Rhythm Wolf Hi-Hats
They utilize both an analog and digital noise generator, and the tune doesnt actually tune anything, and rather sets the mix of these two. R118,113 limit the extent of this mix. The relative volume of each is quite different. The digital noise is around 5Vpp max, whereas the analog is 500mVpp or so. There are two lowpass filters on the analog noise (C19,60) and the digital noise has a resonant lowpass and a regular RC lowpass (C48,54,67). This combined noise source goes to two pretty standard drum 'VCAs' (Q19,39 - one each for open/closed). The envelopes for these are generated with the same gate/accent setup as the bass synth. The envelopes peak value tracks the accent voltage. The decay time is half for the closed hat. The envelopes also go to comparators to shut off the digital noise source. The output of the VCAs go to a sallen-key highpass filter, using a single transistor. The output then goes to a mixer stage and the volume pot.
I find it odd that the digital noise source is connected to digital powersupplies. This means any noise in the digital side will be coupled into the hihat outputs. The amount of current drawn by the HC14 is really low, so i'm not sure it would have been splattering noise onto the analog supplies if it was connected to them. The other thing is that the comparators do not work as intended here. Since there are diodes and capacitors in the VCAs, the emitters of the evenlope buffers sometimes do not get down below the threshold to shut off the comparator. A pulldown resistor is needed at the emitters to ground. i'm not sure this matters much, though. This section could be modified to apply a sort of 'mute' to the voices, so they end more abruptly. This might be good for the closed hat. Another oddity is that as the decay knob is turned down, it loads down the charging circuit, and the envelope actually gets completely eliminated, so the voice pretty much stops sounding at really low settings. R208,202 could be increased to basically set a more reasonable limit on the decay time.
All in all, i like the sound of this, and fixing the decay times is a pretty simple mod. i would like to be able to alter the pitch in some way, and i think the easiest way would be to alter the supply voltage to the HC14. These can run from 2V to 5.5V, and the pitch of the oscillators change with the supply voltage, as the hysteresis increases with increasing supply voltage. A little modulation on it might be nice as well. If i felt that the mute function was needed, then this voltage would be applied to the emitter of Q72, otherwise it would be easier to just remove Q72 and apply it directly to the supply pin of the HC14.