[sdiy] Simple VCA?

[Ingo Debus]

Am 03.05.2008 um 00:49 schrieb ASSI:

>
> #                                 FB ?
> #
> #                     +---------/\/\/\/---+
> #             510R    |                   |
> #                     |     |\            |
> #  +IN/2 o--/\/\/\/---*-----|-\           |
> #                     |     |  \          |
> #                     |     |   \         |
> #                   |-+     |    \        |    OUT
> #                   |       |     >-------*----->
> #  -CV   o--------->|-+     |    /
> #                     |     |   /
> #                     |     |  /	
> #  -IN/2 o--/\/\/\/---*-----|+/	
> #                           |/  	
> #             510R
>
> It can now be seen that the (-) node must not be a virtual ground  
> as it
> were in an inverting amplifier.  Let's for the moment ignore FB and
> pretend we have a bog standard differential amplifier.  The voltage
> developing across the differential inputs is indeed proportional to  
> the
> negative CV - just insert the expression for the R_DS into the voltage
> divider equation.  It can indeed be controlled over several decades,
> three or four should be quite possible depending on the choice of  
> V_DS,
> the lower the better.  By splitting the divider resistors this circuit
> also linearizes the JFET response by coupling V_DS/2 onto the gate if
> you reference the CV to GND.  The opamp should have zero offset and
> very low input current for this circuit to work well.
>

But without the feedback resistor the opamp would be operated without  
any feedback at all. Are there any opamps out there (I mean opamps,  
not integrated differential amps with well-defined gain) that would  
work well with audio in this situation? I think offset and input  
current aren't the only problems here.

I still think the circuit that was posted by Roy here a while ago is  
based on a drawing error. Look at NatSemi's AN-32 (Aaron posted a  
link). There's a "Variable Attenuator", but with the FET in series  
with the input resistor. The circuit description uses almost the same  
words as the description Roy quoted. Perhaps this is the corrected  
version?

Ingo