[sdiy] SSM2164 VCAs

[Tim Parkhurst]

On Mon, Jun 8, 2009 at 5:50 PM, David G. Dixon<dixon at interchange.ubc.ca> wrote:
> (You probably saw this one coming based on my last post...)
>
> I've been looking at the datasheet for the SSM2164 Low-Cost Quad VCA (which
> I just bought a pile of in the form of CoolAudio V2164D clones), and I'm
> thinking about building a -- (wait for it) -- quad VCA!

Hi Dave,

The 2164 is a pretty nifty chip - quiet with a huge control range, and
you get four in a single package. They're also pretty cheap (I have
them at Magic Smoke for $3.50 in single quantities, cheaper if you buy
three or more).

Yes, the control voltage response is "backwards." However, this was
done on purpose, as they assumed that in the vast majority of cases,
users would want to use a CV summing circuit. Having an inverted
response saves an op-amp. One note: even if you don't need a CV
summer, I'd still drive the 2164 from an op-amp as the CV input has a
fairly low impedance (<5k) and you don't want it loading down your
source. The data sheet is a little vague as to what the control range
is, but I'd say you probably won't need more than 10V. Your input
summer circuit sounds about right, and yes, negative CVs will give
gain rather than attenuation. With a carefully designed CV summer, you
can get a full 120dB range (which is more than a lot of the downstream
amplification and recording gear can handle). You'll probably be very
happy with a 100dB range, which means you can stick a diode in your CV
summer circuit to block negative CVs from going to the 2164. Also,
inverting the CV doesn't mess with the exponential response - the chip
is actually made to work that way.

Are you sure you want expo response VCAs? Most synth VCAs are linear
response, and then they get driven by an exponential RC curve from an
envelope gen. Having expo response VCAs means you won't get a
"typical" loudness curve unless you drive the VCAs with linear
envelopes (which makes life a lot easier if you're making a digital
envelope gen). I would also think an expo VCA would produce a more
natural sounding tremolo if driven by a triangle wave LFO rather than
a sine wave. There is a way to get the 2164 to produce a linear
response, but it involves using one section for the CV that drives
another section for the audio. This means that you'll only get two
linear VCAs from a 2164 chip, but that's still pretty good for the
money. This is also the chip I use in the Mankato, and it performs
very well there, so I'd say you'll be pretty happy with the little
buggers.


Tim (Mister Little Bugger) Servo
-- 
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings