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Solid State Amplifiers => Amplifier Discussion => Topic started by: Littlewyan on July 05, 2013, 10:54:13 AM

Title: Serious Blocking Distortion
Post by: Littlewyan on July 05, 2013, 10:54:13 AM
Hi Guys

I have built the circuit in this link: http://redcircuits.com/Page52.htm

And it gets pretty bad blocking distortion when you go above half way on the volume control. On full volume its so bad the amp cuts out and then comes back. Now I have tried just connecting my guitar to the 2nd JFET in the preamp and that works fine, so I think its probably the Pre Amp driving the Power Amp too hard. Any ideas? You can hear the blocking distortion start to come in a bit with the volume at half way by the way and its not the speaker as I'm driving a Marshall 2 x 12" speaker cab.

Ryan
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on July 05, 2013, 06:52:09 PM
Blocking distortion is normal and to be expected in that circuit.

Red Circuits are TERRIBLE :duh , and that's being too kind.

Use that preamp and PSU (such ad it not being a total loss) driving a chip amp such as TDA2050 .

Use the single supply amp example from the datasheet.
Title: Re: Serious Blocking Distortion
Post by: Kaz Kylheku on July 05, 2013, 06:56:36 PM
The base of Q3 depends on the output voltage.

It is biased with a voltage divider that originates in the output, and is grounded through the speaker. The resistors are 470K/33K, so the output has to be at around 10V for Q3 to get 0.7V.

There is a feedback mechanism at play in that if, say, Q3 is cut off because the output voltage has dipped to low, then no currrent flows across R11 and R12, and so the whole VBE multiplier is raised toward the positive power rail. In so doing, it turns on Q5 and cuts off Q6.  But if Q5 turns on and Q6 cuts off, the output voltage has swung to the positive power rail also, and so that must turn on Q3.

For Q3 to be turned on at 0.6-0.7 VBE, the output has to be at around 10V, since the voltage divider if 470k:33k.

Perhaps there are episodes of C10 getting charged up, and then forcing the output voltage down so that Q3 is cut off when it shouldn't be.

Bang it up into a schematic and simulate ...
Title: Re: Serious Blocking Distortion
Post by: Roly on July 06, 2013, 01:03:56 PM
Quote from: Kaz Kylheku
simulate

I put the power amp into LTSpice, and to cut a long story short, it's pretty 'orrible.  It has a number of problems such as asymmetric clipping, a large slew in operating point under overdrive, but one of the more curious "features" is that the gain of the OP stage is a sensitive function of the volume control position.  Some kind of instability with the volume control near maximum position, even with the preamp input shorted, seems almost certain because the main amp gain goes very high at this point - it's a bit of an abomination actually.
Title: Re: Serious Blocking Distortion
Post by: teemuk on July 06, 2013, 01:50:51 PM
Quote
one of the more curious "features" is that the gain of the OP stage is a sensitive function of the volume control position.

Well... It is an inverting amp so the gain is basically defined as Rf / Rin.

(http://upload.wikimedia.org/wikipedia/commons/thumb/4/41/Op-Amp_Inverting_Amplifier.svg/300px-Op-Amp_Inverting_Amplifier.svg.png)

Needless to say, Rin in this particular design isn't constant but depends mainly on potentiometer's setting. With volume up all the way Rin is - in theory - zero and the gain jumps through the roof. I'm not surprised that the amp also becomes somewhat unstable at that point.

Quote
Red Circuits are TERRIBLE  , and that's being too kind.

This man speaks the truth. Crappy, inreliable, unstable and dangerous designs that seem to require a complete redesign and rebuild to offer even mediocre performance. IMO, they are not worth the time to get them "fixed".
Title: Re: Serious Blocking Distortion
Post by: Roly on July 07, 2013, 11:09:14 AM
I couldn't get the sim to do the blocking bit, but the large slew in DC conditions alone is a powerful caution.  After I noticed the undefined gain I couldn't really be bothered even doing a demolition job on this one, as JM says above...

All you need to do is combine this power amp with that bloody FET preamp full of trimpots and you would have a socket-to-socket disaster (perhaps all powered by a noisy SMPS).   :duh

I had a look around that site and it's lousy with such abominations.  Publishing circuits like this is somewhere between a shame, a crime, and a danger to mental health.   :trouble


Now here is something for @Littlewyan that looks like it might actually work as intended;

(http://www.eleccircuit.com/wp-content/uploads/2008/06/circuit-40w-amp-ocl-2n3055mj2955.jpg)

Diff pair non-inverting input, gain internally defined by R4 & R10, moderate voltage rails and 8 ohm load within the capabilities of 2N3055/2955, conservative 40W output rating.  For simplicity all other transistors could be BD139/140's.  Given a sufficient heatsink this amp should actually be capable of a bit more than 40 watts.

The only thing I don't like about this design is the position of the bias setting pot VR1.  If it were to go open circuit, likely with most trim pots, the output stage would be grossly over-biased and no doubt damaged.  The right place for this adjuster is at R8.  A few fuses wouldn't hurt either.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 07, 2013, 05:44:18 PM
Thanks for everyone's feedback regarding this. I only built this amp as it was 10watts, so not too loud to use at band practice but its turned out to be a right pain. I should add that I'm also using a regulated PSU for this that only puts out 24volts, now as hes using an unregulated PSU his will probably provide 33volts under no load and maybe 28volts under load, could this be the issue? Also the blocking distortion starts with the volume halfway and sounds awful. I looked at that Chip but it puts out 32watts which is a bit too loud for what I want. Anyone know of a chip or design that only puts out 10watts at 8ohms?
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on July 07, 2013, 06:57:11 PM
Well, that's a common misconception, so let's clar it.

In fact, the power your speaker gets comes from the PSU and nowhere else, and the amp is merely the "faucet" or "trottle" that lets that power get to your speaker in a controlled way.

So if you built a PSU for a 10W amp, and you connect a "32W" chipamp there, guess what will your speaker get?
a) 32W
b) 10W

If you guessed 10W .... you won.

So go ahead with it.

EDIT: I forgot:
Quote
I couldn't get the sim to do the blocking bit, but the large slew in DC conditions alone is a powerful caution.

That's because you applied a polite , constant, well mannered sinewave.

But apply to it any real World music signal with 10 or 15V peaks, followed by, say, 100 to 500mV RMS music (Guitar/Bass/Hip Hop/Tecno/whatever) and look what happens.
Don't know whether the Sim program is sophisticated enough to simulate this, so I'll save you the waste of time: the power amp will block to death, the speaker out will stick to one rail for a few seconds, and will slowly come to normal while horribly distorting the signal.

Those seconds, specially if you are onstage performing before some people, will be the longest in your life.

How do I know?
Well, a friend of a friend of a friend told me there was this 16 Y.O. "whiz kid" around 1969, trying to design his first SS amps, and he had *big* trouble with this   :-[    :(

Fact is, the transistor base makes a nice diode together with the emotter, which to worsen things is directly connected to ground, so on signals over 700mV peak it charges the coupling capacitor real quick.
This reverse biases the BE diode, so only discharge path is through the base biasing resistor.

Now do the Math, with 47uF (C9) and R14 (33K)
I'll save you some time, the RC time constant above is 1.5 seconds.

How would you like having random 1.5 seconds muting going on and off in the middle of a hot solo?  :duh
Title: Re: Serious Blocking Distortion
Post by: Roly on July 07, 2013, 11:02:36 PM
JM> "polite", "well mannered"

 :lmao:

Not me.  I gave it a nasty flogging,  >:(  but it's a sim, and the best sim is the real world where component models aren't constrained or idealised, or "off Broadway" operation skimped or ignored, so once I saw about 10-15 volts of Q-point shift under drive I just couldn't be bothered following a lost cause any further.  I may not get it to actually blocking-oscillate in the sim but I have no doubts that it does so in real life.  As a sidebar, LTSpice has the ability to both import sound files as circuit drive, and export resulting sound files from sim circuit output.  Neat!   :dbtu:

You are quite right that these is a lot more "dog's breath" on that site; noticed a "150W" amp running a single 2N3055/MJ2955 pair on +/-42V, never mind the device datasheet (cough splutter).  The worst part about this sort of idiocy is that exactly the sort of people who will get caught, newbies, will often assume that it's all their fault they have a cloud of smoke and a pile of dead devices, can't seem to get it to work, yet it's a published and "recommended" design.  It's a pretty awful realisation that there are people around who will swear blind on their sainted grandmother that they built it, it delivers 150WRMS measured, and that it has been delivering awesome levels and tone at gigs for years - when it's entirely a wind-up, not a word of truth in any of it - all bulldust.   :trouble

Red makes play of featuring in the Reader Circuits section of Electronics World (was Wireless World, UK).

Well, as it happens I have some personal history here having been a lifetime reader of WW until it became EW and the standards dropped through the floor.  Between then and when I finally gave it up as a lost cause I had quite a number of e-mail exchanges with Svetlana (the editor, not the valve) about technical issues, mainly arising from these reader contributed circuits which variously wouldn't work, would catch fire, or in one case an electric fence that would kill somebody.  The implication on Red is that their circuits have been subject to some sort of peer review by being accepted for publication, but in reality nothing could be further from the truth.   :duh

{I just wish I could do a hot solo for 1.5 seconds.  :'( }
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 07:59:43 AM
J M

Sorry to maybe sound a bit dumb, but can you explain what you mean with regards to the PSU? My PSU provides 24volts up to 2A, does the voltage determine the power output? As this chip can take up to 25volts, so would I have to lower my voltage a bit?

Ryan
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on July 08, 2013, 11:51:51 AM
There's no dumb question  ;)

To answer with more precision, please post what chipamp are you talking about.

I had *assumed* it was a TDA2050, since it's the only "32W" popular chipamp I know, but since it stands way over 25V you must be talking about some other one.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 12:33:58 PM
Well the TDA2050 datasheet states that the supply voltage is 25V? So does that not mean it would push out 32watts at 25volts?
Title: Re: Serious Blocking Distortion
Post by: blackcorvo on July 08, 2013, 12:39:29 PM
Well the TDA2050 datasheet states that the supply voltage is 25V? So does that not mean it would push out 32watts at 25volts?

It actually says +/- 25v, which means a symmetrical supply of +25 and -25v (50v).

Since you're looking for not-so-loud amplification, you could use a TDA2030 instead of a TDA2050. It's used in most 10W guitar combos out there, and it can handle 24v without any problems (max voltage for it is +/- 18v or 36v).
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 08, 2013, 01:12:47 PM
Just a Little curious about one thing, just to make sure. when you say your supply is 24v, does that mean single or 2x12 with a center tap referenced to ground?


Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 03:40:22 PM
The transformer is a 2x30V, center tapped to ground and goes through a variable voltage regulator that is currently set to 24 volts. I only decided to use a regulated supply as I've never built one before!
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 08, 2013, 03:56:30 PM
Ok, just checking to make sure you have both positive and negative supply. (+/- 12 or 24 v), as if you used a single supply you would need to offset the voltage to 12v if using a single 24 supply, and also the "ground" would need an offset to the virtual ground at 12v.  The red circuit regardless how bad, says +/- 24 v. 48v transformer with center tap.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 04:03:22 PM
Ah now it is a single supply as I'm only taking the positive side of the AC Waveform coming from the two 30V windings on the transformer, which gives me a 30V positive rail that then goes into the voltage regulator. So I don't have a negative supply, just a ground.
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 08, 2013, 04:11:57 PM
That might explain some of the problems with the first Circuit aswell. There will be no headroom avaliable at the negative side of the signal which will distort bad.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 05:19:50 PM
Well I have changed C10 for a new capacitor in case the other one was leaky, also changed the output transistors and now I find I get the blocking distortion ALL the time. Tried increasing the voltage to 26 volts but exactly the same issue occurs. Measured the voltage on the emitters of the two output transistors and it is 13 volts, on the base of Q3 its 0.7 volts exactly. I think these voltages are correct, so perhaps its the preamp causing the issue?
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 05:36:35 PM
I should add that C5 is always connected in my circuit, the 47uF capacitors in the Preamp are actually 22uF which should make the situation better in theory and the line coming from R13/R14 has got a ground line going over the top of it on my PCB, however both are covered by heatshrink.
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on July 08, 2013, 05:37:19 PM
Quote
so perhaps its the preamp causing the issue?

