[sdiy] Simple VCA?

[Ingo Debus]

Am 12.04.2008 um 19:18 schrieb Roy J. Tellason:
> 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.
>
> The caption for this reads "VOLTAGE-CONTROLLED GAIN -- 2N5457 FET  
> acts as
> voltage-vaiable resistor between differential input terminals of  
> opamp.
> Resistance variation is linear with voltage over several decades of
> resistance, to give excellent electronic gain control. Values of  
> resistors
> depend on opamp used."  --"FET Databook", National Semiconductor,  
> 1977, p.
> 6-26 -- 6-36.

I just don't understand how this would work. When the feedback loop  
is working correctly there's virtually zero voltage between the two  
inputs of the opamp. There would be no current through a resistor,  
variable or not, between the two inputs, so this resistor doesn't do  
anything.
What am I missing?

Ingo