An idea for a distortion pedal -- what do you think?

[David]

Oh, no!  An obsession is born!  I've been looking over schematics trying to figure out what distortion pedal to try next ("so many pedals, so little time").  I had a really EVIL idea.  Check this out:  Make a Tube Reamer with half of a dual op-amp.  Use the other half to make an active post-distortion 3-band Baxandall tone control.  In addition to the clipping diodes across the Tube Reamer, add a couple of additional diodes to ground after the distortion stage for hard clipping.  Then, put a DPDT switch in to select the clipping type.

I admit this isn't original.  A lot of it is from Richardo X's "Cook Your Own Distortion" article.  Seems like this might be a way to goose a little more performance out of an op-amp distortion unit, though.

Anybody got any feedback?  Pros?  Cons?  Suggestions?  Warnings?

[Marcos - Munky]

Sounds cool. If you try this, post in the forum.

[Johan]

...or..eh...why not make the active baxandalcircuit very high gain and have the diods in the eq-circuits feedback loop...being able to adjust the cliping in the different frequency ranges decided by the baxandal...woldn't that be kind of sweet?....


Johan

[David]

Johan:

Frequency-dependent clipping?  What an interesting idea!  I'm having trouble wrapping my mind around how you would incorporate clipping diodes into the RC filters.

[idlefaction]

looking here, try putting two sets of clipping diodes in series with the central 22k and 560pF.

i have NO clue what it would sound like, either.  only one way to find out!  
fwiw, people have had much joy using clipping diodes in feedback loops of filters using +ve feedback - like 'resonance' controls for instance.

i haven't played with this either, but paul perry knows a bit about MS-20 filters which use these, amybe he'd have some advice.  [/img]

[Mark Hammer]

The use of pre and post clipping to shape distortion has been an on-again, off-again topic around these circles for well over a decade..  But, since the use of "frequency-dependent clipping" has cropped up, time for me to put in another 2 cents....

If you boost select bands pre-clipping, it brings content in that band hypothetically closer to the clipping threshold (what I like to call "proximity to clip").  That's great  for getting those nice controlled wolf-tones that Billy Gibbons is famous for.  Some folks do it with EQ-ing, others with %^&*ed wahs, and so on.

The "trouble" with this is that the emphasis required to generate additional harmonics for some portion of the range also adds in resonances.  You may want them, but then again you may not.

So howzabout this idea: a complementary resonant EQ?  Works like this.  You boost a given range, pre-clip, but attenuate that same resonant band by the same amount post-clip.  The harmonic emphasis is largely unaffected and the resonance imbalance is reduced.  Easily achieved by using a dual-ganged linear pot for the pre-boost and post boost and wiring them up opposite.

Counterintuitively, such an approach could also produce harmonic *reduction* in select bands, by cutting pre-clip (increasing distance between signal level and clipping threshold) and then boosting by the same amount post-clip to restore tonal balance.  This would effectively create fuzz "dead-bands" where notes in that band have discriminably LESS distortion than content above or below it.

[Mark Hammer]

It occurs to me that you could make an interesting sort of "invisible wah" in this way, by incorporating a complementary pair of identically-tuned resonant filters, one before and one after a clipping stage.  You'd get the swept harmonic emphasis, but minus the squawk.  It would be more like an animation pedal than anything obvious, though I have to admit I'm still having a hard time trying to think what it would sound like.  Probably something more in the zone of what PWM sounds from a synth sound like.

The "Simple, easy parametric.." document at GEOFEX shows pretty much all you'd need, minus the clipping stage.  The gyrator sections are easily tuned via a single variable resistance to Vb, which means you can get a dual-ganged linear pot and use that to sweep two sections at once.  

One section has the same value components as the other but is set for boost and the other is set for cut by the same amount.  In this case, that complementary boost/cut is easy to do.  Instead of using the 10k boost/cut pots that RG shows, mimicking a wiper all the way to one side or the other is as easy as tying the "wiper" lead to one side or the other of a 10k resistor.  Most folks would probably have minimal use for varying the resonance but that too is easily duplicated via a dual-ganged linear pot.

The clipping stage itself can be just about anything you'd like though a TS-style diode-pair stage seems straightforward enough and about as exotic as you need anyways.

[Rory]

I think monkey fx has something like this as well.  Called the Moon Balloon I believe.

www.monkeyfx.co.uk

[Mark Hammer]

Thanks for the link.  From their description, that's not it.  They use band/frequency-splitting prior to clipping, but nothing after the clipping.  It also appears to be LFO-driven and not foot-sweepable.  Interesting idea for an effect, though.  May even deliver some decent "multi-rotor" Leslie sounds.