[sdiy] SSM2164 VCAs

[David G. Dixon]

Thanks Dave, Aaron and Tim for their excellent advice and links.  It would
appear that the Irwin-Gallo-Lamm-Bareille axis is all I need to go forth and
amplify!  It's interesting that the Irwin method of linearizing the 2164
appears to be very similar to the Sims method for linearizing the 13700....

To answer Tim's concern, I was going to put in switches to have the option
of linear or log response, for the very reasons you alluded to (analog
envelopes should feed linear VCAs, and triangles (such as my lovely triple
LFO) should feed log VCAs).

Now I've got to go and study (and possibly simulate) both Sims' and Irwin's
schemes to convince myself that I really understand what the heck is going
on there, and to determine whether it's strictly necessary for VCA
applications and therefore justifies doubling the IC requirements.

Cheers,
Dave

> -----Original Message-----
> From: Tim Parkhurst [mailto:tim.parkhurst at gmail.com]
> Sent: Monday, June 08, 2009 6:27 PM
> To: David G. Dixon
> Cc: Synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] SSM2164 VCAs
> 
> 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