Here is a snippet from Bernie Hutchins. I think this is from Electro Notes 132. Seems to be an interesting clipping scheme. I found this here: https://www.diystompboxes.com/smfforum/index.php?topic=36572.msg258950#msg258950
I’ve been thinking about this snippet for years now, I’m putting it here to remind myself to experiment with it in the future.
I’ll quote the description here:
“There are several design goals we had in mind here. First, there should be a small linear region about zero. Secondly, we want only weak non-linearities when they do start. Thirdly, we would like a non-symmetric non-linearity somewhere in the middle range in order that we may generate even harmonics. Finally, we would like to end up with a somewhat balanced waveform at the maximum amplitude. The circuit…realizes these goals.
For small signals, all diodes are off, and the circuit is linear and unity gain. When the output exceeds +/-o.6 volts, diode pair A and the extra 100k resistor come into play, reducing the gain factor, rounding the top. At two diode drops of input signal (+/-1.2V), diode pairs B come in with their 22k resistor, greatly increasing the gain, and peaking the top. Beyond three diode drops (1.8V), diode pair C comes into play, substantially reducing the negative going gain. This gives the desired non-symmetry. Finally, at slightly over a 5V level, the section with diodes D comes in, effectively allowing no further gain on the positive side, allowing the negative side to catch up if it can.
Thus as the output of the filter (nearly a sine wave) increases in amplitude, the output of the non-linear circuit has harmonics, and the relative amplitudes of the harmonics changes with amplitude (and thus with time). This is the type of response we want.“
This seems like it might do something interesting. The question is whether this circuit will work stompbox voltages. We probably need a buffer. Which is not hard to add, we’re obviously going to use a dual op-amp.
The description says the first diodes begin clipping at +/-0.6V. The next pair kick in at +/-1.2V. That is a good range for effects powered by 9V.
At 1.8V the diodes at C are activated. These are facing one direction so should only activate on the negative swing (I’m guessing here). That’s we’ll within the 9V.
Then the description says “Finally, at slightly over a 5V level, the section with diodes D comes in“. If the power is 9V and our op-amp is biased at 4.5V. It seems like we will never get to 5V. This looks like a diode followed by a 5V zener diode. Maybe a 3V Zener would work here, and make this work on a 9V supply.