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JordanAceto / VCF_plug_in_boards

Various small synthesizer Voltage Controlled Filter pcbs
Creative Commons Attribution 4.0 International
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the feedback VCAs are a bit noisy #4

Open JordanAceto opened 3 years ago

JordanAceto commented 3 years ago

In many of the plug-in boards, I use a VCA made of transistors and an opamp. This works pretty well, but they are pretty noisy if the signal levels are small. I've been testing these with an electric guitar, and there is noticeable noise when the feedback is cranked up in most of the boards that use the feedback VCA described above. When using instruments with large signal levels such as modular synthesizers, the noise is pretty far below the signal so it isn't offensive.

Consider improving the feedback VCA design. The basic existing plan could probably be improved, or you could consider an LM13700 or SSI216x VCA designs.

IMO a linearized SSI2162 VCA might be the way to go. It probably the most expensive route, but yolo. You could also use a linearized SSI2164 VCA and parallel sections to improve SNR, if you don't want to stock both 2164 and 2162 chips. SSI2162 will save a little bit of space if the layout is tight.

JordanAceto commented 2 years ago

The offenders are:

These are the boards that would benefit from a better feedback VCA, the rest of them are fine as-is.

JordanAceto commented 2 years ago

I updated the minimoog and late-MS20 filters with SSI2162 feedback VCAs, see these for reference if updating boards. I also did this for the optical phasor, but I have not done a test build yet, so it remains unverified.