IC3 (mixer) will kill a lot of fizz but will effect the tone. In my experience if you want the sweeter sounds limiting the higher freq makes the amp more playable. Just my 2 cents worth. xP

Phil.

Listened to the MP3 and it sounds like you speak straight into the microphone and then place it  close to the speaker.

That's unreal, people will not listen to your amp with their heads stuck into the cone.

For a realistic test, please put the microphone 1 meter away from the speaker, your head also 1 meter away , start recording, speak and make silence for at least 15 seconds, then repeat 3 or 4 times, so we can "calibrate" our ears.

That way we can compare hiss/buzz/whatever 1 meter away with a standard voice same distance.

Otherwise we have absolutely no reference.

Do NOT fall in the trap of putting the microphone closer to the speaker "to show us how bad it is" .

Even so, hiss will be exaggerated, because very probably your recording compresses audio, so high level (your voice)  gets attenuated, and low level (hiss)  gets amplified, making it look worse (on recording)  than it actually is.

Thanks.

Thanks guys
I will have a think what to do with this amp. I have learnt an enormous amount from working on it, but perhaps it's just a noisy amp. I have other SS amps around, and I'm telling you this one has an annoying hiss. It has been a 'project' - with the fantastic help of you guys - to see if we could quieten this amp. I guess if I got it to a technician maybe the source of the hiss could be found, maybe he'd just say it's a hissy amp.

Criticisms of my recording - unfortunately I could only use my phone, so that's not a great start. Yes I put the phone near the amp, but there's already noise and hiss from the phone, so there was no point in having the phone at a distance where you couldn't separate it's own hiss from the hiss of the amp. Believe me from one metre the hiss on this amp is noticeable.

I will put the amp back together, and play it a few more times, and think about what to do.

For the time being perhaps we should close this case - and to quote Spinal Tap - 'best left unsolved'.

I plan to return to this forum soon with my next project - pimping up a Roland Micro Cube.

Thanks
Jim

Hi Jim,
        I'd be adding that ~100p cap across the mixer FB before you give up, it will kill off a lot of hiss.
I've done this many times as a lot of high freq is not needed for good bluesy sounds.

caps were missing anyway which maybe all that was wrong,,,so worth a try. :tu:

The success of Amps like early Marshall's is due to not having a lot of top end.
Players like Santana have only just enough bandwidth to cover the fundamentals and maybe one extra octave. If you add all the extra freq it just destroys the tone and the magic is lost. :'(

If I recall you wanted a more blues tone anyway so it might actually improve the sound for you.
just a thought, Phil.

  Also, have you tried contacting the designer and asking him about it?  He sounds like a pretty helpful individual and may have some suggestions.

Quote from: Jungle-Jim on February 11, 2015, 07:23:49 AM
Thanks guys
I will have a think what to do with this amp. I have learnt an enormous amount from working on it, but perhaps it's just a noisy amp. I have other SS amps around, and I'm telling you this one has an annoying hiss. It has been a 'project' - with the fantastic help of you guys - to see if we could quieten this amp. I guess if I got it to a technician maybe the source of the hiss could be found, maybe he'd just say it's a hissy amp.

Criticisms of my recording - unfortunately I could only use my phone, so that's not a great start. Yes I put the phone near the amp, but there's already noise and hiss from the phone, so there was no point in having the phone at a distance where you couldn't separate it's own hiss from the hiss of the amp. Believe me from one metre the hiss on this amp is noticeable.

I will put the amp back together, and play it a few more times, and think about what to do.

For the time being perhaps we should close this case - and to quote Spinal Tap - 'best left unsolved'.

I plan to return to this forum soon with my next project - pimping up a Roland Micro Cube.

Thanks
Jim

I wish I'd seen this last year as I could have been helping you.
Considering IC1a and IC1b in your posted circuit diagram you can change the 1Meg feedback resistors to 100k,  the 100k resistors to 10k, increase your added 22pF to 220pF and increase the 10nF not to 100nF as you would expect  but to 47nF for a better sound.
All these ideas can be seen in the later model Award-Session HiFlex140 preamp circuit and found in the S100R112 circuit diagram on the Award-Session site. My own SG30 is now very quiet.

Regards,
Steve