[sdiy] Simple VCA?

[Batz Goodfortune]

Y-ellow Roy 'n' all.

At 01:18 PM 4/12/2008 -0400, Roy J. Tellason wrote:

>It's a very simple circuit,  really.  An op amp inverting amplifier,  one
>resistor going from the non-inverting input to ground,  one input resistor,
>and one feedback resistor.  And,  a JFET connected between the two input
>terminals with the gate lead labeled as being the gain input.

I can't remember where I saw something similar (If I understood the 
description correctly) recently. I've seen similar before though with a pot 
or trim pot instead of a FET.

The problem here as I understand it is that it relies on the common mode of 
the Op-Amp. Which means the attenuation can never be 100%. But assuming it 
could be trimmed and/or matched, might be useful for a VCF perhaps. Or a 
compander. The thing to note here is that it is literally "GAIN" of the 
op-amp you are messing with. When the signal on both inputs are equal, in 
theory they cancel out. But for a number of reasons this will never be 
perfect so expect some leakage.

As an added advantage though, it is possible to modify this kind of scheme 
to provide variable gain in both inverting and non inverting directions. I 
can't remember the exact circuit off the top of my head but I'd seen this 
done with a pot. Such that in the centre the gain was a theoretical zero. 
Turn the pot one way and the gain increases inverted and visa versa. One 
wonders whether the same kind of scheme could be used to make a 4 quad 
multiplier on the sly?

If I can remember where I saw the circuit, I'll post it.

Hope this helps.
Be absolutely Icebox.

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