VCA - does this work?

Discussion in 'General Electronics Chat' started by MachineHum, Jun 30, 2015.

  1. MachineHum

    Thread Starter Member

    Nov 3, 2014


    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.

  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Yes, that is from Randy Sloan's book, Understanding Electricity and Electronics (TAB Publishers). He mentions a use in Wah pedal for guitars - p335
  3. ian field

    AAC Fanatic!

    Oct 27, 2012
    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.
  4. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    THAT Corp. makes some excellent VCAs for precision audio work.

  5. ian field

    AAC Fanatic!

    Oct 27, 2012
    They do a lot of op-amps with variable transconductance - some you can vary both the output and input bias.