Hi Chip,
Thanks for the detailed response, it helps a lot to discuss the architecture and the possibilities as they can directly impact the feasibility of the mods.
I think it will also help if I describe more of what I had in mind, knowing that the single LFO is a bit of a limitation.
As it is, the higher LFO speed is quite useful already - it has made me spend a little more time within the synthesis engine thanks to the new sounds possible. Now, the sounds would be even better if we could make the high-speed LFO effect be strong at attack time and less at sustain time. As an alternative to different sounds, we can also envision sounds which start with low LFO Level and the latter rises up with the sustain portion of the sound.
This means that the ideal implementation of this mod necessitates a re-trigger for each note. Now, we do have an envelope for each note and the Tubbutec mod taps into this. If my understanding is good, the Envelope output has to be buffered to be useful.
Additionally, in your personal setup, you have extra flexibility since you can intercept note triggers: you could do a soft LFO on the Arduino Mega, triggered on each note on, and program it to decrease with time or a soft envelope as well, provided you allow switching that new LFO in in place of the old one in the Polysix. Terje has done something similar so let's see if he can help us understand the electronic implications.
As a peripheral useful mod, I was reading about your detection of a square signal at I16-7. That can come in very useful as well. I am wondering if instead of swapping it in before the next opamp at I16-3, the signal is injected at J on the schematics. There could be a useful combination of the square and triangle signals as well.
I was trying to understand from the schematics how to make the Mod Wheel act upon the VCA and VCF in addition to the usual VCO, but that part stumped me a little. I can see where the signal is switched for MG. For VCO, the schematics are proving hard to decipher for me.