Is this a joke?   :o

There's 2 pages of posts telling you this power amp is cr*p and explaining why and you still think it's something else?

Please re-read the entire thread.

As many times as it's necessary  :cheesy:
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 08, 2013, 05:46:23 PM
As jm says, you should probably abandon that amp for a better suggestion, i just wanted to make the supply configuration clear. As a dual supply with center tap referenced ground is not the same as a single supply.

Edit: you can still split a single supply, but you will need to add a virtual ground, and remove the dc offset created that way. It wont just replace the psu out of the box.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 05:48:02 PM
I understand that, however others have also said the power amp should work to an extent. I just wanted to see if I could get this to work, then I was going to use the TDA2030. Does the TDA2030 overdrive nicely btw? As in when its pushed?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 08, 2013, 05:59:36 PM
Havent heard it myself, but if you got the power supply and a heatsink already, its really cheap to build the single supply circuit jm suggested from the datasheet. 7 caps, 4 resistors, the ic and 2 diodes. :)

In my opinion i think you will benefit more from overdriving the preamp when it comes to solid state circuits.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 06:06:13 PM
Well in that case I think I'll just get hold of the parts and use the chip, as it sounds like the only thing I'll learn from this circuit is how to NOT build a power amp.

By the way, how would I overdrive the Preamp?
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on July 08, 2013, 06:06:40 PM
Go straight to the TDA2030 .... or even better 2050, cost is the same.

Sound?:

http://www.youtube.com/watch?v=0H7TVB81e8w

http://www.youtube.com/watch?v=d_1A9lwA6uk
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 08, 2013, 06:28:54 PM
Sounds good, I'll get onto the parts buying on Wednesday and I'll let you guys know once shes finished. Thanks for all your help. I'll stay clear of RedCircuit in future and will spread the word!
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 09, 2013, 02:43:39 AM
What were the two diodes for by the way? Distortion?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 09, 2013, 03:34:28 AM
The diodes looks like polarity protection or something like that, cant say for sure. Distortion should happen in the preamp, while the power amp should be as clean as possible.
Title: Re: Serious Blocking Distortion
Post by: QReuCk on July 09, 2013, 04:56:19 AM
I'm with stormbringer there:
If you think about it, the real point of SS amps aside from the price and maintenability is the scalability. If for some reason you have your power amp distort, then you miss the biggest tonal advantage of SS amps: they sound almost as good at low level as at near-full volume. The only difference is introduced by psychoaccoustics (our ears do not compress bass and trebbles the same way depending on volume), room reverb (I meen "real" room reverb, you know sound wave bouncing on the walls and the ceilling) and feedback (sound waves from the amplifier exciting guitar strings).
Tube amps sound better at high volume because tube power amps are push-pull stages clipping slowly and symetrically while tube preamps are single-ended triodes clipping kinda slowly and asymetrically (which can sound good to an extent, but doesn't sound quite the same).
Fortunately enough, you can build a fairly pleasing soft symetric clipping into a solid state preamp. You can also put some asymetric clipping if you need it and even design the possibility to mix them together. So there is really no point in trying deliberately to have a solid state power amp sound different depending on the outup volume.
Title: Re: Serious Blocking Distortion
Post by: Roly on July 09, 2013, 12:40:26 PM
Quote from: Littlewyan
I'll stay clear of RedCircuit in future

Wise move.  I try to give credit where it's due, but after reviewing a number of the circuits in all sections on that site it really is a large collection of carefully selected rubbish.  In fact it's almost amazing how few workable circuits there are there.

Quote from: Littlewyan
What were the two diodes for by the way? Distortion?

No, it's intended as a clean amplifier.

The primary function of these two diodes is to prevent the amp output going above the supply or below ground where it might be damaged.

The reason that this is a concern is that a loudspeaker is not a pure resistance but an inductance which can store energy, and in some situations it is possible for this stored energy to be returned to the circuit in such a way as to drive the output above or below the power supply rails, and these diodes are there to prevent that and protect the chip from damage.  In many situations they would not actually be needed, but datasheet circuits tend to try and cover every possibility, no matter how remote.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 10, 2013, 07:02:43 PM
Hi Guys, I've drawn up schematics for both the PSU and the Amp itself and attached them to this post. What do you think?
Title: Re: Serious Blocking Distortion
Post by: phatt on July 10, 2013, 07:58:34 PM
@ Littlewyan.
Your  understanding of PSU needs work.
Ask yourself,,What does the bottom Diode Do?????????

Can I suggest;
http://sound.westhost.com/project27.htm (down the bottom is PSU)

http://sound.westhost.com/index2.html
Go to the site map for tons more stuff. You will be there a long time so take lunch ;)

Or Dload Teemu's PDF Book on SSAmps right here on SS Guitar. winky.

http://www.ssguitar.com/index.php?topic=711.0

Phil. 
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 11, 2013, 02:27:24 AM
Does this not increase the load capability of the PSU? As one diode can take 1A, so surely one on each winding means i can draw 2A?

The first link you sent me btw was a PSU design i planned to use in future, looks much better ;)
Title: Re: Serious Blocking Distortion
Post by: Roly on July 11, 2013, 02:44:25 AM
PSU;

- The cathodes of the rectifier diodes should be connected.

- The 1uF's would be better as 0.1uF's, and right on the regulator pins.

- The voltage adjustment is a bit fraught; if the wiper goes open on the track the output voltage goes to the unregulated max.  It is better to use a fixed resistor that is a bit larger than required to give an output voltage a bit higher than required, then trim it down with a trimpot in parallel.

- the 100k across the output is not required.


With the amplifier; there is no need to have the input resistance as low as 220k.  The first FET looks effectively like an open circuit, so the Gate return resistor may as well be at least 1 megohm, particularly if you will be using it with a passive guitar, and could even go as high as 10 megs for tone reasons.

- The 100nF input cap, 100pF RF stopper, and 100nF power supply bypass, and 0.47uF in the Zobel are shown as polarised, they aren't.

- There should be a bypass cap, say 100uF, on the supply rail after the 1k, on the Drain of the second FET.

- A de-popping resistor would be a good idea across the output to charge up the 1000uF if the speaker isn't connected, say 1k.


P.S. the difference between one diode and two in this case is between a half-wave and full wave rectifier.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 11, 2013, 02:57:32 AM
O god i'm sorry i forgot to connect them, apologies it was late last night i drew these up. Ok i'll make amendments tonight. The trim pot modification on the PSU may be a bit difficult as its already built and bit tight on space in that area but i'll see what i can do.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 11, 2013, 09:00:45 AM
I've amended the schematics and attached them to this post. The PSU I may leave as is as it is already built and working, however I will add the 4.7K Resistor in parallel with the Trim Pot to give a maximum of 24volts. I've also left that 100K resistor on the PSU in case I want to test the PSU without the amp connected.

With regards to the AMP the capacitors you specified are on the schematic as non polarized, they just use the same symbol as the polarized capacitors except for the missing positive sign. I will add an 100uF capacitor by Q2's Drain if need be but it does work ok with that 470uF on Q1's Drain at the moment.
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on July 11, 2013, 04:58:18 PM
It looks fine and should work well.
Have you already tried it?

Only minor problem is that the tone control will not work as intended, since it's trying to shunt highs to ground from a low impdance point, but that's a minor matter.

After you check your amp is fine, I'll suggest a more useful tone control-.

Another minor problem is that those 1N400x PSU diodes are barely usable (1A), I'd replace them with 1N5402 or higher (3A).

But it's already looking fine  :dbtu:
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 11, 2013, 05:09:40 PM
It does work quite well at the moment though? A little too well to be honest it can make the sound so bassy that you can barely hear the high notes!

This amp won't use more than 2amps will it?
Title: Re: Serious Blocking Distortion
Post by: Roly on July 11, 2013, 09:27:27 PM
Quote from: Littlewyan
The trim pot modification on the PSU may be a bit difficult as its already built and bit tight on space in that area but i'll see what i can do.

Meh, your build; can you live with the possibility of raw power finding its way to the chip amp?  I think it's below the maximum, and they tend to be pretty rugged and ignore crud on the supply anyway...  :shrug:

Quote from: Littlewyan
the capacitors you specified are on the schematic as non polarized, they just use the same symbol as the polarized capacitors except for the missing positive sign.

They are an abomination that is neither the US nor Euro stantard symbol.

(http://www.learnabout-electronics.org/ac_theory/images/capacitor-symbols.gif)

There are lots of young programmers who want to reshape the world in their own image, and while "standards are as exciting as a Russian truck" they also allow communication without confusion.

Quote from: Littlewyan
it does work ok with that 470uF on Q1's Drain at the moment.

100uF or 470uF is no matter, but nothinguF was shown on your Mk1 cct.

Quote from: Littlewyan
This amp won't use more than 2amps will it?

DC? No.  But a) 1N400x are 1 amp diodes (http://www.digchip.com/datasheets/parts/datasheet/131/1N4001-1N4007-pdf.php), and b) the peak current that flows on each cycle of the mains to recharge the filter cap is very considerably higher than the DC or average current.  Diodes are rated with this is mind, however that doesn't change the fact that the 1N400x-series are only one amp (average) diodes.

Quote from: Littlewyan
A little too well to be honest it can make the sound so bassy that you can barely hear the high notes!

Which isn't surprising considering that at the extreme it simply connects 1uF across the signal path - a bit savage.  I'd lose that tone arrangement entirely and look at putting a more conventional tonestack in  just ahead of your "Gain" control.  Failing that you could reduce the 1uF to something more like 0.1uF.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 12, 2013, 07:01:59 AM
Alright I didn't mean to upset anyone by picking the wrong symbol, I just used that symbol as it was there in front of me and I understood what it meant. If I was publishing this schematic online (which I most definitely will not) then I may of put a bit more thought into it. I'm not a young whippersnapper trying to reshape the world, I'm just trying to learn about electronics.

It wasn't on the schematic you're right, I just forgot to add it in, that capacitor is currently in the circuit. I'm not saying you're wrong in anyway, I was just pointing out the circuit currently works with the 470uF in it (well barring the power amp obv), sorry I forgot to point out my mistake.

I will add that trim pot modification onto the PSU, just need to work out how due to space etc. It may be below the maximum for the chip but it won't do my Pre Amp much good as those Transistors can only take up to 25volts. Not sure how the guy who originally designed the amp had 33volts going on it as those Transistors should of blown surely.
Title: Re: Serious Blocking Distortion
Post by: Roly on July 13, 2013, 09:41:13 AM
Quote from: Littlewyan
Not sure how the guy who originally designed the amp had 33volts going on it as those Transistors should of blown surely.

And here we are back again at what JM and I were saying above about some of the circuit sites that are around the net being a health hazard.  They walk like a duck, quack like a duck, but are actually turkeys.


I ain't upset - it's a matter of what sort of impression you want to give/leave (and by uploading your circuits here you are actually publishing on the net).  Give Google the right keyword search and it will pull up your circuit, or plonk somebody down in the middle of this thread.

Drawings are another form of descriptive language, and be they architectural, engineering fabrication, hydraulic, or electronic, they all have conventions, and if you step outside those conventions then communications starts to get muddied.  There are some drafting programmes around that have been put together by keen and effective programmers, but some of these haven't bothered to find out what these drafting conventions are, and why, and simply invented their own.  While these work to a greater of lesser extent you frequently find that when you want to do something entirely reasonable in conventional electronic drafting you either can't, or can only do it wrong.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 14, 2013, 07:36:30 AM
You made a very good point Roly, so I've edited my schematics. I shall remember to use these symbols in future.

