Single-Vactrol Sallen-Key Lowpass Wah Filter

[Fancy Lime]

Hi everyone,

I fiddled a bit with a Sallen-Key lowpass design, trying to make a voltage controlled lowpass with a resonant peak with just one control element, namely a NSL-32 vactrol (optoisolator). It's a bit more sensitive to the properties of the Vactrol than I would like it to be but the NSL-32's I have are surprisingly consistent going from >10M dark to pretty exactly 1k light. So I managed to design a filter with a good range and resonant peak. The Q changes with the filter frequency if one resistor is voltage controlled and the other is static. I managed to make it so that Q goes from ~3 at 80Hz filter cutoff to ~20 at 600Hz to ~8 at 4000Hz.

Have any of you guys ever build something like this? Has this been done commercially? Seems like an easy enough concept that might sound nice (have not build it yet) but I've never seen it anywhere. The characteristic looks not too different from the good old inductor wah.

Cheers,
Andy

[Fancy Lime]

To illustrate, here is a little minimum working example of what I had in mind:



Q peaks at 21.7 when both resistors (meaning the equivalent resistors, one made up by the 4M7 and LDR in parallel, the other by the 15k and Range pot in series) are equal. Range can set this point between 376 Hz and 979 Hz, while also shifting the total range of the filter up or down. Come on guys, someone must have done this before, no?

Andy

p.s. these filters are a bit touch to minor value changes, so low tolerance parts are probably a good idea. I would only use film caps or film for the large and silvered mica for the small one. At least no ceramics or tantals.

[rankot]

This looks quite interesting, you can even combine low/high pass filters with two vactrols.