[sdiy] Looking for blend circuit?

[David G Dixon]

> Everyone,
> 
> I'm looking for a blend circuit that at center position there 
> is no output, turn it left from center and you get just input 
> A then turn it right from center you get just input B. Using 
> VCAs I can do this but that would make what I'm doing to big. 
> I need to keep it simple/minimal using op-amps, pots and diodes.
> 
> Any suggestions?

Set up a potentiometer between the +15V and -15V rails, and send the wiper
voltage to a buffer, and an inverter.  Use the outputs of the buffer and
inverter to control two 
linearized 2164 VCA circuits (see here:
http://www.edn.com/design/analog/4336650/Op-amp-linearizes-attenuator-contro
l-response).  Feed A into one VCA and B into the other.  This circuit will
do what you want.

In this circuit, the VCAs only operate with a positive CV.  A negative CV
renders them inactive.  Hence, only one VCA will be active no matter what
the pot setting.  At fully CCW, A will be amplified at unity gain, at fully
CW, B will be amplified at unity gain, and in the centre, both will be off.

Parts count: 1 pot, 1 V2164 chip (Small Bear Electronics), six opamps,
various resistors and caps.