I have now ordered a couple of TDA2050s, so once they arrive I shall dig out the rest of the parts and work out how I'm going to lay out my PCB. I will also change that tone control capacitor to try and improve things, just for the moment anyway, might change it altogether as suggested by yourself.
Title: Re: Serious Blocking Distortion
Post by: Roly on July 14, 2013, 11:48:09 PM
Yo!  Now the instant I look at that it starts to tell me its story (but you still have that 100k on the output of the power supply that you don't actually need).

There is another convention which we have discussed here in the past, and it concerns how you draw wire that cross and connect, and that cross and don't connect.

There are (like caps) a couple of conventions.  One is that crossing wire that join have a dot and those that don't join don't have a dot.  The other is that wires that join simply cross, and wires that don't join have a little hop-over loop.

Straight away we can see a couple of potential confusions, that join in one convention is don't join in the other, and that a missing dot will change a join to a not join.

For 50-some years now I have followed a mixed convention; wires that join cross with a dot, and those that don't hop over.  This is neither convention, but it is also another step away from a possible misunderstanding, and in the end that has to be the dominant thing, a drawing that represents the situation in the best possible way that avoids misunderstanding.

While I may draw my circuits like that whenever I can (and some software can make it very hard to do what you want) the world is still full of people who follow different conventions so you will never entirely escape having to deal with them.

Guitar amp tone controls generally are an area where you can have almost endless fun trying stuff out.
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on July 15, 2013, 10:39:34 AM
That's the clearest meaning convention, and it follows "real world" stuff inside the chassis: where wires join you *do* have a "dot": the solder blob, and where they don't. one "jumps" the other, they are definitely not in the same plane.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 17, 2013, 03:08:37 PM
I've updated my schematics so they are much easier to read now. I've added the dots in and also removed the 1M Pot between the two Pre Amp Transistors as I believe when I had this in the circuit before it simply acted as a volume control.

I've not got round to building the circuit just yet as I've been a bit busy, but tonight I have taken off all the components off the PCB from the old Power Amp so I'm ready to start laying out my circuit.

Now once I've built this, if I want to increase overdrive/distortion within the pre amp, how would I go about it?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 17, 2013, 03:29:56 PM
To increase distortion you could either add more gain stages, amplifying past the headroom voltage avaliable to the transistor, it will then cut the peaks of the waveform creating distorsion, or you could use clipping diodes. There are several ways of doing this, but the simplest form is just 2 facing the opposite direction in parallel shunted to ground. You might wanna add another gain stage after them to keep the signal at the desired level for he power amplifier.

I'm working at a heavy distortion preamp design, which i will upload once it is done, it uses both soft clipping and hard clipping through diodes.
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 17, 2013, 03:36:57 PM
You can also use a boost pedal in front, which usually basically works just like adding another gain stage in front of the others. The result depends on how the circuit works. A nicer distorsion requires some tone shaping in my opinion, filtering out some frequences that would make the sound "muddy" for example. It's not easy making a heavy distorsion, although there are some diy versions out there that sounds great. (Madbean upper decker, j0k3rx's black forest) etc.

For less distorsion, but great sound. Take a look at maxon od808 or any of the tube screamers. Might take some work integrating into your preamp, but good way to learn i suppose.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 17, 2013, 03:55:23 PM
Thanks, I'll see what the amp sounds like once I've built it and go from there. I don't actually like very much overdrive, but then I don't like clean either :D. If I can't hear my open chords ring out, theres too much overdrive!
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 17, 2013, 03:58:58 PM
I'll probably try both the diodes and the extra gain stage, get an idea of the different sounds both methods make. I quite like the fact that a lot of people slag off the diode method when actually companies such as Marshall were using that method in their valve amps :D
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 17, 2013, 04:06:27 PM
I think you would prefer soft clipping then. :) (diodes in the feedback loop of a gain stage)
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 17, 2013, 04:13:14 PM
Ok on my amp would that be between the negative input of the TDA2050 and the 22k resistor coming off the output of the chip?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 17, 2013, 04:50:00 PM
The hard clipping diodes could go right after the 22uF cap after the input stage, but would benefit alot from Another stage after, to bring the level back up, as the clipping will lower the signal volume, so i would add a gain pot Before the diodes, and Another stage after.

Soft clipping, i have only seen done with op-amps. cant really say how well they would work or how to be done with a transistor.

Edit:

Found this at GeneralGuitarGadgets, uses both types of clipping.

(http://www.generalguitargadgets.com/richardo/perfboard/electra.gif)
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 17, 2013, 05:22:08 PM
Right I see, I think I know how to achieve the soft clipping now, however the online schematic website I use is currently down so I can't publish a schematic to show you how I think I can do it  :-\
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 17, 2013, 05:56:26 PM
Site came back up, I've attached an updated schematic. I picked the 1N4148 as a few circuits I looked at used those.
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 17, 2013, 06:38:58 PM
Hm.. I dont think using the clipping diodes in the power amp is a good idea. They should be located in the preamp section, preferably followed by another gain stage to bring the level back up again.

Stage 1: the input stage, raise the level.
Stage 2: clipping stage
Stage 3: recovery stage.

With a gain pot before the clipping stage you get controllable distorsion, without the output volume being affected as you know the total voltage after the clipping stage depending on the diodes used. That way its easy calculating the optimal gain for the recovery stage.
Title: Re: Serious Blocking Distortion
Post by: Roly on July 18, 2013, 12:38:17 AM
May I just chip in here that it is worth experimenting with using different diodes in each half of a clipper to get asymmetric/unbalanced clipping, say a single diode in one direction and a couple of diodes in series in the other.  If you can find some germanium diodes try pairing one with a silicon.  Different colour LED's have different cut-in voltages so something like a red and green back to back is also worth a try.  This imbalance tends to give more even harmonics.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 18, 2013, 01:46:33 AM
Hmm could i do that with a JFET though? Or would i have to add a bjt stage?

Yer i think i saw a schematic where they mentioned using 2 diodes one way and 1 diode the other to get the extra harmonics.
Title: Re: Serious Blocking Distortion
Post by: Roly on July 18, 2013, 06:24:54 AM
Ah, Mk4 and Mk5 we have lost the ground return resistor on the Gate of the second FET.

I'm also not sure that this second FET is doing much for you.  While it lowers the impedance at that point it is already pretty low out of the first stage anyway.  What might be more interesting is to put the Gain control back in after the first stage, and change this second FET stage for something like the electra.gif circuit above.

Re: the diodes in the feedback of the TDA2050; if you do that you will limit the output voltage swing to +/-0.7 volts, or pretty much stuff-all, into your speaker.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 18, 2013, 06:05:35 PM
Right I found a website to help me design the second stage of the Pre Amp, only part I'm not sure about is the 5K Pot as I'm supposed to use a 3K Resistor on the base......
Title: Re: Serious Blocking Distortion
Post by: Roly on July 19, 2013, 07:04:16 AM
Point #1; as a general rule the load of a following stage should be at least about five times the Anode/Drain/Collector load of the driving stage, thus with a 27k Drain load the following resistor (pot) should be at least 100k.  (When we look at this first stage in AC terms the Gain pot winds up effectively in parallel with the 27k Drain resistor).

Point #2; your 5k pot isn't DC isolated from the transistor base, so when turned down it will shunt all the bias to ground and turn the transistor off.  A blocking cap required.

I think you also need a small value decoupling resistor in the supply rail directly above your tone control so that the preamp stages are isolated and protected from signals impressed on the supply by the power amp chip (or you may get some form of instability).  Calculate resistance for a 10% drop in supply voltage at whatever current the first two stages draw.


"376" ohms?  Boy, where did that value come from?  The normal E12 10% value progression is;

10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100. (http://www.logwell.com/tech/components/resistor_values.html)

Outside precision instrumentation you simply don't need resistors that are more accurate than this almost universally available range.  With the exception of precision audio filters and oscillators, the use of 1% resistors in audio circuits is simply not required and comes under the heading of "spurious accuracy" - an advantage which is purely an illusion, and frankly often just to impress.

When you consider the device it is biasing will have a spread of around 30%, and a FET may be anywhere between 3:1 and 10:1 spread, ordinary caps are 10% and electro's can be as wide as -50/+100%, then using 1% resistors is simply an absurd waste in the home workshop.

{and it also suggests that the person who originally specified these values didn't actually know what they were doing.}
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 19, 2013, 01:46:15 PM
That was just the value i got from my equation, i will obviously put a std resistor in there i just couldnt think of what value at the time. I shall take a look later if i have time, got band practice first. Thanks guys for all your input on this.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 19, 2013, 06:27:52 PM
Ok I've redone the schematic again. If I didn't have you guys I would of seriously gotten nowhere  :o
Title: Re: Serious Blocking Distortion
Post by: Roly on July 19, 2013, 09:47:52 PM
Those clipping diodes may still need some work, but it's getting down to suck it and see time.   :tu:
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 20, 2013, 03:55:23 AM
Why is that?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 20, 2013, 06:39:05 AM
There's alot you can do with clipping diodes to experiment and find what suits you best. Right now you have "symmetric" clipping. it clips both positive and negative side equally, by putting one extra diode in series with one of them, or using for example 2 different diodes (maybe a LED and a standard Silicon diode) you might get interesting results. But start out easy, and then you can try out different configurations. :)
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 20, 2013, 07:54:25 AM
Ah right yer I see. Thanks guys :) I'll order the parts now and will hopefully start building next week
Title: Re: Serious Blocking Distortion
Post by: Roly on July 20, 2013, 02:10:28 PM
...aaaaand also, other possible variations (depending on what you find in practice) are;

- insert an electro in series with your clipping diodes so the stage can be biased without the diodes conducting,
- placing a resistor in series to soften the clipping,
- placing the diode pair in different positions, such as across the base to ground, or, again with a series cap, across the signal path from collector to ground,
- switching diodes in/out.

These depend on how quickly the stage goes into clipping and what you desire, what sounds good to you.  You could spend a month just trying out different diodes in different positions.  (A little point about LED's - just be aware that they are also light sensitive.)

{And this is where rolling your own has it all over just going out and buying somebody else's compromises - you get to tweek, diddle, and futze with it until it pleases you, and if you ever change your mind you can get into it and give it a rewiring it'll never forget.  ;) }
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 21, 2013, 05:53:43 AM
This is very true. I want to get this amp just right so i can use it at practice if my other amp dies or even at small gigs or even recordings.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 29, 2013, 08:07:50 AM
Would it make a big difference if I used a 220uF Cap on the Output instead of 1000uF?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 29, 2013, 08:33:28 AM
It only affects the low cutoff according to the datasheet, and my calculation says 16 hz original, around 70hz with 220uF. You can try it, and if you later feel you lack low end you could increase the capacitor later.
Title: Re: Serious Blocking Distortion
Post by: Roly on July 29, 2013, 12:18:18 PM
Quote from: stormbringer
It only affects the low cutoff according to the datasheet

The problem here is that the chip datasheet isn't wrong, just incomplete.

Electrolytic caps are rated for capacitance, for maximum working voltage, but they also have a lesser known rating for ripple current, or maximum AC current that may pass through them without excessive heating.  The ripple current rating is loosely related to the capacitance so it rises as the capacitance rises.

This amp chip is rated for a 35 watt output into 8 or 4 ohms.  We can work out what the required AC current is for this power in both loads;

P = I2R

P/R = I2

I = Sqrt(P/R) = (P/R)0.5

I = (35/8)^0.5 = 2.09 amps

For 4 ohms;

I = (35/4)^0.5 = 2.95 amps

So there is actually quite a lot of AC current flowing through this output coupling cap at full power.

