Well, I’ve at last completed my modified Turing Machine LPG/MIX module with switchable Pulses/Gates connections - much later than I’d hoped:
Everything works as expected including the Pulses/Gates switching under manual or CV control.
The only problem was that the LPG output was so short that it really wasn’t possible to hear the filtering as the sound faded to silence too quickly, although it was clearly happening on the scope. I was about to write it off as inevitable given the speed of my home-made (ultrabright red LED / 5528 LDR) vactrols. Then it struck me that the signal going into the output buffers (U2A, U2B) is via a potential divider -
the vactrols’ resistances which feed into SUML and SUMR and then either the reactance of C5 or the resistance of R11. So it should be possible to increase the time for the signal to fade out by either increasing the value of R11 or decreasing the value of C5. This is indeed the case. With my vactrols, using 47k instead of 10k for R11 (for Vactrol Mix output) and 10nF instead of 220nF gives a nice decay of about 1 second and the LPG effect is very clear when used in that mode - a great result! (I should note that the schematic shows one channel built as an LPG and the other as a Vactrol Mix to illustrate the components in question - I wouldn’t recommend building it like this).
So if anyone is using home-made vactrols in either a Vactrol Mix or a Worng TM-LPG-X type device, do try different values for these components - they have a profound effect on the output. Anyone building in a larger format (I’m Eurorack) could add a pot to adjust the decay time for Vactrol Mix output and a switch to have a choice of capacitors for LPG output. Happy experimenting!