[sdiy] Simple VCA?

[Magnus Danielson]

From: "Roy J. Tellason" <rtellason at verizon.net>
Subject: Re: [sdiy] Simple VCA?
Date: Sun, 13 Apr 2008 16:58:28 -0400
Message-ID: <200804131658.28434.rtellason at verizon.net>

> On Sunday 13 April 2008 13:11, ASSI wrote:
> > On Samstag 12 April 2008, 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.
> >
> > What you have there is a "condensed" version of an adder/subtractor
> > circuit.  When the JFET is off, you get just the normal inverting
> > amplifier, when it is conducting, you mix the inverted with some
> > portion of the non-inverted signal.
> 
> There wasn't any non-inverted signal,  just that resistor going to ground...
> 
> > In this particular configuration the input resistor for the non-inverting
> > input is shared with the non-inverting input.  Besides saving a resistor it
> > keeps the voltage across the JFET near zero, which presumably helps getting
> > a control law that is independent from the input signal (you'd suffer from
> > that if the JFET was in the feedback path).
> 
> I'm still unclear as to just how this would work.  What control voltage would 
> you be applying to the gate of that JFET for zero output?  For maximum?  What 
> sort of a characteristic would varying this control voltage have?

You guys should check out the EMS Synthi-A schematics. There is a bunch of
these in there... including the Filter Q feedback system. :)

Cheers,
Magnus