I came across Tim Escobedo’s Bronx Cheer (an “envelope waveshaper filter”) some time ago and finally gave it a go. It’s a very simple high gain Darlington transistor fuzz, with a filter attached to it’s simple collector to base biasing. The filter is a simple low-pass “double pi” topology, with its two resistors cleverly replaced by diodes that work as variable resistors that conduct when the signal gets past their forward voltage. The result is a resonant fuzz where the filter frequency changes when one hits the notes harder, and changes again as the notes decay.
I made only minimal changes in the design, replaced the power capacitor to 100μF, and the filter capacitors to 1.5nF, removed the switch that did not make much of a difference, and replaced the MPSA13 that I could not find anywhere locally with the higher gain MPSA14 (the similar MPSA28 works just as good). The quirk of the circuit is that it expects to see the high impedance of guitar pickups at the input. The impedance mismatch can result in the most horrible hum and noise when connected to the output of a synthesizer. Escobedo suggested the trick of connecting the primary windings of a small transformer at the input to mimick the impedance of a guitar pickup. I tried locating the transformer he indicated in more than a dozen local shops and none had anything similar. So I tried different small transformers and inductors (the ones you find in “wall-wart” switching power supplies and fluorescent lamps) and they all worked, with no discernible difference in the sound.
