Source: http://www.next.gr/audio/amplifiers/voltage-controlled-variable-gain-amplifier-ii-l12486.html



Hey guys/gals, I found a schematic for a VCA (Voltage controlled amp) online. It uses a FET to very the gain with V- (Gain control pin) This could be a simple solution for what I'm looking for. I wrote in some of my math off to the right, I'm not sure if it's actually doing what I says it's doing. It seems to me more like an offset then anything, my cct analysis is a little rusty so if anyone want's to chime in that would be cool

"k" would be a constant of the FET, I mostly ignored polarities as I'm concerned with magnitudes.

Thanks

 

@MachineHum
Yes, that is from Randy Sloan's book, Understanding Electricity and Electronics (TAB Publishers). He mentions a use in Wah pedal for guitars - p335

 

Source: http://www.next.gr/audio/amplifiers/voltage-controlled-variable-gain-amplifier-ii-l12486.html

View attachment 87883

Hey guys/gals, I found a schematic for a VCA (Voltage controlled amp) online. It uses a FET to very the gain with V- (Gain control pin) This could be a simple solution for what I'm looking for. I wrote in some of my math off to the right, I'm not sure if it's actually doing what I says it's doing. It seems to me more like an offset then anything, my cct analysis is a little rusty so if anyone want's to chime in that would be cool

"k" would be a constant of the FET, I mostly ignored polarities as I'm concerned with magnitudes.

Thanks

Its an NS appnote that's been floating about for donkey's years - I assume it works as described, but never actually tried it.

The old favourite was the CA3080, but that's long since unobtaneum - I believe the current favourites are the LM13700/13600 types.

One I've had some success with in a 8-DIL package is the A suffix variant of the TDA7052 BTL amplifier which has DC controlled volume control.

Its designed for full bridge drive of a low impedance speaker, but you can use one of the outputs single ended at reduced power - still plenty for small signal use.

 

THAT Corp. makes some excellent VCAs for precision audio work.

ak

 

THAT Corp. makes some excellent VCAs for precision audio work.

ak

They do a lot of op-amps with variable transconductance - some you can vary both the output and input bias.