VCA - does this work?

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

  1. MachineHum

    Thread Starter Member

    Nov 3, 2014
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    Source: http://www.next.gr/audio/amplifiers/voltage-controlled-variable-gain-amplifier-ii-l12486.html

    upload_2015-6-30_12-20-36.png

    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
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @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
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    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

    Distinguished Member

    Aug 1, 2013
    4,542
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    THAT Corp. makes some excellent VCAs for precision audio work.

    ak
     
  5. ian field

    Distinguished Member

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