Minus the coupling capacitors, both seem to follow the same one transistor plus diode approach. And the second has biasing for the base, which seems a good idea anyway?
What puzzles me though: The first feeds CV to the base, the second to the collector.
From the little I know about transistors, base current controls collector-emitter current so the first approach sounds reasonable since here CV controls the signal. In the other one, the signal controls CV? Is there a reason for doing this?
This is a “swing type VCA” that effectively modulates the envelope (when positive) by the signal. But it’s taken from the internals of a drum machine (808?), which is a controlled environment where the signal levels are well-known, so not sure how generally useful this approach is…
Well I breadboarded both solutions. In this case, e.g. in lack of a real voltage controlled amplifier I actually liked the second one, i.e. the “swing type”, because using it I can use a high CV to actually make the output signal louder than the oscillator on the input. It’s a very hacky approach, though, but a good thing to keep in mind, I guess.
