Need help deciphering my amp's built in overdrive circuit.

Bill Mountain · December 09, 2011, 08:08:41 AM

I have an old Traynor bass combo amp that I use for at home practice. Whether it's intentional or not, it has a sweet overdrive tone when the preamp is cranked. This is a very popular sound in some circles. So much so that, there is even a preamp pedal out (Tronographic Rusty box) that is a copy of this preamp.

When looking at the schematic here: http://www.lynx.bc.ca/~jc/810407_TS60B.gif (this is my amp. The rusty box copies a slightly different amp) what is making the overdrive sound?

I used to think it was something in the power amp but I realized that I can get the sound with the gain up and the master almost off.

Also, there are 6 transistors labeled "MPS 8598" that are set up in a configuration I'm not familiar with. Is this part of the overdrive sound? Do these transistors serve a different purpose?

Thanks!

Mark Hammer · December 09, 2011, 09:09:42 AM

I'm with you on that. Everything up to that "ladder" makes sense, and then my WTF-reflex kicks in.

Explanations.....please?

Bill Mountain · December 09, 2011, 09:12:43 AM

Quote from: Mark Hammer on December 09, 2011, 09:09:42 AM
I'm with you on that. Everything up to that "ladder" makes sense, and then my WTF-reflex kicks in.

Explanations.....please?

Well...if you're stumped then I'm f'd!

DavenPaget · December 09, 2011, 09:19:20 AM

Well ... I always believed an overdrive section kicks a *tube amp* tube's into clipping ...
But it isn't tube , and i'll help you examine later .

gmoon · December 09, 2011, 12:22:41 PM

Looks like three parallel stages, each one fed by a different filter. With the output all summed back together afterwards... Followed by a 4-band equalizer.

Something like that...

slacker · December 09, 2011, 12:45:15 PM

The ladder thing looks similar to what some synth designs use as a triangle to sine wave converter, I think technically they are linear to log converters? If that's what it is it would add "soft" clipping, could be completely wrong though.

Looks like an interesting circuit, I like the tonestack.

CynicalMan · December 09, 2011, 03:41:17 PM

Looks more like differential amplifiers to me. Maybe a multiband distortion stage? Spicing...

Edit: OK, I think it is three amplifiers in parallel. Each of the amplifiers has this form:

I'm not sure whether that's a differential amplifier, strictly speaking. But it's very close anyway:

The filters on the input of the amplifiers are a three-way crossover. So each amplifier is amplifying a different band. Then they're mixed together and some of the high end is taken off with the 10n capacitor.

So my guess is still a multiband distortion stage. And this is where PRR or RG comes in.

Earthscum · December 09, 2011, 08:53:39 PM

The high and low pass sections look to be the same cutoff frequency, what's the bottom section filtering through it?

Ronan · December 10, 2011, 03:52:33 AM

Looking at the trannies on the right hand side of the ladder, their collectors are tied to -15V, and their bases tied to ground through the 1K5 resistor. So their emitters must be around 0.7V with respect to ground. These trannies are holding the lower end of the 18K resistors at 0.7V, so the trannies on the left hand side of the ladder have only around 0.7V on their emitters, and are only just biased on with their bases tied to ground through various resistances. Once the input signal swings high those trannies on the left will go into cutoff, resulting in assymetrical clipping. That's as far as I can go with this one.

slacker · December 10, 2011, 12:13:34 PM

Quote from: CynicalMan on December 09, 2011, 03:41:17 PM
Edit: OK, I think it is three amplifiers in parallel. Each of the amplifiers has this form:

Seeing it again drawn like that, it looks like the input section of a discrete opamp http://sound.westhost.com/project07.htm so it's possibly like one of those running "open loop" with no negative feedback. If you search for Joe Davisson's diode comp opamp which is a similar sort of thing you'll find some stuff from R.G saying that clips pleasantly run like that, so maybe it's the same for this.
It's definitely where any distortion is coming from.

PRR · December 10, 2011, 11:41:35 PM

> a multiband distortion stage. And this is where PRR or RG comes in.

Why? You nailed it.

The longtail diff-amp has well-defined clipping. It is used in FM IF strips, and in many Fender guitar amps.

The theory is complex, and does not tell you how to tune for "tone".

I think you could copy the whole thing from gitar jack to the coupling cap after the 8K2, and call it "preamp". If going to a guitar input you want some attenuation after this.

> the trannies on the left ...are only just biased on
> in assymetrical clipping

No. Nominally each pair splits the 18K's current equally. Clipping is nominally symmetric.

Bill Mountain · December 11, 2011, 12:31:35 PM

Thanks for all of the comments guys. This is something I'm going to have invest some time in because the distortion is very complex and satisfying. Any clues what other circuits use differential amplifiers in this type of setup?

ayayay! · December 11, 2011, 01:15:34 PM

No one's mentioned the 1.5M in the NFB??? I don't think I've seen that using the gain pot that way to control it. I'd think this could be a massive pedal with some clippers right before the tone stack, then eliminate the "ladder." I know the ladder is probably where the coolness factor comes in though...

That tonestack and simple boost look way neato.

PRR · December 11, 2011, 08:01:37 PM

> No one's mentioned the 1.5M in the NFB???

What's to mention? It's the DC NFB. AC gain is set by 0-100K pot against 2K, and this is a very standard arrangement when you want constant bass-cutoff (unlike the many fuzzes which want less bass at high gain).

> some clippers right before the tone stack

It's been done.

> eliminate the "ladder."

Not a "ladder". Yes it is drawn ladder-like, but it is three stages in parallel.

> invest some time

Somebody surely already invested much time in this. And you like it. Just get a +/-15V supply and copy the known-working design.

Bill Mountain · December 11, 2011, 08:55:07 PM

Quote from: PRR on December 11, 2011, 08:01:37 PM
> No one's mentioned the 1.5M in the NFB???

What's to mention? It's the DC NFB. AC gain is set by 0-100K pot against 2K, and this is a very standard arrangement when you want constant bass-cutoff (unlike the many fuzzes which want less bass at high gain).

> some clippers right before the tone stack

It's been done.

> eliminate the "ladder."

Not a "ladder". Yes it is drawn ladder-like, but it is three stages in parallel.

> invest some time

Somebody surely already invested much time in this. And you like it. Just get a +/-15V supply and copy the known-working design.

Where's the fun in that?

I actually just brought out my old Hewlett Packard 60 volt variable power supply for different preamp project so it looks like I'll be doing some higher voltage distortions as well!

Ronan · December 12, 2011, 06:12:42 AM

To try and simulate a part of this cct I breadboarded a pair of ZTX751 with 10K feeding the emitters and a 4K7 on the collector of the input side, with +/-9V supplies. With 1kHz sine wave 72mV rms input, I got 1.57V rms out, gain of 22. The output signal was clipped symmetrically and with very rounded tops and bottoms. Similar to the output from an AMZ minibooster cct. Clipping started with around 35 to 40mV input signal. To my newby surprise, adding a 4K7 resistor between the collector and -9V of the opposite transistor made no difference to the gain or shape of the output signal. So, I learned something new, thanks Paul for correcting me.

ayayay! · December 12, 2011, 10:02:59 AM

Quote> No one's mentioned the 1.5M in the NFB???

What's to mention? It's the DC NFB. AC gain is set by 0-100K pot against 2K, and this is a very standard arrangement when you want constant bass-cutoff (unlike the many fuzzes which want less bass at high gain).

My mistake.

News:

Need help deciphering my amp's built in overdrive circuit.