[sdiy] very simple VCA circuit

[David G. Dixon]

> > I noticed long ago that the difference between a "happy hacker" (such as
> myself)
> > and a "real EE" (you know who you are!) is that the professionals always
> think in
> > terms of current. Things may have been different in the days of tubes -
> but I doubt it.
> 
> 
> I always tell my students that op amp-type circuits often "represent"
> signals in terms of voltages, but they "compute" by manipulating currents.
> And, yes, I say that with a lot of hand-waving. ;)


When I finally bit the bullet and set about to understand filters, I did so
by deriving the transfer functions for various filter topologies.  This I
achieved by writing current balance equations at the important nodes (which
are the nodes where the OTA meets the cap-and-buffer, or where the VCA meets
the integrator input).  These become voltage equations (i.e., transfer
functions) when the constitutive equations are plugged in.  For the VCFs I
was interested in, there were only two constitutive equations:

Resistor equation (Ohm's law):
I = V/R

Capacitor equation:
I = sCV

With this analysis, I was able to confirm that the transfer functions for my
2164 Korgasmatron VCF and the late-model Korg MS-20 are identical, and I
finally understood the exact role that the OTAs and VCAs play in these
circuits.  Before I did this, I was just groping in the dark.