Looking at the RB caps listed in the Jaycar catalogue a 220uF/50V has a ripple current rating of only 430mA, even 1000uF/50V is rated at only 900mA, and we don't find a rating in excess of 2 amps until we get to the 4700uF/50V.  The danger with using a cap that is being consistently driven in excess of its ripple current rating is that it may literally explode.

This is also an important consideration with power supply caps in high powered amplifiers.

Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 29, 2013, 05:01:01 PM
I really dont intend to hijack the thread, but just for the sake of it, how would parallell caps act of used for DC blocking? :) I have been taught for example that often 2 smaller caps can be a better choise than one bigger for example as filter capacitors after the bridge rectifier in a Power supply, due to the lower ESR. What if you would use, let's say 2x470uF in parallell instead of 1x1000?
Title: Re: Serious Blocking Distortion
Post by: Roly on July 30, 2013, 05:38:04 AM
This question is spot on.  You will notice above that if you use five 220uF caps in parallel you will get the datasheet recommended value of ~1000uF and a ripple current capacity of 5x 450mA;

Strictly;

5 * 220 = 1100uF, and

5 * 450 = 2250mA or 2.25 amps

A particular reason why multiple parallel caps can be better than a single big one is the ratio of volume to surface area which is much larger with multiple caps, and therefore can get rid of heat better.  (they may also be easier to physically arrange in a build)
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 31, 2013, 06:48:44 PM
Well guys just to give you an update, the amp is now built..........................and it works! Mostly. I couldn't turn it on for very long as it was half 11 at night when I finally finished it, but I did have it on long enough to find that at full volume it wasn't quite as loud as it should be.

Now I just realised that the 1M Resistor on the Gate of the JFET is actually a 220K at the moment, I know this probably isn't the cause but I guess it doesn't help either. Also I have left out the tone control as well as I'm not too bothered about having one at the moment, may add one later, so that should surely help give a bit more volume as tone controls tend to lower the gain of the circuit. I have also got the PSU still set to 24 volts, so I'll try raising this to 28 volts tomorrow night as the TDA2030 datasheet specifies 28volts (14+/-) for 8/9 watts.

Once the volume is sorted I may try to add a bit more overdrive as it didn't sound like it had much, but one step at a time. I'm just thrilled that it worked first time, as you all know most circuits tend not to work at first due to things being missed!

Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 31, 2013, 07:04:19 PM
Cool. :)

Actually the overdrive level might be volume related, have you made an audio probe? it's usually very helpful in finding out where the signal weakens.

The 220k Resistor does lower the input impedance, and might be what lowers the volume.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on July 31, 2013, 07:12:11 PM
How would i use an audio probe? Sorry i've not used one before.
Title: Re: Serious Blocking Distortion
Post by: Roly on July 31, 2013, 07:16:43 PM
One fifth the input resistance won't help at all.

8 or 9 watts is actually pretty damn loud - into a reasonable speaker.  So the obvious question is, what speaker are you using?

What impedance (ohms) is it?  If it's 16 ohms I'd expect it to sound a bit weak.

If you are using a typical Hi-Fi type speaker with an efficiency of around 80dB/W then I would expect it to sound quite gutless, but if you are using a modern guitar speaker in a reasonable enclosure with an efficiency of 95-100dB/W, then you have some sort of problem.

You won't hear the difference between 24 and 28V supply.

If you are using a passive guitar (one without a battery inside) then changing the input resistance from 220k to 1Meg will improve the frequency response and make it sound richer, as well as giving you more raw signal level.
Title: Re: Serious Blocking Distortion
Post by: stormbringer on July 31, 2013, 07:19:04 PM
(http://music.codydeschenes.com/wp-content/uploads/2013/02/audio_probe.gif)

that is a simple, but awesome tool. you just connect it through any audio Equipment with a sound input, i usually use it through my Zoom R16 Sound card/portable studio, as it has LED VU-meters which will show me the signal level aswell as letting me hear the sound.

But also, i Think replacing that 220k resistor with a 1M or higher will be a good thing to test.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 01, 2013, 03:20:30 AM
I'm testing it through a Marshall 2 x 12 8ohm cab so the speakers are fine. I'm definitely not getting 8/9watts out of it at the moment, although i dont think its far off to be honest. I'll change that gate resistor tonight and will then do some proper testing.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 01, 2013, 01:23:38 PM
Ok gate resistor is now 1M, this has helped however the output still isn't as good as I would like it to be, also the sound wasn't great. So I upped the voltage coming from the PSU to 26 volts and it started to sound better tone wise. Now I measured the output voltage and it reached 8 volts at the most, which divided by 8ohms (speaker cab) gives us 1 amp, multiplied by 8 gives us 8 watts so maybe I should increase the voltage further to get a bit more volume? I know this is a crude way of measuring it but its the best I can do with the tools I have.

Also when I turn down the 100k pot even just a tiny bit from max the sound starts distorting badly, I'm guessing a larger pot is required? Turning down the 10K pot does what it should and just decreases the volume.

Also the distortion is a bit harsh so perhaps I should try different diodes on the BJT? Its also slightly too bassy so I'm going to try decreasing the resistor in series with the input to help with that.

Apart from that the blaady thing works! Can't thank you guys enough for all the help you given me, learnt so much.
Title: Re: Serious Blocking Distortion
Post by: stormbringer on August 01, 2013, 01:30:28 PM
I Think you got the 100k pot reversed if turning it down creates distortion. You could absolutely try different diodes, (Try leds, diodes in series etc). Nice to see you got it up and running! Good job. :)

Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 01, 2013, 01:41:04 PM
Well no its correct because the volume does decrease, it just seems to distort even more. Very weird. Also I don't think the preamp is driving the power amp hard enough, as after the 1.5K resistor I've only got 18volts, whereas I designed the preamp to be run on 24volts. So may change the collector resistors for slightly lower values. Also I'll probably remove the 22k resistor just above the two diodes as its doing nothing :P
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 01, 2013, 03:30:52 PM
Also when I say distort its almost unrecognizable, as if a transistor is just being pushed stupidly hard.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 01, 2013, 04:18:53 PM
Ok firstly I changed the collector resistor on the BJT from 1.8K (all I have at the moment) to 1K and this just caused the same distortion effect I get when I turn down the 100K pot. So I changed it back to 1.8K and all is well again (except for the pot issue). I then changed the collector resistor on the JFET from 27K to 22K and wow what a difference. Plenty of overdrive which sounds great for single notes but not too great for chords. Now I'm getting 12.5 watts out of it which sounds great! Next thing to try is removing the two diodes on the BJT to see what effect this has, may even fix the pot issue I'm guessing?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on August 01, 2013, 04:33:26 PM
Well, you changed the bias for the jfet, the question is if it clips now adding overdrive, as it is an input stage, i Think you would want it to stay Clean, but if you like the overdriven sound, maybe replace the bjt/diode stage with Another jfet stage.

Problem with overdriving the first stage and removing the clipping diodes will be that the 100k pot will not be a gain pot anymore, but Another volume pot. But as i said, maybe the jfet distortion is the sound you are looking for, and in that case you could just replace the clipping stage with a jfet instead.

You should probably check the level of the sound coming out of the jfet, and listen to it with the audio probe. Maybe the level is way too high, and hence the diodes clip no matter what level you turn the gain pot to? Or maybe the Bias on the BJT is incorrect? what's the DC level of the input/output signal?
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 01, 2013, 04:37:46 PM
You are right with regards to the 100K pot becoming a volume control, however now that I've removed the diodes the overdrive sounds great. Only quarrel I have with it is the amount of bass at lower volumes, so I was thinking of removing the 100K pot and then putting my tone control back in (only have two holes for ports on the case). A gain control would be nicer obviously but can't work out why that 100K pot wouldn't work properly.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 01, 2013, 05:30:38 PM
Added a tone control in and now it sounds pretty sweet at all volumes, I'll upload an updated schematic another day as its 22:30 here. I'm going to give the amp a proper test at band practice tomorrow night before I say its finished, may not be loud enough yet. I want to use this as my backup amp at gigs :D
Title: Re: Serious Blocking Distortion
Post by: Roly on August 02, 2013, 11:19:05 AM
If you have a supply of 24 volts then you have a peak swing of +/-12V or an RMS load voltage of 0.707 * 12 = 8.484V.

P = E2/R

so roughly

8*8 = 64 /8 = 8 watts

Gain of the TDA2050 is about 22k/680r = 22/0.68 = 32 times

If output = 12Vpk then input will be 12/32 = 0.375 or 375mVpk

The main cause of the bassy response is the low value of the tone pot, 5k.  I changed it to 100k 'tho it may not need to go anything like that high, perhaps 20k to 50k, but really should be about 10x the reactance of the tone cap at 500Hz (~3k ohms).

The transistor stage is drawing much more current than it needs to because of the low resistor values.  The Collector and Emitter resistors are increased x10 while the bias resistor is increased to keep the collector voltage about half the supply.  This all has the incidental effect of increasing the input impedance and reducing the loading on the FET stage, increasing signal levels somewhat.

With the current drain of this stage much reduced the preamp supply rail goes much higher.  This allows more headroom and also shifts the bias of the FET stage to a more reasonable point, about 10 volts or again about half the supply.

Reducing the FET Source bypass from 22uF to 1uF has the effect of giving some bass rolloff below about 500Hz (instead of the current bass boost), again brightening the response.

The first stage is still clipping at a fairly low 300mV input, and part of the reason for this is that the first control isn't actually a Gain control but simply another volume attenuator.  If the 4k7 Source resistor was changed to a 5k pot (now surplus from the tone control) and the 1uF connected from wiper to ground, this then really would be a variable gain stage and could be turned down so it didn't clip even with volts of input, or up so it clipped easily.

HTH

Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 02, 2013, 11:57:20 AM
Thanks for that Roly, I've currently got the amp working using the design I've attached to this post. It works quite well but obviously I don't have a tone control. I'm taking the amp to band practice with me tonight to test it and will know then if I definitely like the sound. Although I may adopt your design anyway!

With regards to having the 4.7K Source Resistor as a 5K Pot, isn't this bad?

O by the way since I put the tone control back in the bass response as gone down slightly which is nice, but I won't know for sure until tonight when I can actually give the amp a proper going through. Also I used a 1uF Solid Tantulum in the end as the 100N Cap I tried didn't make any difference whatsoever to the tone which is odd, unless its because I'm using a 5K Pot!
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 02, 2013, 07:15:19 PM
Tried the amp tonight and found that it still has too much bass, so I'll try what you said Roly and lower the 22uF Cap on the JFET Source to 1uF.

Also I found it overheats on full volume pretty easily! So I've just ordered a much bigger heatsink for the Voltage Regulator as the current one has a rating of 21C/W, compared to the heatsink I just ordered which is 12C/W. When the amp is at full volume the regulator will be dissipating 6.65watts, so multiply that by 21 and you get 139 Degrees C, which is above the regulators 125 Degrees limit. With the new heatsink it should run at a maximum of 79.8 Degrees C.

Other than that it sounds good, even my rhythm guitarist liked it and hes very picky!
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 03, 2013, 11:40:25 AM
Also Roly what is that simulation program you're using?
Title: Re: Serious Blocking Distortion
Post by: Roly on August 04, 2013, 12:13:36 PM
LTSpice, available for free download from Linear Technology, no strings, Getting Started pdf, fairly compatible with other flavours of Spice.  Also Yahoo LTSpice group stuffed with models and an active forum.

