Completely erratic distortion thing needs tone control

[KarenColumbo]

I'm down with some hideous flu so all I'm capable of is sitting in bed an tinkering with the breadboard in a state of half-consciousness. So I somehow put together something that distorts what's put in it without really knowing what I was doring obviously.

I started out with some circuit I found somewhere I don't remember. Then I read a bit of AMZ lab notebook. Then I studied the Wampler Triple Wreck schematic. Then I just passed out or something. It sounds just like a real distortion (almost), but produces very strange waveforms. See this screenshot (blue is my Les Paul clone, red the emanations of my circuits output):



With a last effort before sleep I put together a schematic



What I humbly ask of you: Could you point me to a good insert position in the schematic for some sort of tone control? I wanna give it the Low/High pass treatment. Or something like it.

Oh btw: If I crank up R3 ("Drive" or something) it produces these waveforms which I find intimidating somehow:

[Cozybuilder]

There are some problems with IC1. You are biasing it to ground through R1, not Vref (4.5V), that op-amp will be happier if biased to 4.5V. It looks like you meant to have a pulldown going to ground, that should be on the input side of C1. R2 (the 4K7 to ground on pin 2) needs a cap in series with ground to isolate that pin from DC (at ground level). You might want to rethink your diodes/LEDs/pot arrangement on pin 1.

A tone circuit would be good between the IC3 stages, you could easily up the gain on IC3B if necessary since you are using IC3A as a buffer.

[KarenColumbo]

Thank you Russ, I will unf#%%$ it as you suggested. The hilarious LED clusterdork at IC1 is actually something I copied from AMZ lab book, but I lost all reason on the way back to breadboard I will try some other combinations, I think.

[Mark Hammer]

I find the red traces perfectly understandable.  The intent of that LED pair is to achieve some crossover distortion.  When R4 is set to minimum resistance in parallel with the LED pair, they don't do anything and the waveform you see is primarily the action of D-thru-D8.

When R4 is shifted over to provide a large resistance in parallel with the LED pair, then the LEDs act as a sort of gate.  Max R3 and the signal is more like to pass through that gate but only if it exceeds about +/-1500mv, which is expecting a lot from IC1A.  With a max gain of 22x, IC1A will get some of the signal past the LED pair, but not all of it.  Which, to my mind, is why you see spikes alternating with essentially background noise.

I think a much better strategy is to replace that LED pair with a back-to-back pair of Schottky diodes, or Germaniums.  The role of that diode pair should be to trim the "tails" of the waveform, but no more than that.  And to do that, you need a low Vf.

I think I get the gist of your overall strategy, which is to progress from higher Vf to lower Vf as you go from an LED pair (equivalent to a 3+3 silicon diode complement) to a 2+1 complement and finally a 1+1 pair.  That strategy would make eminent sense if they were allclipping the peaks of the wave.  But The LED pair are set to clip the rise/fall of the waveform, not the peak, so a high Vf ends up yielding a gate-like function.

FWIW I whipped up a version of the old Gretsch Contrafuzz , and incorporate a variable crossover distortion, similar to what you attempted here, the major difference being that I included it after a clipping stage, as opposed to before one, as you did here.  In any event, you can hear what the addition of modest crossover distortion adds to the tone in this video.  In principle, it is a kind of pulse-width adjustment, so you can expect it to generate differen proportions of harmonics.  To my ears it sounds great with a bridge pickup but nothing attractive with a neck pickup.  The X-Over control is in the upper right-hand corner, so watch for my hand to move over there.

But your question was about where to insert a tone control.  There are two approaches to take here.  One is to insert tonal adjustment where it can affect the quality of the distortion produced.  The other is to insert it where it can fine-tune and trim the undesirable aspects of the distortion tone produced.  C3 and C5 look like they will do a decent job removing the irritating harmonics, and some of the background fizz and hiss, although it seems to me that your overall gain structure seems kind of weak.  IC2A is unity gain.  Where's the fun in that?  You should at least hike it up to 10x to make decent use of the diode trio that comes after.  So make R6 100k and C5 1nf instead.  Same rolloff but more "push" to those diodes.

I also question why you chose to omit an input resistor on IC3B.  Will it work properly like that?  In the absence of an input resistor, does it revert to open loop gain somehow?  These sorts of perimeter-of-functionality things are beyond me.

IC3A doesn't appear to be doing anything especially useful.  If the output of IC3B was inverted with respect to input, then I could see using IC3A to restore phase, but it seems to be just sitting there.  All of which suggests that it could be put to good use supporting a standard Baxandall cut/boost circuit.  Or, you could simply make R8 an SWTC treble cut.

You might want to consider converting C2 to something like the "Contour" control used in G&L and Reverend guitars, and as Joe Gagan also used in many of his excellent early Fuzz-Face-based designs. Make C2 18nf, so that C2/R5 yield a default rolloff below 880hz.  Now, stick a 50k pot and 1uf cap in series with each other and put that in parallel with the reduced-value C2.  At max pot resistance prominence will be accorded whatever can pass through C2.  As the pot resistance is reduced, more and more bass passes through successfully. Because this will vary how much bass is in the signal to be clipped, it will change the character of the distortion.

[pinkjimiphoton]

karen,
just on a side note... feel better. i had that. knocked me on my arse for weeks.

that said, i humbly suggest either mark's stupidly wonderful tone control, or the big muff one. if ya go with the big muff, add a midrange pot. it can really help you define the distortion tone and cut thru the mix.

or a three band. maybe it's unnecessary, but i find midrange emphasis in dirt to be important sometimes.

[KarenColumbo]

I find the red traces perfectly understandable.  The intent of that LED pair is to achieve some crossover distortion.  When R$ is set to minimum resistance in parallel with the LED pair, they don't do anything and the waveform you see is primarily the action of D-thru-D8.

