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Something interesting about the IC Big Muff is the active lowpass filter before the clipping stage. Look at the second half of the 4558. The 560k and 62k resistors in the negative feedback loop control gain (and also Q of the filter). The stuff attached to the positive input (the .0047 uF and .01 uF capacitors, and the 10k and 47k resistors) form the Sallen Key filter. The filter starts rolling off the high end at about 1k.
The 10k pot labeled "FUZZ" and 47k resistor attached to it are just a level control into the next stage.
Some interesting possibilities for mods could be to increase the resonance of the filter, or change the cutoff frequency; or you could make either adjustable. Making the frequency adjustable involves changing two resistors at once; you could use a dual pot or a voltage controlled resistor of some sort (like LDR or JFET).
Use a 100k dual potentiometer for R1 and R2. Make R4 a 100k resistor in series with a 100k potentiometer. Make R3 a 100k resistor. Use 10 nF capacitors for C1 and C2.
This gives you a variable resonance from Q=1 up to the limits of the op amp. R4 = 190k gives Q=10 in the simulation program. The cutoff frequency is variable from real super high (but really you should put a resistor in series with each section of the dual pot, because you don't want R1 and R2 to go all the way to 0) down to 159 HZ.
I've been rolling around ideas for converting the traditional op amp Muff into a circuit that use a quad op amp. And I was wishy-washy about what to do with that last op amp.
