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

[Roy J. Tellason]

On Monday 22 September 2008 22:21, Robert Lorentz wrote:
> 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

Interesting circuit...  How did you happen to end up with a load of those 
chips?

> 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?

I'd use a trimmer there.  Lots of op amp stuff shows similar,  and you may be 
able to omit it if your output signal ends up being ac coupled at some point.

> #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?

Lookinng at the datasheet,  which you can find here:

http://www.classiccmp.org/rtellason/mfr-b.html

it looks like when it's sitting at -15V there's no bias on a bunch of 
transistors,  so bringing it toward 0V would tend to increase the gain,  it 
looks like.

> My source for the VCA CV is a 555 based ADSR that is not inverted.

So tie the "ground" for the 555 to the -15V line,  and the "Vcc" to ground.

> 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.

I wouldn't mind getting a hold of one or some small number of these to play 
with,  but the datasheet I have seems to indicate that it's only available in 
surface-mount packaging,  and I'm just not ready to go there yet.  Are the 
ones you have through-hole parts?


-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin