Help with voltage-controlled distortion circuit design

Thread Starter


Joined Sep 29, 2017
I'm trying to make a voltage-controlled distortion circuit (see attached schematics). The idea is similar to PAiA's Quadrafuzz module:

..except I'd like to add voltage-control (or CV in the synth world) to each channel. I don't want to blindly build PAiA's design, but instead re-design it based on my understanding how the circuit actually works - which isn't too complicated (besides the extra circuitry at the output for feedback and loop send/receive which I don't quite understand). And I'm also adding a wet/dry circuit to for mixing distorted/clean signal.

So the idea.. the input signal splits off into 4 frequency bands/channels, then each one's distortion is independently voltage controlled, and then all channels are summed and re-scaled at the output. Note, my attached schematic has the input/output section on page 1, and only one of the four freq + distortion channels on page 2. There's no global (input-to-output) feedback or anything either - don't really need it (..I think?). So it's a pretty straightfwd circuit.

The part of the circuit which adds the distortion is a variable feedback resistor based on the LM13700, wired as a floating resistor per the datasheet's cryptic wiring (see figure 28 - bottom of page 16 on the datasheet:

This circuit actually works, but there are several (some big, some small) problems. Here are just a couple for now..

1) Distortion is subtle and not very prominent. I'm using LEDs in series with 4148 diodes to do the clipping. But I want the clipping to be at a higher voltage. Should I just add more diodes in series with the LEDs? Also, I'd like the LEDs to be brighter when they’re clipping (right now they're quite dim).

2) The wet/dry circuit (on page 1) doesn't really work, because the signal is always pretty wet. There has to be a better way of mixing wet/dry signals.

Any advice, suggestions, or comments are welcome :)

Thanks you!