When R4 is shifted over to provide a large resistance in parallel with the LED pair, then the LEDs act as a sort of gate.  Max R3 and the signal is more like to pass through that gate but only if it exceeds about +/-1500mv, which is expecting a lot from IC1A.  With a max gain of 22x, IC1A will get some of the signal past the LED pair, but not all of it.  Which, to my mind, is why you see spikes alternating with essentially background noise.

I think a much better strategy is to replace that LED pair with a back-to-back pair of Schottky diodes, or Germaniums.  The role of that diode pair should be to trim the "tails" of the waveform, but no more than that.  And to do that, you need a low Vf.

I think I get the gist of your overall strategy, which is to progress from higher Vf to lower Vf as you go from an LED pair (equivalent to a 3+3 silicon diode complement) to a 2+1 complement and finally a 1+1 pair.  That strategy would make eminent sense if they were allclipping the peaks of the wave.  But The LED pair are set to clip the rise/fall of the waveform, not the peak, so a high Vf ends up yielding a gate-like function.

FWIW I whipped up a version of the old Gretsch Contrafuzz , and incorporate a variable crossover distortion, similar to what you attempted here, the major difference being that I included it after a clipping stage, as opposed to before one, as you did here.  In any event, you can hear what the addition of modest crossover distortion adds to the tone in this video.  In principle, it is a kind of pulse-width adjustment, so you can expect it to generate differen proportions of harmonics.  To my ears it sounds great with a bridge pickup but nothing attractive with a neck pickup.  The X-Over control is in the upper right-hand corner, so watch for my hand to move over there.

But your question was about where to insert a tone control.  There are two approaches to take here.  One is to insert tonal adjustment where it can affect the quality of the distortion produced.  The other is to insert it where it can fine-tune and trim the undesirable aspects of the distortion tone produced.  C3 and C5 look like they will do a decent job removing the irritating harmonics, and some of the background fizz and hiss, although it seems to me that your overall gain structure seems kind of weak.  IC2A is unity gain.  Where's the fun in that?  You should at least hike it up to 10x to make decent use of the diode trio that comes after.  So make R6 100k and C5 1nf instead.  Same rolloff but more "push" to those diodes.

I also question why you chose to omit an input resistor on IC3B.  Will it work properly like that?  In the absence of an input resistor, does it revert to open loop gain somehow?  These sorts of perimeter-of-functionality things are beyond me.

IC3A doesn't appear to be doing anything especially useful.  If the output of IC3B was inverted with respect to input, then I could see using IC3A to restore phase, but it seems to be just sitting there.  All of which suggests that it could be put to good use supporting a standard Baxandall cut/boost circuit.  Or, you could simply make R8 an SWTC treble cut.

You might want to consider converting C2 to something like the "Contour" control used in G&L and Reverend guitars, and as Joe Gagan also used in many of his excellent early Fuzz-Face-based designs. Make C2 18nf, so that C2/R5 yield a default rolloff below 880hz.  Now, stick a 50k pot and 1uf cap in series with each other and put that in parallel with the reduced-value C2.  At max pot resistance prominence will be accorded whatever can pass through C2.  As the pot resistance is reduced, more and more bass passes through successfully. Because this will vary how much bass is in the signal to be clipped, it will change the character of the distortion.

Thank you, dear Mark, for your answer. And thanks for the vid - this is a very unexpected, yet satisfying sound you made there! Very cool indeed! Duly noted and entered in my "Things to get my head around" list My tubes and lungs are still full of rattling nails (and my brain is addled with paracodin) so it will take a couple, but I'm looking forward to doing this asap.

As to the "topology" of my thing:

1. I thought I'd make IC1A a simple buffer, then let the as of yet unused IC1B do what A is supposed to do, but then I forgot - or was it the meds? Anyway, I'll make it right according to what y'all trouble-shooted. I thought I'd give him hell already with a gain of over 20 (I thought those red flat noise lines on my capture were evidence that I overloaded something and it just lies there and dies). I will have a sweep over similar schems to get my bearings.

2. I will crank up IC2A, too. As mentioned in 1., I thought I already had way too much gain (it got pretty loud at the output already). Good to know that there's enough headroom for more mayhem. ARRRR!

3. As of IC3A & B: Exactly! I wanted to put phase back in order, tried to implement an inverting unity gain stage - alas to no avail, the thing was dead. So I just wired it to at least let sound through again and promptly forgot/went to sleep again. I will study the basics one more to get this aligned properly.

4. I will incorporate ALL of your tone forming suggestions - it's a breadboard after all. Maybe I will stumble upon the ideal cmbination - the "luck of the bug-sick" or something

karen,
just on a side note... feel better. i had that. knocked me on my arse for weeks.

that said, i humbly suggest either mark's stupidly wonderful tone control, or the big muff one. if ya go with the big muff, add a midrange pot. it can really help you define the distortion tone and cut thru the mix.

or a three band. maybe it's unnecessary, but i find midrange emphasis in dirt to be important sometimes.

Dear Jimi, yeah, this has been going around here in Vienna/Austria for months now - everybody I know and care for has/had it, if you live in a relationship as I do (married) there's a whole lotta cross-infecting going on. Wife and I are at round #3 at the moment lol.

You got it with the "stupidly wonderful" tone thing - I found this, but then I lost it again because of my addled brain. Thanks for mentioning it - I will have a look at this. Midrange is an excellent idea, too. I'm "testing" the thing with an "acus" acoustic guitar amp - because it's absolutely clean. Doesn't give me an idea what it sound like over a "normal" amp, though. Gotta plug in the good old AC4TV again.