[sdiy] very simple VCA circuit

[David G. Dixon]

> Using currents is very useful. You run out of voltage in a real hurry

You know, I use to curse the fact that 2164 must feed into a virtual ground.
Now, however, I've embraced it.  A lot of my circuits (including this new
VCO) now have currents at virtual ground going all over the place which are
only converted back to voltages at the very end.  There's very little error
with current, since electrons are perfectly conserved and leakage currents
are typically much less significant than voltage offsets.