[sdiy] VCA response - continously variable between linear andexponential

David G Dixon dixon at mail.ubc.ca
Sat Mar 2 05:21:10 CET 2013

> i'm looking for some advice/pointers on how to get a VCA 
> response curve to be continuously variable between linear and 
> exponential. i can do switchable between the two, but is 
> there any trick to getting everything inbetween?
> if it helps, my VCA is lm13700 based but i don't mind 
> starting with something else.

I don't know how to do it with an OTA, but it's pretty easy with a 2164 VCA,
if you start with Mike Irwin's circuit from EDN -- the last article in this
PDF: http://www.edn.com/file/18017-72502di.pdf

The thing to do is to add a separate CV inverter which feeds the amplifying
VCA's VC pin directly for the exponential response.  The CV output from this
and the CV output from the linearizing servo can be connected to the poles
of an SPDT switch, with the centre terminal going to the VC pin of the
amplifying VCA, or the switch can be replaced with a pot, with the wiper
going to the amplifying VCA's VC pin.

I'll send you the schematic (sorry, I have no website).

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