VCA Vactrol choice effect on envelope's attack time

Presumably. But for one LED or maybe two you probably wouldn’t need the transistor, the LM324 can source 20 mA min, 40 mA typical.

None I can think of. It’s often good practice to have a 100k pulldown resistor on module outputs anyway. (For one thing, if a module is unpowered, then its op amp outputs are probably floating and can cause trouble for a powered module they’re plugged into.) Though I rather rarely see it done. And a pulldown after a series diode is definitely called for in many cases. I think this is one. You might want to add a note suggesting it to Annotated Simple DIY Envelope Generator.

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