Murphy's Law of heatsinks - they are never big enough (you always need more than you thought).
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 04, 2013, 06:24:40 PM
If I change the resistor values for the BJT to ones you have specified won't that lower the output of the BJT?
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 04, 2013, 07:13:59 PM
Ignore the last question, downloaded and ran LTSpice, seems that I get increased output due to the lower current consumption of the BJT. However I found that after adopting the values for the BJT that you specified it didn't actually seem to do much at all! Audio Output was the same before and after the BJT. So what will it actually be doing if I adopt the values you specified?
Title: Re: Serious Blocking Distortion
Post by: Roly on August 05, 2013, 09:48:13 AM
The BJT stage is drawing much more current than it needs to, and in conjunction with the decoupling resistor pulling the supply rail down considerably.  By simply scaling the resistors up and current down the supply rail goes higher giving more headroom and incidentally biasing the FET stage better.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 05, 2013, 11:37:20 AM
But the audio output from that BJT is the same as it's input if i use those values so what is it doing? With my values it increases the audio signal but with yours it stays the same. I understand the reason for using your values but what does the BJT do?
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 05, 2013, 01:01:42 PM
Also the overheating issue is now (hopefully) fixed. I have fitted a much better heatsink which has a rating of 12C/W and the difference even at low volumes is extreme. So I shall give the amp another thrashing at band practice tomorrow night to see if it will hold up. I must say the chip I'm using for my output stage runs very cool considering its pushing out quite a bit of volume!

I shall now modify the resistor values to Roly's spec.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 05, 2013, 05:45:16 PM
Nearly there guys, here is an updated schematic for the amp so far. I'm pretty happy with it apart from there is still a tad too much bass, so I'm going to replace the JFET Source Cap with a 0.68uF Cap later in the week if I can.
Title: Re: Serious Blocking Distortion
Post by: Roly on August 06, 2013, 09:04:31 PM
Quote from: Littlewyan
what does the BJT do?

Currently, not a lot; it's just lurking until you decide you want to give the diode clipper another go I guess.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 07, 2013, 03:23:11 AM
O well i'll just leave it in there. Tested the amp at band practice, plenty of volume, still bit too much bass so i'm going to try a 0.68uF cap on the JFET source.
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on August 07, 2013, 08:40:47 AM
Try this:
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 07, 2013, 02:15:18 PM
Thanks for that J M. Would .68uF be too much on the Emitter of the BJT? As I don't have a cap as low as .1uF.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 07, 2013, 03:52:14 PM
Here is the amp now, I have made some of the changes you specified J M, simply because I just tried the additional resistor and capacitor to begin with and that made a world of difference. The amp is so much better now. I need to test it at band practice really though to hear it at a good volume. My only possible quarrel with it is the overdrive is quite harsh but as I understand it (correct me if I'm wrong) this is a characteristic of transistors. I know Marshall have made a very good Transistor amp that has nice soft valve clipping but their amps are pretty complicated compared to this one! Is there any way to soften the overdrive?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on August 07, 2013, 04:50:28 PM
In my opinion you can get quite warm distortion from jfets, i would suggest trying different types, and maybe make the gain adjustable.

There really isn't any magic about distortion, filtering Before and after to shape what frequencies get the majority of the distortion etc.

let's take a well known tube amp (analysis is from ltspice, not sure how accurate):

The first stage is a Clean amplifier stage that increases the signal level and rolls of frequencies below 70 hz.
the second stage clips the + side of the signal, filter after rolls of some of the highs
the third stage clips the - side of the signal, filter after increase high mids
fourth stage clips the entire signal, and drives the tone stack. which schoops out the mids and goes to a buffer stage.

This is a high gain amp, but as you can see, there is alot of filtering going on, not just bringing the signal up to clipping.

But also, as i said: try a different jfet aswell, mpf102 and 2n5457 are quite easy to obtain and used in lots of guitar effects, you will have to rebias them though.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 07, 2013, 06:09:46 PM
I have just changed the jfet source cap to .68uF which has made the amp sound better, as for the distortion I'll have to take it to my next band practice so I can crank it. May try the MPF102 as I'm sure I read somewhere that its a bit more warm sounding, then again different circuits will sound different etc. I'm having fun experimenting though!
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 07, 2013, 06:13:21 PM
Also has anyone ever done what this guy has done with the two zeners? He says its to protect the JFET from blowing.

http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp3.htm
Title: Re: Serious Blocking Distortion
Post by: Roly on August 08, 2013, 12:25:43 PM
Looking at the emitter of the second stage you're starting to get some serious tweekology going there.

I use MPF102's because I've got them in stock, they are cheap, available, and come in a tasteful black ... but not for their thermal qualities, which I would frankly take with a fairly large grain of NaCl.

Back-to-back zeners across the input will certainly give the FET some protection, however they will only do so if there is some resistance in series with the input.  An equally effective way of protecting a FET input is with a couple of common-or-garden signal diodes such as 1N914's.  One is connected between ground and gate with the cathode (band) towards the gate, and the other between the gate and the supply, again with the diode pointing northwards with its cathode connected to the supply.

In either case there must be a resistor in series with the input, say 10k to 68k, to limit any surge current through the protection diodes or you will simply have a shorted protection diode instead of a shorted FET, but it will still stop the show.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 08, 2013, 01:07:19 PM
I'll probably use the zener option as I know I can definitely get hold of a few 5.1V Zeners.

Title: Re: Serious Blocking Distortion
Post by: Roly on August 09, 2013, 10:41:21 AM
Fine, but be mindful of what I say about including a resistor in series with the input to limit any overdrive current through the zeners.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 09, 2013, 12:58:55 PM
I've already got  a 33K resistor in series with the input before the Zener Diodes so should be ok. I ran it through LTSpice to make sure, great program that is!
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 13, 2013, 05:44:00 PM
Here is the amp as it stands. I have added the two zener diodes to protect the JFET and increased the voltage from the PSU to 30volts to increase the volume. I may change the JFET source resistor to 5K6 to lower the gain slightly, which may also give me a fuller tone. Do you think I will benefit at all from changing the 10K Pot to a 100K Pot? I should see an increase in volume shouldn't I? Actually I think the preamp will just be stupidly overdriven.
Title: Re: Serious Blocking Distortion
Post by: Kaz Kylheku on August 13, 2013, 11:38:05 PM
Here is the amp as it stands. I have added the two zener diodes to protect the JFET and increased the voltage from the PSU to 30volts to increase the volume. I may change the JFET source resistor to 5K6 to lower the gain slightly, which may also give me a fuller tone. Do you think I will benefit at all from changing the 10K Pot to a 100K Pot? I should see an increase in volume shouldn't I? Actually I think the preamp will just be stupidly overdriven.

I would say that the thing to do is to revise the impedances so as not to have to use these ridiculous 22 uF capacitors in the signal path.

They are actually already too large.  You have a 22 kOhm impedance into the TDA stage input. If the coupling capacitor is 22 uF, the -3dB frequency is 0.33 Hz, which means you have nearly flat frequency response down to 3.3 Hz (10X the -3dB frequency: this is a rule of thumb).

That is ridiculous even for hi-fi, let alone guitar.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 14, 2013, 03:17:31 AM
Good point Kaz, I didn't realise they affected the tone that much, I thought as the Source and Emitter capacitors were 0.68uF it shouldn't matter. I'll change these tomorrow night for 0.68uF capacitors as I have plenty of them. Unless you think 1uF would be better?
Title: Re: Serious Blocking Distortion
Post by: J M Fahey on August 14, 2013, 06:01:44 AM
Quote
I would say that the thing to do is to revise the impedances so as not to have to use these ridiculous 22 uF capacitors in the signal path.
Agree.
I suggested you realistic capacitor values some 10 posts back, yet you keep those ridiculously large ones ... then complain about blocking distortion  ::)

FWIW "Magic" has no place in Electronics.
Using ".68uF because Marshall uses them" is nonsense, the proper parameter is the RC value.
If you concentrate on C but ignore R, you will never solve the problem, because alone it's meaningless.
All value corrections I posted cut useless bass below, say, 60Hz or thereabouts.
Having 0.33Hz bandwidth, as Kaz Kylheku pointed, is musically useless and can only lead to trouble.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 14, 2013, 06:46:57 AM
You did JM but I was too naive to use them. I'm still learning  ::)

Also I'm not using 0.68uF because Marshall uses them, I went to 0.68uF as 1uF was still too much and as I have about 20 of these capacitors now I may as well use some as coupling capacitors.

Apologies JM I thought I remembered you used 0.1uF as the coupling capacitors in your modded version of my pre amp. I shall use 1uF capacitors instead of 0.68uF. What I shall do is first change the coupling capacitors and then move onto the others you've specified. I like to hear the tonal differences as I change these things :)

Again also I agree there is no magic in electronics.

I shall make changes to the circuit tomorrow night and will post the results. Once I'm happy with the circuit I'll see if I can create some audio clips and take some photos.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 14, 2013, 02:42:34 PM
On LTSpice when I run a simulation using the AC Analysis, I get these two lines. The solid line I believe shows how the volume increases with the frequency, however I cannot work out what the dotted line is?
Title: Re: Serious Blocking Distortion
Post by: Kaz Kylheku on August 14, 2013, 04:58:54 PM
On LTSpice when I run a simulation using the AC Analysis, I get these two lines. The solid line I believe shows how the volume increases with the frequency, however I cannot work out what the dotted line is?

Note how the vertical units on the far right of the LTSpice plot are degrees.

The dotted line gives the phase shift.   Frequency response is a complex number: it is two dimensional. Not only does it have amplitude, but it has phase.

Signal processing devices can change the amplitude in a frequency dependent way, but also shift the phase in a frequency dependent way, and the two are inter-related.

So frequency response has two dimensions, and this is why the output of the Fourier transform is in the two-dimensional doman of the complex numbers (even though the input is a real-valued time-domain signal).

Title: Re: Serious Blocking Distortion
Post by: Roly on August 14, 2013, 10:38:50 PM
{ooooh boy, here we go ...}   ;)
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 15, 2013, 02:24:21 AM
Haha Roly :P

Ok i've done some reading and am i right in saying the Phase Shift is basically a change in the audio signal? Like a delay perhaps? And that its for mixing two signals together? Which for a guitar would be the original note that was picked and then the harmonics that come after?
Title: Re: Serious Blocking Distortion
Post by: Roly on August 15, 2013, 08:25:19 AM
Well yes, and no.

The amplitude and phase of a signal are two different properties of the same signal, not two different signals.  {I will now attempt the impossible, explaining AC theory in less than 100 words with no algebra, vectors, or Complex Numbers}.

Consider this circuit;
(http://commons.bcit.ca/math/examples/elex/logs_exponentials/image257.gif)

Initially the capacitor is uncharged and the switch is open.  At time t=0 the switch is closed.

Initially there is no voltage, Vc, on the capacitor so a current, i, flows according to Ohms law, 100 volts and 2k gives 50mA, but as soon as the current starts flowing the cap starts to charge up, the voltage across it rises so the voltage across the resistor, Vr, falls, and therefore so does the current.

Compared to the input, the battery, the voltage rise on the cap takes place delayed from the closing of the switch.  In the DC case the voltage on the cap rises ever more slowly as it comes up to the battery supply, but with an AC sinewave signal instead of the battery the supply starts to reverse before the cap can fully charge up, so it has to discharge again to follow the incoming sine wave, and so is continually chasing the incoming voltage, is effectively delayed from it.

Similarly, because the input voltage is now continuously changing and the voltage on the cap can't catch up, its peak value or amplitude is also smaller compared to the applied signal.

So in terms of the applied AC sinewave the voltage on the cap is also a sinewave but it is both smaller in amplitude and delayed in time, or as we say, phase.

Since a sinewave can be represented as 360 degrees over one whole cycle we express the phase of the output across the cap in terms of the angle between where the input, E, crosses zero and the output, Vc, crosses zero as degrees of phase delay.

With the resistor in series and the capacitor in shunt across the signal path we have a low pass network, and with only a single C and R it's called a first order or single pole network.

(http://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/1st_Order_Lowpass_Filter_RC.svg/250px-1st_Order_Lowpass_Filter_RC.svg.png)

If we exchange the positions of C and R we create a first order high pass network, and I will ask you to accept on trust that similar reasoning leads us to the fact that the output signal is phase advanced on the input.

