Any way to get more pulse width variation out of the Great Cheddar? Swells?

[Processaurus]

This is one of my favorite classic sounding fuzz designs, I've been thinking part of it may be how it varies the pulse width of the fuzz with your picking.  So I'm wondering if someone with a better understanding of transistor circuits had the time to explain how this happens with the different feedback paths in, and if possible, if there is a way to get some control over this parameter, for hypothetically getting a more obvious, envelope filter like behavior, or getting more of the neat fuzz cancellation thing where you pick hard, and it actually gets quieter, like its struggling to make a sound?  Kind of like some of Neil Young's raunchier sounds.

By the way, thanks, R.G., for the publishing the Great Cheddar page at Geofex, and to Dan Coggins for the original BC design work, without them I would be one step closer to sitting around cross legged playing acoustic in front of coffee houses...

Schematic:
http://geofex.com/Article_Folders/grtchedrschm6.PDF

[Processaurus]

Bump in the night, with a little bit I found with the search:

A discusion of the fuzz section by RG in '03:

The basis of the circuit looks to be a dual-feedback shunt-series pair. This has a casual resemblence to a FF, but includes another feedback path from the collector of the second transistor to the emitter of the first; the FF has only one feedback path, emitter of the second transistor to the base of the first.

The chief characteristic of the shunt feedback input stage of both the FF and the shunt-series pair is that they are very low impedance inputs, and can easily be thought of as current amplifiers. The gain depends on the impedance of the driving source. This is one reason the FF is so sensitive to what drives it; guitar pickups have impedances in the tens of K's, and that's what sounds good. Driving it with an opamp output overdrives it pretty solidly.

The BC uses the same input, but puts a weenie 0.047uF cap in front of the input, between the base of Q1 and the driving opamp. This gives a steadily rising driving impedance with frequency, and is small enough that the bass frequencies don't overdrive it (See - The Technology of the Tube Screamer at GEO for some discussion of bass rolloff keeping distortion from being flabby).

Near as I can tell, from both scope tracing and simulation, the shunt-series pair produces a slope-sided square wave for any reasonable input. The opamp just blasts the input *hard* and the pair saturates both ways. Thankfully, the SS pair is simple enough that it doesn't do some of the ugly things that can happen from overdriving things like diffamps.

What it *does* do is produce a rectangular wave at Q2's collector that varies in duty cycle if not shape or size with the magnitude of the signal driving the input. It's touch sensitive over the entire range of reasonable guitar inputs. The actual unit has a variation from smaller to 50/50 and back smaller as input drive goes up. The simulation only shows the small to 50/50, so there's another reason to only trust simulators a little bit.

The BC is perhaps more interesting for the response of the SS pair under multiple feedback. The linear response (if you could put in a signal that small) has a response peak at about 200Hz, then flattens off out to about 10KHz. The SS pair shows some promise for adaptation to some other things than distortion pedals...

R.G.

[Processaurus]

Bumplestiltskin.  The kind of swells I'm thinking about aren't bonafide volume swells, but when the pulse width gets so narrow during the initial pick attack, it sounds quieter...  There was a boutique effect called the Wylo Moonrock that was related to the Scrambler supposedly, but it had a knob that would dial that kind of effect in.  There was a prescription electronics fuzz too that had some sort of crude swell thing going on.