Little Phatty overdrive

I can't remember if I've written about this before, but I've certainly looked at it.

EDIT: Turns out I did, in great detail! https://atosynth.blogspot.com/2024/01/moog-overload-circuit.html

I'm trying to retrofit overdrive to my ladder filter, so I had a closer look at how the little (and slim) phatty does it. The slim phatty schematics are available online but the circuit is spread across multiple pages so it's a bit harder to see what is going on.

Here is a simplified schematic:

 

There are a few surprises here:

- there is no feedback from the filter output to input, which is the way the minimoog achieves overdrive.

- it uses a pretty standard soft clipping circuit with CV-controllable clipping - the overdrive of the minimoog filter, if I recall correctly, happens in the transistors of the differential amplifier at the end of the filter.

- the output of the overdrive is fed back to the oscilllator mixer and returned to the soft clipping circuit

- the mixer is not a normal inverting summer op amp - it's just an op amp buffer, with all inputs connected to the positive input. This is possible because the OTA outputs are current, not voltage outputs

- the differential amp of the filter is realised using op amps, which won't give soft clipping when overdriven. 

- the filter has an additional output gain OTA.

It's a pretty neat circuit, and all three OTAs - feedback, distortion and post-filter gain - are driven from the same CV. 

 All CVs are biased in various ways, to +5 or -5, I haven't studied exactly how they work in conjunction. Neither is it clear to me if the post filter OTA contributes to the overdrive in any way or if it just makes up for lost gain during overdrive.

 

Major take-away

Distregarding the post filter gain, all distortion happens before the filter, just as I'm currently doing in my synth. I don't have the additional feedback OTA though, whatever that does.