(https://upload.wikimedia.org/wikipedia/commons/thumb/f/fe/High_pass_filter.svg/210px-High_pass_filter.svg.png)

In both the low and high pass cases the attenuation or loss of amplitude from input to output is related to the reactance or effective AC resistance of the capacitor, Xc ohms, which changes with frequency according to;

Xc = 1 / (2 Pi f C)

A rule of thumb to remember is that when the reactance of a capacitor is equal to the resistance in a CR network the amplitude response is half, and the phase angle is 45 degrees.  We normally take the values ten times above and below to be where the phase angle falls to zero or goes to the maximum 90 degrees; similarly where the output amplitude is effectively the same as the input, or effectively zero.


There are a couple of handy graphs for estimating amplitude and phase response here (http://www.ozvalveamps.org/maths.htm#Filters) under "Response estimation".
Title: Re: Serious Blocking Distortion
Post by: Kaz Kylheku on August 15, 2013, 04:24:33 PM
Haha Roly :P

Ok i've done some reading and am i right in saying the Phase Shift is basically a change in the audio signal? Like a delay perhaps? And that its for mixing two signals together? Which for a guitar would be the original note that was picked and then the harmonics that come after?

Every sinusoidal has phase. Frequency and amplitude are not enough to completely specify a sine wave. You have to know the phase. At a given point in time, a sine wave could start on a crest, or in a trough, or anywhere in between.

A phase shift is not exactly a time delay. Whereas, of course, a time delay will shift the phase, circuit components like amplifiers and filters do not introduce delays (at least not delays that matter in audio): the signals move at close to the speed of light through the circuit.   An amplifier's or filter's phase shift is a frequency-dependent response.  It is not caused by a time delay, but by a response lag (or its opposite, "eagerness", or lead).

For instance, a low-pass RC filter has lag.  Lag does not mean that there is a time delay. When we send, say, a step signal into the circuit's input, the output begins to react instantly, but it does not change instantly: it moves slowly, because the capacitor has to charge through the resistor to achieve the new voltage. It cannot follow the step.  This lag is why the circuit is a low-pass filter; higher frequencies shake back and forth more quickly, and so the lag suppresses them more than it suppresses slow oscillations.

If we look at your graph, we see that you have a positive phase shift over most of the range, which goes hand in hand with the high-pass-filtering that is going on.  In a high pass filter, the phase shift goes the other way: it leads the original signal. This is because (in the case of a simple RC high pass filter) the output is proportional to the current passing through the capacitor. Suppose we send a step signal into a such an RC filter. The coupling capacitor begins to charge instantly, and so there is an immediate, fast voltage spike on the shunt resistor. The spike then dies down as the current diminishes and ceases. The eagerness of the high pass filter to respond causes it to produce a leading phase shift over sinusoidal signals.

Phase shifts are important in audio, and at the same time they are unimportant.  If you have a stereo (or surround) system, and the signal is inverted in one of the channels, this is bad; it means that every frequency is shifted by 180 degrees. When you sit in the ideal listening position, you will get strange sounding cancelations and comb filtering.   Phase shifts affect bass.  A 90 degree phase shift in a 30 Hz signal represents a time shift. It can throw off  a kick drum in a perceptible way.

Phase shifts are not audible in the higher frequencies (except allegedly in some contrived listening tests with specially crafted material).

All filters have some kind of phase response. For example, the narrow filters in a 31 band graphic equalizer seriously affect the phase coherence of material. Yet, audio engineers and musicians use them anyway.

Phase is the subject of a lot of audiophoolery. Some claim they can hear several degrees of phase shift in a 10,000 Hz signal, and the like, which is nonsense. Snake oil devices exist which supposedly reconstruct the phase coherence of a signal.

Phase is crucially important in the stability of amplifiers which use negative feedback.  All amplifiers have a limited frequency response, and due to multiple parasitic filter poles in their stages, at some frequency they accumulate a 180 degree phase shift. Negative feedback is also 180 degrees, and so this phase shift turns negative feedback into positive feedback.  If the amplifier has a gain of 1 (or, practically speaking, at least one) at this frequency, then it will oscillate.

Phase can tell you things about the response of your circuit. For instance if we look at your graph, the amplitude response looks fairly flat toward the right. Only if we look a little closer do we see that it actually is curved: it reaches a maximum and starts to fall. So there is a frequency response peak hiding there. Where is that peak? The phase response provides a clue: right around the frequency response peak, the phase response drops down to zero degrees, and crosses into the negative (lead turns to lag). That's about the place where high pass switches to low pass.  It is clearer in the phase shift plot; the phase shift clarifies and confirms something to us.


Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 15, 2013, 05:01:54 PM
Ah ok thanks guys I think I understand it a bit more now :)

With regards to the amp I have changed all coupling capacitors to 1uF which has improved the sound greatly, I'm sure I can hear a bit more clarity in the distortion now (weird saying I know). I also changed the input capacitor from 100n to 4700pF as JM advised but I may change it back as the sound is a bit trebly for my liking now. However I have also changed the JFET Source Resistor to 5.6K from 4.7K which has lowered the distortion a bit but it is still a tad high, so I'm going to try swapping it for a 6K or even a 6.6K as I get distortion even at low volume! So I'm thinking once I've lowered the distortion the amp may not have as much treble, but we shall see, one step at a time.

As always I've attached an updated schematic, I noticed an error in the schematic where I included an extra capacitor that isn't present in the circuit, apologies.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 18, 2013, 01:53:14 PM
I've now changed the Source Resistor to a 6K2 value from 5K6, however I haven't been able to test the amp at high volume (parents and neighbours get annoyed!). I'm hoping to get the amp to a point where I have a clean sound at low volume and distorted at high volume. This should be possible by increasing the Source Resistor shouldn't it? I'm sure I can do that on the 1 watt amp I built earlier this year.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 18, 2013, 06:38:47 PM
The other thing is it doesnt really clean up with the guitar volume knob, i'm guessing this is because of the high gain?
Title: Re: Serious Blocking Distortion
Post by: Roly on August 18, 2013, 08:23:32 PM
Check the voltages on the drain of the FET and the collector of the transistor.  The operating sweet point ("Q point") we are looking for is that while idle the voltages on these two points should be about half the supply voltage (so that the signal has equal headroom for both +ve and -ve swing).  The ideal point is actually with equal voltages across the device and the load resistor to account for the supply voltage lost across the bias resistor, so a little high on half should be about right.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 19, 2013, 02:40:49 AM
The JFET is currently at 19volts and the BJT is at 13 volts. If I change the BJT Collector Resistor from 10K to 6K8 then the collector voltage goes up to 15.7volts which is about right. So I'll just try that first, if it still doesn't sound right then I'll change the JFET resistor from 22K to 27K which will put it at 17 volts. Does this sound ok Roly? If I need a higher value resistor on the JFET then I can get hold of one.
Title: Re: Serious Blocking Distortion
Post by: Roly on August 19, 2013, 10:06:02 AM
In general when we want to alter the operating, bias, or Q-point of a transistor (or indeed valve) stage the resistance we vary is normally the emitter/source/cathode resistor.

The resistor that is in the collector/drain/anode is called the "load" resistor and normally remains unchanged.  The reason is that the load changes the Q-point alright, but it also changes the stage AC signal gain. Because the emitter/source/cathode resistor is normally bypassed for AC using a capacitor changing this resistor only changes the DC gain, and thus the bias point without changing the AC gain.

Because your FET stage is the more conventional I'll use that as the example (since the simple bias method on the transistor stage has a few gotchas which complicate matters a bit).

Looking at the datasheet for the 2N3819;

http://www.digchip.com/datasheets/parts/datasheet/343/2N3819-pdf.php (http://www.digchip.com/datasheets/parts/datasheet/343/2N3819-pdf.php)

...we find that the VGS(off) (Gate-Source Voltage for cutoff) is given as -8 volts, and as we want a linear Class-A stage let us assume our bias target point is half this, or -4 volts.

We will assume an available supply of 30 volts, and will decide (guess) that a suitable device current will be around 1mA.

Your circuit has a 4k7 Source resistor, and 1mA through a 4k7 will give 4.7 volts, pretty close.

Your load resistor is 22k and 1mA through 22k will give a drop of 22 volts.

If we subtract the load resistor drop from the available supply we get an idle Drain voltage of;

30-22 = 8 volts.

This is a bit low on the desired half-supply for best unclipped voltage swing on the Drain, it will bottom out pretty quickly because the FET is already elevated by the Source bias resistor to around 4 volts, so the available negative swing is only 8-4 = 4 volts.

We could reduce the Drain resistor but this would also reduce the stage gain, which we generally don't want to do, but what we can do is increase the bias so that the device draws less current at the idle Q-point.

As a rough calculation we can now find a target Drain voltage of the supply less the bias voltage, 30-4 = 26 volts, divided by 2 to put it in the middle, or 13 volts.

13 volts from the supply is 30-13 = 17 volts drop across the Drain resistor, and the current that will give that is;

I = E/R

17/22 = 0.77272727 (k ohms/volts = mA) or around 0.8mA.

Now we can go back to the Source resistor and work out what value will give the -4 volts bias with this new current;

R = E/I

4/0.8 = 5(k)

So we only need a small increase in the bias resistor to push the Drain voltage up to the desired point and could try changing the 4k7 for a 5k6 (or we could try adding a 330r in series with our 4k7).

Keep in mind that all these devices have a parameter spread, and that FET's in particular often have a very wide spread in the required bias voltage for a given current.

Now if we want to play around with the AC gain of the FET stage we can change the Drain load, increasing for more gain and reducing for less, then resetting the mid point DC conditions using the Source resistor.

We need to keep in mind however that an upper limit is set by the fact that the input impedance of the following stage effectively (for AC) appears to the FET to be in parallel with its Drain load - you can't just make the Drain 10 Megs and get a monumental gain, because the input impedance of the following stage is likely to be somewhere between 100k and 1Meg and this will be the effective upper limit.

This exercise shows that the FET stage isn't too far off the correct bias point ('tho it may be a fair bit lower at around -2.5 volts in reality), and a bit of fiddling in LTSpice shows that this stage doesn't really seem to be much of a problem.  It shows a voltage gain of around x24 which is actually a bit on the high side.  We normally assume that a guitar pickup will output around 200-500mV average with peaks to around a volt, so we would normally be looking for a gain where the stage could cope with that without clipping.  In this particular case reducing the Drain load somewhat could help in both centering the stage Q-point and reducing its gain a bit.

Something that looks good is that this FET stage starts to limit in a fairly nice way, softly at first, and by flattening out the positive peaks a bit.  This would give Hi-Fi-ists the galloping jeebies, but for guitar this sort of distortion gives a "nice" bit of second harmonic, a bit valve-like.

Taking out the bypass cap across the FET Source resistor drops the gain to about x11, but still leaves the following transistor stage clipping badly at 1V in; in fact it still clips at around 500mV in.

Something that doesn't look good is that the transistor stage is hard clipping the negative peak early, and need some attention.  This, I think, is the basic cause of your 'distortion at all levels' problem.

At the moment the transistor stage isn't doing anything useful, just distorting early.  I suggest that you re-arrange this stage to be an emitter-follower directly coupled to the FET Source Drain for bias, then at least it will be performing the function of a buffer/impedance transformer to isolate the FET load from the following low impedance tone and volume controls.

Some detail on transistor biasing;
http://www.electronics-tutorials.ws/blog/biasing-transistor-tutorial.html (http://www.electronics-tutorials.ws/blog/biasing-transistor-tutorial.html)

HTH
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 19, 2013, 03:12:48 PM
Thanks for that Roly. I've attached a screenshot of what I've put together in LTSpice, these are the changes I will try I have made tonight based on your recommendations. Only thing I didn't do was connect the BJT to the JFET's Source as this seemed a bit bizarre as I won't have any voltage gain at all if I do that? Or am I misinterpreted?

