Overdrive from Peavey standard (reconstruntion from last forum crash)

[tca]

So here goes the info that I managed to save from the last forum crash.

The schematic:



Ref.: http://www.peavey.com/media/pdf/manuals/standard.pdf

A user said that a similar distortion can be found on Dean amplifiers (what
model?) and possible from a valvecaster.

PRR argues that this is different from the original Peavey schematic. My answer:
Ok, it is just a simplified version of that with the same type of distortion.

On a reply to a comment I said:

(about T1) It is very asymmetric indeed. T1 is just a "bad bias" amplifier. The asymmetry
is some what tamed by the clipping section.

- D1/D2 limits the input signal (this was part of a preamp); I do ear some
  differences with them in and on the circuit, but I'm not sure they are
  working as I think they are.

The feedback network is clever
- D3/D4 work works as soft limiting
- the 100k to power sets the transistor almost conducting with the collector
floating

these two parts sets the signal near the knee of the transistor (at the
collector the voltage looks like a log amp output) and it is added to the
input, thus the peak at the negative part.

I also posted that this is probably a earlier incarnation of Peavey's distortion
circuit of the T-Dynamics. Teemuk's book on SS guitar amplifiers has the image:



Lets hope that we can put this back on track.

Cheers.

[teemuk]

Well, I too have to argue that you have drawn the schematic differently.

... and by doing that, very incorrectly.


It really isn't that fancy design when simplified properly:

- Gain control potentiometer - which you have totally omitted - works by adjusting both negative feedback ratio and output signal's magnitude. A pretty usual setup for amplifiers of this vintage. In essence the gain stage could be replaced with any circuit or device that has moderate open loop gain as the (closed loop) gain is really set by feedback loops. Today folks basically do this with opamps instead of using discrete circuitry. The performance is essentially the same.

- Clipping network - by principle - is just a standard diode network in the feedback loop. Nothing extraordinaire.
Yes, there are few additions: The transistor is simply a switch (it really does nothing else) and yes you have drawn the scheme totally wrong depicting something the circuit isn't doing. The transistor's base circuit simply receives DC signal, which then controls whether transistor is on or off conduction. Depending on state, the signal inputted to the emitter (and attenuated to suitable magnitude, hence emitter resistor) is either flowing to the distortion adjustment potentiometer or not (and from there on closing the other feedback loop). As you see, this is a standard feedback diode clipper with few basic additions that allow remote switching and crude control of the amount of negative feedback coming from the diode network.

If you make this circuit correctly the output signal is nothing resembling the plot in OP's post but plain symmetric soft clipping. Since all circuitry is pretty much standard I wouldn't be surprised at all if one would find something similar from some of the Dean Markley amps.

The series/parallel channel switching is really the unique feature of these amps (and I think an idea found from some Dean Markley amps as well). As far as I know, the idea was invented by one of the Acoustic Control Corp. designers and soon adopted to dozens of other guitar amps.
It also looks like the germanium diodes in the input are only toggled when certain input jacks are used. For example, they are effectively out of the circuit when the "effects" input is used but the cascading channel setups seem to be toggling them.

[tca]

Well, I too have to argue that you have drawn the schematic differently.
... and by doing that, very incorrectly.

That is a very strong statement... I did sim the complete Peavey schematic and
also breadboard a very similar version (clean and distortion). And this simple
circuit has the same harmonic characteristics of the original circuit. With and
without the clipping feedback, etc...

- Gain control potentiometer - which you have totally omitted - works by
adjusting both negative feedback ratio and output signal's magnitude. A pretty
usual setup for amplifiers of this vintage. In essence the gain stage could be
replaced with any circuit or device that has moderate open loop gain as the
(closed loop) gain is really set by feedback loops. 

Agree, always full gain.

Today folks basically do this with opamps instead of using discrete
circuitry. The performance is essentially the same.

The details can be surprisingly different using discrete devices.

- Clipping network - by principle - is just a standard diode network in the feedback loop. Nothing extraordinaire.
Yes, there are few additions: The transistor is simply a switch (it really does
nothing else) and yes you have drawn the scheme totally wrong depicting
something the circuit isn't doing. The transistor's base circuit simply receives
DC signal, which then controls whether transistor is on or off
conduction. Depending on state, the signal inputted to the emitter (and
attenuated to suitable magnitude, hence emitter resistor) is either flowing to
the distortion adjustment potentiometer or not (and from there on closing the
other feedback loop). As you see, this is a standard feedback diode clipper with
few basic additions that allow remote switching and crude control of the amount
of negative feedback coming from the diode network.

I think differently. It does more than switching. Did you ever breadboard it?

If you make this circuit correctly the output signal is nothing resembling the
plot in OP's post but plain symmetric soft clipping. Since all circuitry is
pretty much standard I wouldn't be surprised at all one could find something
similar from few Dean Markley amps.

I think it is correct the way it is.

Cheers.

[tca]

The transistor is simply a switch (it really does nothing else)...

NO!

It really isn't that fancy design when simplified properly.

Hmm, lets us see then what it *really* does. Here goes a more "modern" and
*properly simplified* version of the same idea, taking your suggestions into consideration.  



Which clearly shows what the BJT is doing in the feedback path. The diodes
account for the soft-clipping part and the BJT adds some elongation to the
positive part of the signal (or negative depending on the connections, as my
first schem shows). Do you see what this little circuit is trying to emulate?

Cheers.

P.S.

Since all circuitry is pretty much standard...

They are not That's your shortcoming.

[teemuk]

Ah, I see it now; In these particular designs there is not enough base voltage for switching "headroom" so high enough input signal will actually overdrive the BJT switch resulting into depicted asymmetric clipping. It's a similar design quirk one can find from some older Marshall amps as well (the ones featuring BJT switches). In fact, something like this will always occur at the point where semiconductor -based switches get "overdriven" past their designed region of operation. I'd guess that in this case the distortion from the BJT was probably more or less unintentional.

