[sdiy] THAT 2181C VCA's

[WeAreAs1 at aol.com]

In a message dated 11/3/03 8:06:44 PM, snd_audio at yahoo.com writes:

<< Hello,
 Wondering if anyone here has worked with the THAT
2181 VCA's. I have 200+ of these and would like to use
a couple of them for VCF's and VCO's as well as VCA's.
Any idea's, suggestions...? >>

Hi Steve,

Isn't the 2181 an exponential-only control response?  That makes it very easy 
to use it in audio apps such as compressors and volume control, but makes it 
a little complicated for use in filters, VCO's, or for attenuating control 
signals such as envelopes and LFO's.  Yes, it's true that we want exponential 
response on our VCF's and VCA's, but we need to have a lot of control over the 
design parameters of the exponential amp, and if I'm not mistaken, the 2181 
doesn't really give you much access to the expo amp section.  If you want to use 
it a VCA for attenuating (DC) control signals, it's almost always better to 
have a linear response VCA for that purpose.  There might be some way to 
"linearize" the control input on those chips (by building some kind of external 
anti-log amp), but that also seems like a lot of trouble, and I'm not sure you could 
ever really get the linearity to exactly track.

Maybe you could build a nice analog mixer fader automation system with all 
those chips!

Michael Bacich