"That" compressor

From the vendor

"That" compressor

Jun 30, 2023 10 min read

Updated: Aug 1, 2024

I had wanted to make a compressor circuit for a long time, because it's a popular and somewhat easy to make effect, but the existing DIY offer left something to be desired. You should be ready to accept some compromise, in the form of: noisy and hard to find OTA chips; expensive and hard to find (in THT at least) VCA chips; hard to find (again in THT) and fiddly JFETs; circuits that underperform in some way or another, or; circuits that aren't well translated to instrument input and stompbox format, in the form unbuffered gain controls and bipolar power supplies. I'm not claiming that making a good VCA is easy, and the VCA chips of THAT corporation deserve some credit anyway, for their incredible performance and how easy they make a precise gain control. I own a compressor using one of such chips, and while I can't find any flaws in it, I still think a more accessible DIY alternative should be possible, without any major compromise.

I was then confronted with a choice between LDR optocouplers and JFETs as control elements: the former offer very good distortion performance, and I've used them successfully in the VCA-1 and the Trembling Satellite; I still wanted to give the latter a chance, because of their intrinsically higher speed (attack time can always be made slower with the side-chain, but there's an intrinsic lower limit) and the better defined voltage-resistance relationship. I thought they were still a decent option, if the circuit wasn't picky about specific part numbers, given also the possibility of using SOT-23 devices.

I've actually made and tested a JFET circuit before this one, but I was forced to face a distortion problem that's hard to overcome: I thought instrument levels were small enough to be fine, but the peaks (otherwise, what's a compressor for) aren't! There are ways around it, but an upper limit on amplitude handling without distortion is given by Vp, the pinch-off voltage. You can't use a part with a very negative Vp on a 9 V single supply or you won't be able to use much of its resistance range. Moreover, the necessary local negative feedback (usually) halves the control voltage that reaches the gate, making things even worse! This is how, after a month, I ended up with the following circuit, using a trusty homemade LDR-LED optocoupler, with the LDR is still available at low prices despite its small Cadmium content, a situation I hope doesn't change.

That's not to say my troubles instantly ended, having made this choice: they started even earlier, due to the general lack of useful technical information about compressor theory, and they persisted, while I was looking for a circuit that gave me the results I wanted. THAT corporation has by far the best collection of resources, but while it's all useful on a general level, most of it is best applied to their own chips. You start getting into this and the world seems in your hands, but then you wonder h