It also explains the germanium diodes in the input: They limit the output signal (that is coming from the cascaded preamp stages) to suitable magnitude, therefore preventing overdrive of the BJT switch. When plugged directly to "effects" input (sans cascaded preamp stages) the signal amplitude is (in average) in millivolt ranges (e.g. 20mV - 200mV) and therefore already low enough to render those limiting diodes somewhat unnecessary.

Try your simulation / breadboarding with more reasonable input signal amplitudes (ones you could typically expect from a guitar pickup) and you get the usual symmetric soft clipping performance I described. Also, try it with higher base drive voltage (e.g. 30VDC) and with different BJT types. This design is very sensitive to picking proper transistors that can do the job. Some BJT's in such circuit will overdrive more easily than some others. Needless to say, today folks would probably use FETs instead, which introduce less aforementioned quirks. Back then it probably just wasn't an option.

I still fail to see how this is nothing but a diode clipper with remote switching. (Granted that the said switching device has inherent flaws that introduce peculiar side effects when typical input signal amplitudes are exceeded).

[tca]

Here goes a more "old fashion" way of doing the same thing, much more like the original.



I still think it is a  made-on-purpose inherent flaw. Firstly the note of 0.6V at the base of the switching transistor (original schematic) and secondly something that I heard when I was a teen... Some one told at the time that there was a guitar amplifier that added a peak to the input signal to add distortion to the guitar signal, for what it worth, that are my reasons. Not much I know.

BTW, it does sounds great.

Cheers

[Johan]

If you look at the slope of that peak, if followed through it would end up double the original frequency, making it a really strong second harmonic....interesting...or am I reading it wrong?...j

[tca]

If you look at the slope of that peak, if followed through it would end up double the original frequency, making it a really strong second harmonic....interesting...or am I reading it wrong?...j

It has s very warm tone, the 2nd harmonic is clearly heard.

[tca]

Another thing about this circuit is that it has a small slew rate, from my sims
<=1 V/µs, which gives a nice roll-off of the highs when gain is increased. When
I first breadboard the circuit I notice some different freq response when
varying gain but did not quite get it. This can be heard when playing with the
circuit.

Cheers.

[psychedelicfish]

I noticed with my dean amp that I get a sort of "clean bleed through" effect when I feed it a strong signal

[tca]

^ What do you mean by "clean bleed through"? What model is it?

Cheers.

[psychedelicfish]

DGX1025RV is the model of the amp, as for clean bleed through, a clean signal is clearly audible above the distortion

[psychedelicfish]

The switching in that amp is pretty terrible, so it could be that causing the clean bleed through

[teemuk]

When I first breadboard the circuit I notice some different freq response when varying gain but did not quite get it.

Of course it does that. The way the gain control pot is set introduces at least two RC filters which's time constants depend on potentiometer's setting.

First half of the pot forms a variable resistance in the negative feedback "divider" circuit, with a series capacitor. The second half of the pot varies the shunt impedance in the output so the output coupling cap will interact with this varying resistance. Basically, at higher gain settings the circuit reduces gain of lower frequencies much more than it does at lower gain settings.


This setup later evolved to Peavey's "Saturation" circuit, which supposedly mimics the bandwidth limiting effect of a saturating transformer. Again, at higher gain settings these amps reduce the bandwidth more than they do at lower gain settings. Other manufacturers have used the same idea countless times but most of them never bothered to give it any fancy names.

[tca]

^ yep, that sounds like it. The same trick can be used in any configuration, even in a simple BJT stage.

[tca]

Here is a sound sample. I've played with the knobs... at the end of it you can ear me pushing it with a clean mini-booster (my version of it...).

P.S.
Recorded with a small mp3 recorder.

P.P.S.
The amp is a SS amp, only clean sound comes out of it!

[tca]

ah, just found this effect on a tube amplifier:



It is called 'nipple distortion' for obvious reasons... Valve_wizard says it sounds like bad distortion? Does it?

Ref.: http://www.freewebs.co.uk/valvewizard/cathodyne.html

[teemuk]

Tone preferences are a matter of taste so I won't go into that argue.


...but, considering that such effects ["nipple distortion", frequency doubling, etc.] are VERY rare in traditional guitar amplifiers and usually avoided by all costs (as can be read in between the lines from the Valve Wizard site) I wouldn't have very high hopes that these kinds of distortions would sound all that great. One thing's for sure, that kind of distortion will add plenty of high order harmonics and probably sound like scraping glass.

I'm still not convinced that the Peavey amp does anything similar when driven with usual guitar signals. It can take pretty high input signal before the switching transistor actually gets overdriven and under normal operating conditions the output from that clipper is simply symmetric soft clipping.

[tca]

...but, considering that such effects...

What about that picture on page 273 of your SS amp book? It does seem to introduce a lot of odd higher order harmonics?

[teemuk]

It simply portrays the effect of feedback loop becoming less effective and thus gain increasing and skewing the signal.

The plot shows the output waveform of the first gain stage of T-Dynamics circuit so don't forget that this wave is immediately CLIPPED in following stages, which effectively removes the "nipple" part.

...basically, this is also what would happen in a tube amplifier because during the "nipple" the power tubes would clip the signal in cutoff state. You did catch that part from Valve Wizard site, did you?

Likewise, the distorted output signal of a T-Dynamics circuit is basically symmetrically soft clipped with dynamically varying amount of crossover distortion. Like in many tube amps. No "nipples" there at all.


I state it again: Such output waveforms coming out from a generic guitar amplifier are extremely rare and designer's try to avoid such distortions. Probably because they don't sound all that nice.