The Sound Semiconductor SSI2164 quad vca chip was selected to be used as the mute VCA. Typical Application from the datasheet looks like this:
In essence, we convert the incoming voltage to a current through the 220 Ohm / 1200pF RC Network (the VCA is current based), we control this via Vc1, which is a -33mV/dB exponential voltage input which is capable of +20 to -100dB gain with -660mV to 3.3V.
Since we are utilizing this as a +0dB to -100dB VCA we can avoid the need for negative voltage and only require the 0V to 3.3V input, which due to the IC's nominal impedance of 10kOhm with an internal 10:1 divider we can run directly from the Teensy's digital output pins (0-3.3V).
Coming out of the VCA, we put it through a 0dB buffer stage with the TL072 and out the jacks.
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