[sdiy] SSM2164 to use as ring modulator...

[David G Dixon]

The standard way to linearize 2164 VCAs was published by Mike Irwin in EDN
in 2004.  The article is available on the internet.  As I mentioned above, a
2164-based ringmod (which is essentially a linear crossfader processing a
signal and its inverse) requires all four VCAs on the 2164 chip.  The
balance is quite good.  You can even adjust the balance between 2-quadrant,
4-quadrant, and negative 2-quadrant.  This is what I did on the uMod.

> I don't know how would actually sound the result of X*exp(Y) 
> but you may definitely try.
> For linear gain response of 2164, I'd recommend expo/lin 
> converter using one of 2164 cells in feedback path of an 
> opamp, so as a result you get voltage that when connected to 
> another 216 cell control input, would control its gain 
> linearly with the main input voltage.
> You need 2 of such converters, because one will control 
> positive control voltages, and the other negative voltages. 
> And they both go to separate VCA cells, which outputs are 
> simply tied together in summing amp. And one of those cells 
> is supplied with reversed "X" signal.
> So in fact you get 1 ballanced modulator (who knows how well 
> or bad ballanced anyway) made of one 2164. Not a big bonus 
> although I think 633 quite expensive anyhow, when compared to 2164.
> 
> Why do you need separate VCA for positive and negative? One 
> input, let's call it X, that is treated as attenuated input 
> in typical 2164 aplication, can go + or - voltages and that's 
> how we leave it. But the other signal to multiply, let's call 
> it Y, carries only information of the VCA gain, but not 
> polarity. It vill never reverse the X signal, so we need 2 
> amps. One is carrying normal X signal, and the other one - 
> reversed. When one VCA is working at + end of Y input, the 
> other VCA is attenuated to -120dB, and vie versa.
> I know a schematic would be recommended here, but I don't 
> really have time for this today, sorry.
> 
> Roman
> 
> W dniu 2014-02-13 21:27, Jean-Pierre Desrochers pisze:
> > Would it be a good idea to use the venerable SSM2164 for a ring 
> > modulator project (dual, quad)?
> > The modulation could be log (by default) or lin with some 
> tweakings..
> > I could also use the AD633 but I'm just wondering.
> > I already have plenty of 2164 chips so..
> >
> > JP
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