[sdiy] VCA from BA6110 - should be simple right?

[Robert Lorentz]

I've got a load of BA6110 chips and I'm trying to make a bunch of
(simple) VCA's based from these. We're talking synth use here, so
let's not wander to the realm of hi-fi (or I wouldn't be using these
chips anyhow).

Everything here relates to this schematic from the app notes:
http://lucidsoft.org/~rcl/vca.png

There's not a whole lot to the circuit except for an Offset Adjustment
and a variable voltage.

#1 Vr (Offset Voltage): This looks like it will bias the signal above
or below 0v. Should this just be a trimpot and I calibrate it to keep
dc bias out of my amplified signal? Or does it need to be a pot to
change bias of different signals?

#2 Variable Voltage between -15v, going to Pin 7 as Icontrol through a
30k resistor. To me it looks like I would hook up a *0 to 15v* voltage
source ground to the BA6110's -15, and the hot of the voltage source
to the 30k? OR, should I be feeding in a *0 to -15v* inverted CV
there?

My source for the VCA CV is a 555 based ADSR that is not inverted. If
one needs an inverted cv (I see this mentioned a lot), how do you do
it? I know how to set up an opamp as an inverting amplifier, but the
minimum gain looks like it is 1 + [resistor ratio, 1 at lowest]... so
there must be another way to do this without changing (maybe clipping)
the signal...


Thanks for any advice, I could just hack around on a breadboard until
I find something that works but I want to get the real explanation so
that I understand it.  I have been chewing on this for a bit and just
can't wrap my head around it.

Robert