[sdiy] Vactrols subbed for dual-gang pot

[lanterma at ece.gatech.edu]

On Jul 11, 2010, at 4:46 PM, cheater cheater wrote:

> Yeah, I know all about non-linearities - but you can usually get some
> sort of general idea anyways. I guessed low-pass-ish, but I wonder if
> it's more 12dB/Oct or 24dB/Oct or ... :)

Well, you need to be careful to specify exactly what it is you're talking about modeling. If you're talking about the audio path of a typical VCF, that's one thing, and makes it easy to talk about 12db/Oct or 24db/Oct or whatever. If you're talking about the *control* mechanism of such a beast - say a Buchla LPG - that would be trickier to characterize.

You can, it seems, do tricks to effectively speed up the vactrol response (roughly speaking), if you use a couple of vactrols; the amplitude modulation circuit of the Buchla 259 and the ring mod circuit of the Music Easel both use this trick. I can sort of follow the 259 circuit but the Easel circuit makes my head spin. Grant Richter probably has the best understanding of how the Easel circuit works. It's all based on the sort of datasheet trick that Harry Bissell pointed out a few days ago - the idea is to use one vactrol in a feedback loop do basically "undo" the non-ideal behaviors of another vactrol.

- Aaron