There’s that saying that you’ll never have enough VCA’s in your modular system. IMHO you never have enough of anything in a modular, but let not digress. VCA’s are useful little buggers that you simply can’t do without. I’ve build a variety of them in the past, mostly incorporated into other more complex modules, but I thought I might as well just make a few standalone ones. I’ll probably post more approaches to the VCA issue in the future.
What’s a VCA: For those yet unclear about the concept, it’s a voltage controlled amplifier. It’s indeed as boring as it sounds, but it’s invaluable in any synth. What it lets you do is use a voltage, like one generated by an ADSR for example, to control the ‘level’ of whatever sound you’re putting through the VCA. No voltage = no sound, the higher the voltage, the louder the sound gets. That’s the basic idea.
This VCA is based on a long-tailed pair differential amplifier. The biggest, and arguably the only, benefit of this design is the availability of components. It uses no rare or hard to get parts, heck it doesn’t even use anything most people won’t have around if you have done a bit of DIY already. Otherwise you could source all parts from any electronic components provider which carries even the bare minimum. This alone makes it a valuable approach to a VCA. Most other approaches to a VCA require some kind of ‘specialised’ component. Being it an OTA, vactrol or dedicated IC’s like the V2164.
I’m not going to into the technical description on how this works, if you’re interested in the math, use google. There are quite a few comprehensive write-ups on how it works online. What you have to know is by using a differential amplifier setup in such a way we can modulate the amplification by controlling the current at the emitter of both transistors. Hu? Effectively making it a controllable amplifier.
The 2 transistors forming the long-tail pair should ideally be matched, but in my experience it’s not a really that critical if you use 2 transistors you bought at the same time. Mostly they’re close enough, but if you have the possibility to use matched ones, why not do it. The CV-reject trimmer can be used to make minor adjustments if you find yourself having a DC offset.
As you can see, it’s a fairly simple design, and you shouldn’t have too much problems getting it going.
The BOM and schematics specify a BC550 as a transistor, but a transistor like the 2N3904 would work equally well.
RV1 is a trimmer used to offset the control voltage in such a way that the VCA attenuates fully when closed. If you encounter some sound still coming through when closed you should trim this until it disappears. If you find that CV does little, trim it as well until you start hearing sound coming through without any CV input and then back off a bit. RV1 is set in the BOM as 10K, but you could use almost any value you want 50K, 100K, 500K will all work equally well.
RV2 can be used to adjust for CV bleed and compensate for transistors which aren’t matched. While the value isn’t all that crucial you should keep it low, 1K or 5K will work just fine.
In order to fit everything on the PCB you should mount the potentiometer and jack sockets to the backside of the PCB.