FET based eurorack VCA

I’ve been thinking about what to add to my ISMS lately. After adding the wonderful Sputnik Modular Quad VCF/VCA it got me to thinking about VCA’s that can be dual purpose. I came from the guitar world where overdrive pedals and amps overdriving are really important to crafting one’s ‘sound.’ To my eye it looks like there aren’t a ton of eurorack VCA’s that overdrive, but I was just wondering if there a company or VCA out there that used a FET instead of an opamp?

I’ve been doing some DIY modules lately and I basically had the idea to try and make a VCA with overdrive intended to color the sound. Basically just adapting a simple guitar pedal circuit to see what it sounds like.

Maybe not exactly what you are looking for but there are certainly one or two tube based VCAs kicking around.
Definitely colour the sound.
This is the one I have experience with, albeit not in Euro format, but I think there might be other (probably DIY) ones available.
http://www.cgs.synth.net/modules/cgs65_vca.html

this one? http://www.l-1.su/TubeVCA.html

no FET but another Tube VCA…

also this: http://ladik.ladik.eu/?page_id=62

Vca’s are usually specialized vca chips, not op-amp. It’s the voltage control of the amplification that are the tricky part (current mirror etc). If I recall correctly it’s easier to build the circuit bit FET than bipolar transistors so I guess the majority of the vca chips already uses FET. I could be completely wrong and maybe you mean that you want a discrete VCA circuit based on FET. In that case ignore my statement.

I really like the sound of the saturation circuit in the WMD multimode VCA http://wmdevices.com/collections/eurorack-modules/products/multimode-vca. The same circuit is used in their compressor, I think. Slight aside, the most musical distortion I have found, thus far in eurorack is the Rossum Evolution filter. The way the distortion interacts with the ‘genus’ and the Q gives an almost infinite varieties, and I found myself liking nearly all of them. http://www.rossum-electro.com/products/evolution/

Could you a schematic for a eurorack VCA? Most I saw had opamps, but I’d love to look at some more schematics though!

If you are looking for distortion, have a look at the various cp3 mixer modules our there. There is a manhattan analog pcb that is available, cheap to build and sounds great.

Yeah I’m going to build one of those in my other case. Those sound great! I’m trying to get just a couple more VCA’s in to the ISMS and seeing if I can get dual use out of the VCA similar to how the Sputnik Modular does with the VCF/LPG.

Thanks for the resources! The WMD is almost exactly what I am talking about. I wish I could have a look at the schematic on it though.

I don’t have that many schematics. And exactly zero with discrete vca’ so. If y want to look at schematics, Mutable instruments have all their hardware open sourced. Olivier utilizes V2164 in a couple of modules. VCAs from THAT seems to be a popular choose as well.
My comment was formed out of memory but the Wikipedia page confirms my fuzzy memory from class.

Please note that the amplification part of the vca circuit is still probably is a bipolar transistor, it’s the negative feedback loop that controls the gain that a JFET based. As always it’s the whole circuit (ICs or discrete) that forms the sound more than individual components.

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If you emailed WMD I wouldn’t be surprised if William shared it with you. He seems like a great guy.

Totally agree with that the circuit forms the sound more than the components, but with the case of distortion I know that components used for the gain stages do have certain characteristics when clipping which is what people are going after. I don’t have much knowledge in this area but I’m trying to learn more but I believe this is why FET’s are used in the gain stages of some overdrive pedals.

The JFET for most of the VCA’s I’ve been looking at are part of the VCR and not the gain stages so it’s not quite what I was looking for.