Anyway the amp sounds much better now and cleans up perfectly when rolling back the guitar volume. Only issue now is the tone control doesn't work, as I should have suspected really as its in parallel with the BJT's Emitter Resistor. Going to look into how to get around this another night.

Just had a thought, I could change the Tone Pot configuration to be the same as the volume control, so the middle wiper goes to the output, one side connects to the input and the other then to the capacitor that goes to ground. Would that not work as a tone control?
Title: Re: Serious Blocking Distortion
Post by: Roly on August 20, 2013, 11:53:36 AM
Cleaner?  Good.  :tu:

Oops, yes, no, I dun goofed - Drain, not Source.  :duh

aaaaahhhh ... eeeerrrr ... uummmm

A simple top cut control should still work.

Try increasing the emitter resistor, say to around 15k.  This will reduce the current through the transistor (which doesn't need to be that high) and increase the output impedance of the emitter follower a bit which will then give your top cut something to work against.

Or try a Big Muff type single knob control.   8|
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 20, 2013, 02:47:18 PM
I've got an 18K resistor so I'll install that, that should drop the current down by at least 3ma. Although going by LTSpice this won't make much difference to the tone control, however that Big Muff tone control looks very interesting so I may give that a go :)
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 20, 2013, 03:42:15 PM
Used the Big Muff Tone Control in LTSpice on my amp and the output went from 5V +/- to 900mV +/-. I'm guessing I'd need another gain stage for this tone control?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on August 20, 2013, 03:45:33 PM
You will Always lose signal level with a passive tone stack. So either add a recovery stage, or increase the level Before the tone stack.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 20, 2013, 03:58:50 PM
Well the good thing about my old Tone Control was that I didn't lose much signal at all, not compared to this one anyway. When you said Tone Cut Roly did you mean the Tone Control I'm currently using or a different one?

O thats not right, I've lost it  :P

Actually I just checked using LTSpice and whereas the Tone Control did lower the output it would only go from 5v +/- down to 3v +/- which obviously isn't a massive drop compared to the Big Muff Tone Control. Back to the drawing board me thinks.....

I'm wondering if perhaps I should just lose the Tone Control and maybe add a different option in. Perhaps a switch to go from Clean to Distortion? Could switch between Source Bypass capacitors, from 0.68uF for distorted to 0.1uF for clean perhaps?
Title: Re: Serious Blocking Distortion
Post by: Roly on August 21, 2013, 08:47:47 AM
The simple cap-in-series-with-pot is called a TOP cut control because that's what it does, it shunts higher frequencies to ground.

Currently your signal level isn't being dropped by your top-cut control, and your control isn't tone controlling; so yep, that's consistent.  The reason it isn't dropping much is because the output impedance of the emitter follower stage is very low, particularly compared to the impedance of your top-cut control, so you could try inserting some series resistance after the emitter-follower before the tone control to give the control something to "work against".  Values to try are between one tenth and one half of the tone pot value; and yes, this will also lead to some overall signal loss.

As @Stormbringer says, the only thing a tone control can do is drop the level; "boost" is a misnomer since "boost" actually means "less cut" than the neutral or flat position.

You can play around with the component values in the Big Muff (or any control for that matter) so that you get less "insertion loss" than 5V down to 0.9V, but at the expense of available "boost".  Or as suggested you could follow up the tonestack with a recovery amp, another transistor or FET stage.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 21, 2013, 09:48:05 AM
No i mean i simulated the tone control LtSpice and didnt get as much drop, obviously at the moment i don't really get any signal drop as its doing sod all at the moment. I might remove the Tone Control and put a switch in it's place to switch between different capacitors on the JFET Source. Need to do a few tests first to see if it'll do what i want it to though.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 21, 2013, 02:16:45 PM
Ok here is what I propose to do. I'm going to take out the tone control completely and in it's place on the chassis I shall install a SPDT that has three positions. The switch will basically change the JFET Source Capacitor to give me three different 'Channels' if you will. I've attached a picture showing what I plan to do. Not sure about the clean channel at the moment though, wondering whether its best to have that part of the switch not connected to anything to clean up the sound as I imagine the 220uF will make the amp VERY bassy. Also I may need a resistor in series with the input as when I switch to the 0.68uF Cap I imagine it will pop as the capacitor charges up.
Title: Re: Serious Blocking Distortion
Post by: stormbringer on August 23, 2013, 05:01:53 PM
if you place the tone stack between your jfet and bjt and increase the bjt gain you have your recovery stage done.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 23, 2013, 05:53:31 PM
I think i want to remove the Tone Control Stormbringer. Do you think what i proposed will work?
Title: Re: Serious Blocking Distortion
Post by: stormbringer on August 24, 2013, 10:20:10 AM
I think it would work more as a treble/prescense control than gain, you could switch between 2 different source resistors instead and probably get a more satisfying result. The source bypass capacitor helps with the gain as it has a low impedance path to ground, but the size affects frequency rather than gain.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 25, 2013, 06:16:29 PM
I tried this in LTSpice, starting from 6K2 on the JFET Source and then lowering the resistance as a Pot would and this just seemed to increase distortion. I think I may just put the BJT back how it was minus the Emitter Bypass Capacitor and just keep the tone control where it is. As in this configuration I lose very little gain and the tone control works well. I've attached the circuit I used in LTSpice to test this.
Title: Re: Serious Blocking Distortion
Post by: Roly on August 26, 2013, 12:19:40 PM
Have a look at the preamp circuit here;

http://www.ssguitar.com/index.php?topic=2912.msg22358#msg22358 (http://www.ssguitar.com/index.php?topic=2912.msg22358#msg22358)

Fust point; have a look at the arrangement on the Source of the first FET.  A variation is to use a pot for the Source resistor and connect the cap to the tapping.  These are two ways to alter the AC conditions without changing the DC conditions.

Secnt point; notice the pot declarations;

Rtot=<VAL> wiper=<VARIABLE>

and the parameter value lists

.step param <VARIABLE> list <LISTITEMS>

When you do the simulation run it takes each of the list items in turn and uses them as the variable, building a nest of curves for each list item value, in this case pot position.  You can use all ten 0 0.1 0.2 ... 0.9 1.0 but it gets a bit busy so I normally use just five or six; 0 0.2 0.4 ...
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 26, 2013, 01:15:00 PM
Ah i see now. Shouldn't i use the configuration hes using on his JFET source? I see what you mean about replacing the JFET Source with a pot which would work, until the pot is turned all the way and the cap is effectively shorted. I'm on holiday this week so can't try anything until i'm back on Saturday but will definitely give this a go. Also i've done a bit of reading about valve sounding JFET preamps and it seems the best way to achieve this is by using multiple JFET stages all softly clipping the waveform.
Title: Re: Serious Blocking Distortion
Post by: Roly on August 26, 2013, 01:39:26 PM
That "he" is actually me, that's my design.

Quote from: Littlewyan
valve sounding JFET preamps

Yes, well, there's a lot of writing out there, and some of it even matches up to reality.  The characteristic slope of a triode valve and a FET are similar but not identical so the distortion they will produce is similar but not identical.

IIRC the slope of a FET is proportional to x2 while the slope of a triode is proportional to x2/3.  In both cases this results in a sinewave becoming slightly peakier on one side and slightly flattened on the other.

When it comes to guitars and distortion, an awful lot has to do with personal taste.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on August 26, 2013, 02:23:30 PM
O right sorry didn't see that!

This is very true. When it comes to overdrive/distortion no one can really describe it as it'll be an opinion not necessarily shared by others.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 01, 2013, 06:03:46 PM
I believe I finally have the amp the way I want it. I tried the gain control Roly but didn't like the way the sound went very bassy when I reduced the gain. So I used a switch to go between two capacitors. I'll attach a schematic to this post. Basically one capacitor gives me less bass and in turn less distortion, whilst the other gives me pretty high distortion. Also the switch I used had a middle position and this just leaves the source resistor by itself without a bypass capacitor.
Title: Re: Serious Blocking Distortion
Post by: Roly on September 02, 2013, 09:12:19 AM
Cool; you have to try stuff to see how possibilities work out in practice - that's what rolling your own is all about.   ;)

I would make one little suggestion 'tho, if you are getting clicks when you change the switch position it's because the two caps are not staying charged to the same voltage that is on the FET source (and why should they if they aren't connected?)

The fix is quite simple, a couple of high value resistors, say 100k to 1Meg, but at least x10 the Source resistor value, connected across the switch between the Source and each cap.  This will keep them charged to the same voltage even when they are disconnected by the switch, so little or no click on switching.  This little de-clicking trick is worth pasting in your hat for similar situations.

Now you have got there, how about some sound samples?   :dbtu:
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 02, 2013, 01:18:20 PM
Ah good idea Roly. I was going to ask about the clicking, I thought it may of been due to the capacitors quickly charging up and could be solved by having a small resistor in series with the source and switch, but your solution sounds like it'll actually work :P. I'll implement this fix and will get the sound samples soon as possible. Unfortunately as I still live with my parents and they aren't big fans of loud guitar so I'll have to wait until they're out.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 05, 2013, 02:40:16 PM
Well guys I created a few sound clips, unfortunately they were a bit rushed and recorded using my iphone. But they should hopefully give you an idea of how the amp sounds. These recordings are only of the .68uF capacitor setting.

Also it no longer pops or clicks when I switch between capacitors, added a 100K resistor between the source and .68uF Cap and a 820k between the source and .22uF cap. Reason for the different resistors is the 820K worked for the .22uF but wasn't enough for the .68uF, so I just changed the .68uF's cap and left the .22uF's as is.
Title: Re: Serious Blocking Distortion
Post by: Roly on September 06, 2013, 11:03:54 AM
hmmmm ... could it be that your 0.68uF is a bit leaky?  New, or ex-equipt?
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 06, 2013, 06:57:17 PM
Brand new, electrolytic by the way, the .22uF is a Polyester I think. Its Orange and Rectangle shaped
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 14, 2013, 04:38:03 PM
How would I lower the distortion in this circuit? Is it just a case of trying different JFets? Or if I increase the IDSS Current then the JFET will be able to handle more before distorting?
Title: Re: Serious Blocking Distortion
Post by: phatt on September 15, 2013, 10:20:34 AM
Hey Littlewyan,
               I just had a listen of those 2 clips and I've heard far worse from some brandname amps so considering it's only one fet you have done well so maybe quit while you are ahead. :-X

You have some good basic sound building blocks you will need more stages if you want to go further into tone shaping.

I don't know why you want cleaner as it seems fairly clean in places.
Bare in mind even clean electric guitar has some clipping and done right it can enhance the result.

Maybe go find some classic Shadows hits, most will assume it's all super clean electric guitar but if you listen carefully you will notice there is an edge to a lot of the notes played.
The classic triode rattle as the signal slightly deforms at higher levels.

Bare in mind that most guitar pups made today are far more powerful (bigger output swing) than days of surf music and that won't help if you are trying to get a super clean sound.
Phil.
Title: Re: Serious Blocking Distortion
Post by: Roly on September 15, 2013, 10:43:37 AM
Swapping FET's around is shooting in the dark; better to persist with a given FET and try to get the biasing and gain where you want it.

Quote from: Littlewyan
How would I lower the distortion in this circuit?

Well first up it would be helpful to know where the distortion is arising, and why.

For example, is it the FET stage that is clipping, or is the gain of the FET stage causing the chip amp to clip?

Looking at the TDA2050, what input signal level would cause it to clip?

Assuming a supply voltage of 30 volts (adjust the following to suit the actual supply voltage);

The output will clip when the signal level is 30 volts pk to pk, or 15 volts pk.

The approximate gain is set by the ratio of the 22k feedback resistor and the shunt to ground, 680 ohms;

22000/680 = 32 times

So the input that will cause it to clip is the output voltage divided by the gain;

15/32 = 0.46875 volts peak

So it is already pretty sensitive and we would expect a guitar signal at this point to be able to fully drive it, however we have the volume control to hold this under control.

