featherpillow
Well-known member
Okay, so over the weekend, I had the chance to do some prototyping of a simple VCA compressor. I wanted to use parts I had laying around, and keep the cost low. I'm certainly not an expert in circuit design, but I do know a bit about VCA compression, so I thought I'd give it a shot. Plus, THAT has very good datasheets.
It's nothing fancy, it's a feedback design (I like the way these sound in comparison to feedforward), and it uses a 22M resistor tied between pins 1 and 7 of the 2252 to compensate for errors. In terms of control, it only has Ratio (1:1-1:infinity) and Makeup Gain (-20dB to 20dB), although it's easily expanded. I used an OP275 in the CV signal chain, since they're low noise/low distortion, although one could easily sub a 5532 or TL072.
It uses a 2N3819 for an input buffer, and, instead of using the traditional "opamp as a trans-impedance stage" for the VCA, it uses a FET/PNP Sziklai follower. It's designed with assuming an Idss of roughly 5ma, since I have tons of 3819's with roughly that Idss.
Overall, I think it's a pretty nice and simple comp, it's a soft-knee design (see D2), but it's easily changed to something faster if needed. It doesn't distort or pump at all, due to the values used for timing and bias on the 2252. It probably won't out-do an SSL clone.
I was able to build it on a proto-board. It's designed for use with a +/-15vdc supply because I have plenty of those, although it's easily adapted for use with +/-24vdc supplies.
Anyway, if anyone else is interested, the schematic is here
It's nothing fancy, it's a feedback design (I like the way these sound in comparison to feedforward), and it uses a 22M resistor tied between pins 1 and 7 of the 2252 to compensate for errors. In terms of control, it only has Ratio (1:1-1:infinity) and Makeup Gain (-20dB to 20dB), although it's easily expanded. I used an OP275 in the CV signal chain, since they're low noise/low distortion, although one could easily sub a 5532 or TL072.
It uses a 2N3819 for an input buffer, and, instead of using the traditional "opamp as a trans-impedance stage" for the VCA, it uses a FET/PNP Sziklai follower. It's designed with assuming an Idss of roughly 5ma, since I have tons of 3819's with roughly that Idss.
Overall, I think it's a pretty nice and simple comp, it's a soft-knee design (see D2), but it's easily changed to something faster if needed. It doesn't distort or pump at all, due to the values used for timing and bias on the 2252. It probably won't out-do an SSL clone.
I was able to build it on a proto-board. It's designed for use with a +/-15vdc supply because I have plenty of those, although it's easily adapted for use with +/-24vdc supplies.
Anyway, if anyone else is interested, the schematic is here