LPG/VCA circuit

Wasn’t there a point in the GBMM build process where you (Sam) were looking at VCA options and one of them ended up being essentially a transistor? For some reason I’m remembering you saying it was more of a low-pass gate. Does that ring a bell? I’d love to see a schematic for that.

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Hey! I’m guessing you mean this one?


yep thats the VCA I use in the fart box and the synth bike, its good for audio. however just using any npn should work to an extent!. even ditch the diode haha


I’ve been trying this circuit with all the transistors I have but the best I can get is a constant sound (bleed?) with a boost in level when voltage is applied from envelope generator.

Should it be near-silent when no voltage on CV?

If I can check the pins here: CV->DIODE->Base, IN->EMITTER, OUT->COLLECTOR?


It looks like you have the pins right.

Does your input go negative?

I think this circuit is meant to handle only positive going inputs for both the CV and signal.
I won’t pretend to understand how this “VCA” works, but as soon as the input is lower than the CV, the transistor will start conducting. the larger the difference the more current will pass.

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That’s a good point-it’s not really amplifying anything, is it @antoine.pasde2? It’s more like a voltage-level gate. But don’t get me wrong-it’s effective at what it does. I have all those vactrols sitting on the bench for a low-pass gate array, but I’m toying with the idea of a toggle on each line to switch between those and a “ghetto” VCAs like this or a butted diode pair.

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OK that’s interesting. I really am quite ignorant of what was happening.
So maybe if I can add offset the input dc then take if off after, it would work better? Or put a diode on the input too, which would clip but give less bleed? Thanks for your help!

I tried this VCA and had horrible results, but I expected that. I think the reason is that the diode doesn’t actually limit the current to the base enough. Then I found this:


Even without the 10uF cap at the base, it still performs way better than the circuit above. Don’t get me wrong, you still get what you’d expect from such a simple passive circuit, but worth considering if you need something quick and dirty.