VacTrol VCA/VCF

vactrol_vca_03_jpI was about to make a clone of the buchla low pass gate, but cloning isn’t very interesting. Too much boredom and not enough grey matter.  I’ve some 60’s components in stock stock to a PCB from a Silversound wha/crap/fuzz. That thing was horrible in all aspect. I decided to salvage the components and build something out of them. I guess Mother Earth would be greatful for that. The idea is to keep the buchla spirit, but not a clone, and 60’s components oriented.

It’s nothing but a 2 pole low pass gate using a sallen key topology. Remove the caps form the circuit and you get a VCA. I don’t really want to bother myself with DPDT switch and I haven’t space for it on the 4cm large panel anyway, so a CD4066 will be nice. The plan is to build two of them (if the pannel allows it and I find another suitable vactrol). Also, it remains two switches from the CD4066. 60’s oriented, maybe a germanium distortion could be a nice addition.

There’s a input called “pluck” which generates a very short pulse to strick the led into the vactrol, create a pluck effect. The magic being that the photoresistor take a large amount of time to discharge. Scaling the voltage for the led is quite tricky if you want a proper nice amplitude of the potentiometer (hence the offset at IC1B output), if one doesn’t care, let it as it.

There’s an issue which is that output signal is 1.2V less than than the input because of the two Base-Emetter junction. I don’t really care although it also gives a bit of offset (that can be avoided in AC coupling the circuit).

I find myself more interested in those alternative sounds rather than the usual substractive synthesis which can be boring at times. And I guess those kind of VCF can be interesting when used with gritty digital VCO as well.

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3 Comments

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3 responses to “VacTrol VCA/VCF

  1. Andrew

    As you mentioned Germanium fuzz, I thought you might find this link usefull, if you have not already seen it….Hope it is of some use.

  2. Gordon

    Vactrols are great fun for building synth / sound modification type circuits. I built a similar 2 pole vcf, but with opamps instead of transistors. The slow speed of the LDR’s in the vactrols creates distinctive dynamic effects.

    • I also really like the sound of the optocoupler. The “ringing” when attacked with a pulse is very nice. I haven’t built the circuit yet, because I’ll actually use op amps and a resonnance control and get rid of the drawbacks of the transistors.

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