Rhythm Wolf Mixer Section
The mixer section is a pretty straightforward summing amplifier followed by a differential output driver for the headphone output. The synth output is sent to a seperate jack before the mix, and is only included in the mix if no plug is inserted. There is a mute circuit on the outputs which is controlled by both the powersupply board and the microcontroller. This seems a bit redundant, as the microcontroller also controls the the analog rail generation, and i cant see why you would want to un-mute the output before turning on the analog power supply. Between the mixer and output driver is the "howl" distortion circuit. This has asymmetric clipping, a low-pass filter, and a less than 6dB/octave slope high-pass filter.
The howl function being applied to all voices makes no sense, and i would either use it on the kick or the bass. It was a tough decision, but eventually decided to put it on the bass synth. This was a simple matter of cutting the trace from the bass pot, removing R106 and R122, and then wiring the bass to U12B and back again to its normal input in the mixer. The output from the mixer then is wired to bypass U12B and go straight to U11A and the output. A similar thing could be done for the kick drum.