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

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: 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

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 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

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

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

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

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

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

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

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

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

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

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