A to Synth breadboards two versions of the distortion circuit, both with input attenuation and output amplification to keep a 10Vpp signal almost untouched.
Pre-distortion amplification
First I did the version with pre-distortion amplification. With distortion CV at 0V the distortion circuit sees around 30mVpp. This is heavily amplified to distort.
At the other end, a second OTA amplifies the signal. The output has unity gain for a 10Vpp input when CV is around 2.5V.
OTA and parallel resistor in feedback loop
Then I tried breadboarding the OTA-in-feedback version. I had lots of trouble and could not get it working as expected at all. After much experimentation I ended up with a well functioning circuit. I then started documenting the changes, and realised what was going on: I had inadvertently put a 33k resistor in the feedback of the distortion op amp. This works in tandem with the signal fed back through the OTA, so it completely changes the amount fed back.
See the circuits and full treatment on A to Synth.
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