Adding gain control to a circuit.

Discussion in 'The Projects Forum' started by Fenris, Jul 2, 2009.

  1. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Hi All

    I built this voice modulator some time ago. It is built onto a PCB which I etched myself. Now because of the way the unit is used feedback can be an issue. Careful placement of speakers in relation to the mic's position is used as well as any acoustic padding to help reduce feedback as well. Also 'baffling' the rear of the mic helps and keeping the volume down is of course the other part of it.

    Now as I learn little bits here and there I have seen mentioned that the gain can be adjusted by the use of a trimpot fitted into the the pre-amp of the circuit. The idea I have is to help further reduce pickup of the audio output which can cause the feedback and thereby maybe increase the usable range of my volume control.

    So Having googled a bit and looked at data sheets I think I have Identified where a trimpot would go to facilitate the adjustment of the gain. I presume I would need to change the value of R4 downwards and the trimpot should exceed the current value of R4.

    If I have got it right (added trimpot in series with R4) does a higher resistance increase or decrease the gain of the circuit? I attach a schematic of the circuit. I also understand I may have got it totally wrong and stand ready to be lined up against the wall and shot. :/

    regards

    Fenris
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The junction of R3 and R4 is supposed to be half the supply voltage to bias the opamps so that their inputs and outputs are at half the supply voltage for a wide signal swing.
    Therefore you do not want to add a variable resistor in series with R4 to ground.

    The gain of the first opamp is the ratio of R7 and R6 and is (470k/33k) + 1= 15.2.
    To reduce the gain then increase the value of R6.
     
  3. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Hi AudioGuru

    Well I missed it by miles then :( Thanks for pointing me in the right direction.

    regards

    Fenris
     
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