Dickering around with feedback resistance in a Fuzz Face topography

[Mark Hammer]

I put together a clone of the ZVex Wooly Mammoth a year or so back, using the Madbean layout (thanks for that!), and one of the more interesting aspects of that circuit is Zach's use of a variable feedback control between the emitter of Q2 and the base of Q1 in what is essentially a silicon Fuzz Face adapted for bass.  Indeed, a great many effects use some version of a Fuzz Face topography, where there is feedback from the emitter of a 2nd transistor to the base of a first one.  And there is much variation between them, both in terms of what they might do in the manner that feedback is provided, and the feedback resistance used, much of it well beyond my understanding of transistor circuit behaviour.

But I know this: I like the way the feedback and gain controls on the WM interact.

I had made a clone of the Jen Fuzz III, which is essentially a cousin to the (fixed gain) Muff Fuzz and (diodeless) Fuzz Face.  The Jen uses a 150k feedback resistor.  I decided to dicker around with that value and installed a 3-way toggle such that I had a 150k in series with 470k (620k total), and could use the toggle to either bridge the 470k (stock) leave it unbridged, or bridge it with 390k (213k in parallel + 150k = 363k).  Provided three easily detectable ranges of "seriousness", with the greatest "seriousness" accompanying the highest feedback resistance.

If you have anything employing a FF-like structure, this is a parameter worth playing around with.  And not just series resistance, but I imagine that different caps in parallel, or running to ground, can provide interesting changes in the qualitative aspects of the distortion produced.

As an aside, I also took the "saturation/limiting" feature found on some distortions and applied it to the Jen, using the same 3-way toggle logic I used for the feedback control.  That is, the diodes connected to ground through a 20k resistor, which I used the toggle to either bridge (direct connect to ground), unbridge (20k to ground) or partly bridge (4k7 in parallel with 20k drops ground resistance to 3k8).  Those two toggles yield lots of useful sonic variation.

[midwayfair]

There's a bunch of cool things you can do with that negative feedback resistor, and I agree that it's one of the least touched components when I see the latest "super fuzz face" booteekery. Split it in the middle and put a cap to ground and you get a brighter fuzz, for instance.

One of the more interesting negative feedback pedals is the Fuzzstainer: http://tonemachines.blogspot.com/2012/02/sam-ash-fuzz-stainer-197.html.

[Mark Hammer]

Thanks for that reminder. 

And of course, one of the always-interesting aspects of a simple series resistance is that it lends itself very nicely to use of photocells. 

I'm trying to imagine a pedal with a photocell in the feedback loop mounted on the top of the pedal, where you can put your foot or hand over it if you want.  One pot sets a resistance in parallel with the photocell, and another sets a pot in series with it, so that the range, and amount of foot/hand modulation can be adjusted.

[FiveseveN]

Had a bit of fun with the FF platform myself, called it the Mammoth Face Factory . Many textures are offered when you mess with the feedback. But now that you mentioned the LDR idea, I made another connection: envelope control! A "direction" switch could be employed, to offer either decay that "blooms" into cleaner tone or that becomes gated and sputtery. I'm sure the LFO pundits could also add something interesting.