According to my sim of your "AMP - final" circuit the FET stage provides an output of about 2 volts peak for about 0.5 volts peak input, so it has a gain of about;

2/0.5 = 4 times

This is with the bypass cap set to zero, the centre position of your three position switch.

Again according to my sim this stage won't clip until the input is around 1.8 volts peak, which is a fair bit more than I would expect from a guitar pickup, so with the switch in the middle position this first stage really shouldn't clip.

I would however expect the chipamp to clip with the volume set anywhere above about 2/10.

With the switch in the 0.68uF position the FET gain is quite a bit higher at 440Hz and the input clipping level falls to around 250mV peak, which ain't much at all.

Traditionally you would explore this gain distribution using a signal generator and oscilloscope, but LTSpice should give you a reasonable approximation.

If you want to reduce the gain of the chipamp then you can either reduce the 22k feedback resistor, or increase the 680 ohm shunt.

HTH
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 15, 2013, 01:18:19 PM
Thanks Phil, means a lot :).

Roly, as usual very informative. I will say that the amp did sound best to my ears when the capacitor wasn't in the circuit with the volume turned up, I guess thats because it isn't driving the chip quite so hard. Now if I change the 22K resistor to 18K then that will lower the gain to about 26.4 which will then give me 0.56818 volts peak, however I'd probably lose a bit of volume so I'd then increase the supply voltage to perhaps 32volts to make up for it so then I'd have 0.6 volts peak before the chipamp starts clipping. How does that sound? Am I going in the right direction?

Hang on a second, according to the datasheet in the single supply circuit if I decrease the size of the 22K resistor then that increases the gain? So either the datasheet is wrong or we're using the wrong equation to work out the gain.

Ah no apologies Roly, whoever wrote that datasheet got it wrong. Checked one of my books and checked the TDA2030 Datasheet and decreasing the 22K resistor does decrease gain.

So looks like if I decrease the gain slightly and then up the supply voltage by 2 volts then I could reduce the clipping on the input of the chipamp.
Title: Re: Serious Blocking Distortion
Post by: Roly on September 15, 2013, 09:30:46 PM
Quote from: Littlewyan
however I'd probably lose a bit of volume so I'd then increase the supply voltage to perhaps 32volts to make up for it

Taking it backwards, the supply voltage to the chip amp determines the available voltage swing on the output, thus for a given speaker load impedance, the amount of power that the chip amp can deliver before clipping (keeping in mind the other limiting ratings of whatever chip is being used).

By increasing the feedback (by reducing the feedback resistor or increasing the shunt resistor) we reduce the gain.  But seeing that the output swing is determined by the supply voltage, the overall effect is to increase the amount of input signal required for full output - the power amp becomes less sensitive.

Now in your case the chip amp is set perhaps to be a bit too sensitive, so by reducing the chip amp gain a bit we will come closer to the classic "one volt" signal level that has been the traditional input level for main amps since Noah built a stereo.

You don't need to compensate for reduced gain with more supply voltage because your basic problem seems to be excessive gain along the chain leading to unwanted distortion.

At the moment it looks to me like you have too much overall gain even when the Source bypass cap is switched out, going from clean to clip at perhaps 1 or 2/10 on the volume.  In a stereo we don't want distortion so the gain is set up so that clipping starts at perhaps 9/10, but a guitar amp is a different animal and we want a range of both clean and clipped tones, so a reasonable place for clipping to start (assuming the mythical "average" signal) is say around 5/10.

Another way of looking at what we are doing now is adjusting the scale of the volume control to give you a nice spread between clean-ish below 5/10 with a nice transition into dirt at higher settings.  At the moment it seems you are getting into dirt pretty much right off the stop - it's a bit "Formula 1", just touch the accelerator and you have wheel spin.

Even with the overall gain reduced by several times, say 4:1, you are still going to have ample gain for dirt up high, and even moreso with either of the caps switched in.  There will be no lack of raw horsepower, just that with reduced gain the volume control will give you a more useful and less touchy "accelerator".

Something that might both reduce gain and give you more tonal flexibility is to introduce something like a Fender tonestack between the emitter follower and your volume control.  Tonestacks generally are lossy, around 20dB, so this would make good use of the excess gain in the first stage and also give you two or three extra knobs to twiddle (and of course endless fun tweeking tonestack component values, which is where Tone Stack Calc comes in to its own).  Something to ponder.
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 16, 2013, 01:25:48 PM
Hmm I'll have to have a think as I don't really have enough room to add an extra three knobs for tone controls as I wanted to keep the amp side of things as far away from the PSU as possible. You'll see what I mean in the photos attached. I could perhaps double up the switch so as I switch between different JFET Source Capacitors it also switches between different RCs for tone......

Roly how did you get the TDA2050 chip into LTSpice? Been trying to figure a way for ages!


Worked out how to include an opamp so I can see the extent of the issue and now it has all become clear why the amp sounds like it does! So to begin with I'm going to change the shunt resistor to 1K and the Feedback resistor to 18K and see where that gets me. I should have about 0.8V before clipping then.

In the end I just replaced the 680ohm shunt resistor with a 1K5 and now the Chipamp can take a nice 1 volt before clipping. The result......a now fantastic sounding amp! It stays pretty much clean with no source bypass capacitor, starts to distort slightly at about 9 on the volume but not much. Then with a bypass capacitor it starts to distort at about midway on the volume. Perfect :).
Title: Re: Serious Blocking Distortion
Post by: Roly on September 17, 2013, 11:58:01 AM
Ok-ay!  That's a really nice looking build.   :dbtu:

I think you still have enough front panel room to at least get a two-knob tonestack in there.


Now I'm going to get a bit picky about some details.

I'd like to see some heatshrink over all the exposed terminals on the back of your mains switch.

A plastic cap, coat of paint, or blob of silicone goo on the head of the bolt that holds your toroid transformer would be a good idea too (if that accidentally gets shorted to the chassis it makes a shorted turn and will give you a nice big welding splat).

The knot in your power lead will stop it pulling through if it is tripped over, but it won't stop it turning around and possibly stressing wires internally.  There are several methods of anchoring power leads, clamping grommets, clamp glands, but a simple and effective solution is a small plastic gizmo called a "P-hoop" (http://www.jaycar.com.au/productView.asp?ID=HP0754) which bolts down inside and holds the cable firm.  Of course it still needs the protection of a grommet or similar where it passes through the chassis.

Your board mounting nuts are not retained.  You can use star washers for this, 'tho my own trick is to use a bit of cutoff plastic Rawlplug on the screw thread after the nut.

(http://i.ebayimg.com/00/s/MjI1WDIyNQ==/z/6YIAAMXQ56ZR8KBO/$T2eC16hHJHkFFl70jm2oBR8KBNrL!Q~~60_35.JPG)

This both holds the nut from coming off and protects your hands from the sharp thread end biting you while working on the amp.

For your next build (and amp building is about as addictive as crack) a few points for the planning stage.

Normally it's a good idea to have only a single point of connection of the amp common to the chassis to avoid forming earth loops.  This means that we either select a point near the power supply common, or at the input socket.  Where the ground point isn't either input or output then it is normal to use sockets that have their common insulated from the chassis.

It appears that both your regulator and chip amp heatsinks are supported by their respective chips.  This means that their leads carry the weight of the heatsink as it twangs around during transport.  There is also the question of where they are getting rid of their heat to - the air trapped inside the case.

When planning your next build you could make a starting point to mount your heatsink(s) on the back panel outside, cutting holes for the chips to mount on them inside, then grow your layout around that.  That way you will have the back panel supporting the heatsink, which in turn will support the chips.


But it is a very nice looking little build.  It has been an interesting journey to get to this point, and pleasing that you are getting to be satisfied with the amp in operation - but I warn you, now homebrew amp or stomp is ever "finished"; there are always new ideas to try.   ;)
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 22, 2013, 06:10:23 AM
Thanks Roly

I will get some heatshrink on that as I must admit I do get bit nervous when I have to go near that switch when working on the amp whilst its on :P.

Now do you mean the bolt and the washer so it doesn't conduct with the cover on top of the transformer?

I'll take a look at that as I'll probably have to drill a small hole for the P Hoop.

I can get hold of star washers so thats not a problem :).

With regards to the Amp common it is only connected to the chassis near the PSU, unless I'm missing something?

I did have some issues with heat in this amp with the voltage regulator, it seems to be coping now however I obviously haven't tested it in muggy, sweaty gig conditions. I did try and find a chassis with vents but struggled surprisingly. Will definitely be considered next time though.

My next project is actually going to be my first ever valve amp, although it won't be from scratch it will be a kit from ampmaker.co.uk, the PP-18 to be precise. I want to start off with a kit as I've never built a valve amp before and making mistakes in valve amps can be very expensive. If you make a mistake in an SS build then you might blow a couple of pence of components (Unless of course you cook the mains transformer or somehow destroy the chassis), if you make a mistake in a valve build and blow components there will be tears (Valves, Output transformer etc). Also if I start with a kit then I'll have a good sounding amp as a starting point for many modifications.

Like you said, amp building is as addictive as crack ;)
Title: Re: Serious Blocking Distortion
Post by: Roly on September 22, 2013, 01:08:55 PM
Around power mains "nervous" is good.

The amp chassis, torroid mounting bolt, and top plate are electrically connected, and constitute about 3/4's of a turn through the torroid.  If by mischance the circuit were to be completed from the top plate back to the chassis you would have a shorted turn around the tranny core, and one hell of a lot of current would flow.

The P-hoop could possibly use one of the existing board mounting bolts, down against the chassis.

Quote from: Littlewyan
unless I'm missing something?

Yes.  The sockets you have used for input and output have their commons connected to the amplifier common, but they also are metal attached to the metal chassis, thus forming two paths back to the common earth point, and a couple of earth loops.  If you replace these with sockets that have bodies insulated from the chassis you may notice another reduction in hum level.

As I said, the heat has to get out of the chassis somehow, or it just keeps getting hotter.

I don't know the exact valve kit but I can tell you a couple of things based on experiences with a large number of Lamington (http://www.ozvalveamps.org/ava100/ava101lamington.htm) and similar builds.

Apart from a few dropped valves, or crunched-in-transit, nobody has blown anything up worth mentioning.  This is partly because valves tend to be rather more forgiving of mistakes than transistors; so you can forget the bias connection, have the power stage anodes glow red, yet it will come back as if nothing happened when the problem is fixed.  Put a transistor in backwards and it's goodnight at power up before you can even say " :grr ".

I hope you will post a new build thread here with pix so we can keep up with your progress.

Yours is a healthy attitude; build something basic and get it going, then dick around with it.   :dbtu:
Title: Re: Serious Blocking Distortion
Post by: Littlewyan on September 22, 2013, 02:40:46 PM
Ah right I see I shall look at painting the bolt for the transformer and using a P Hoop.

With regards to the sockets conducting with the chassis that could explain something, as I have found that I sometimes get a bit of hum until the lid is on, would this be connected?

I shall keep you all updated.

Thanks Roly
Title: Re: Serious Blocking Distortion
Post by: Roly on September 23, 2013, 08:25:45 AM
With regards to the sockets conducting with the chassis that could explain something, as I have found that I sometimes get a bit of hum until the lid is on, would this be connected?

That's a definite "maybe".

Earth loops are sensitive to magnetic fields, and while torroids generally have very little external field there are plenty of other sources external to the amp.  Assuming the lid is steel it would do a pretty good job of shielding the innards from external fields, but just to be confusing it will also screen electric fields getting in to high impedance circuits.

About the only way you can tell the difference is that magnetic screening will occur just with the lid in the way, while electric field screening is essentially capacitive and requires that the lid be actually electrically connected to the rest of the chassis.  This can give rise to the effect where the hum suddenly stops as you tighten down a retaining screw.

In very loose terms magnetic fields arise from current, while electric fields arise from voltage.