Volume Control - Gain vs Potential Divider

Discussion in 'General Electronics Chat' started by dannybeckett, Feb 23, 2015.

  1. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
    163
    0
    Hi guys,

    I had a (admittedly very) quick search and couldn't find any info on a question I've been wondering to myself the past few days. From a signal noise perspective, is it better to vary the gain of an input amplifier to alter the volume (by having a pot form R1 and R2 in a non-inverting op-amp circuit) , or is it best to have a fixed maximum gain amplifier and have a pot form a simple potential divider at its input? I'm thinking the variable gain option is better, due to the fact you won't be amplifying noise at the same rate at lower volumes.

    Cheers,

    Dan
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the normal way is to use the volume to vary the input if you try to vary the gain by varying the bias on an op amp, you rwill get very large noise spikes as the pot is varied.
     
  3. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    You must be talking about the noise of the input stage of the amplifier.
    Most designs use the separate volume control but I can see your point. If the amplifier is always at a closed loop gain of 100, it's always amplifying its own noise by 100. If you change the gain with a pot, the closed loop gain can be low for a high amplitude signal, but a high amplitude signal already has a high signal to noise ratio so you are getting the advantage when you need it the least.

    I was just about to get to pot noise when alfacliff posted. Pots tend to go open when they fail so I would use the pot in the ground position and wire it as a rheostat. If the whole pot goes open, the gain drops to 1. If the wiper goes open, the gain drops to the minimum design gain of 1+ Rf/Rc where the Rc is that extra resistor in the second drawing.
     
    JohnInTX likes this.
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    and any dirty or worn spots on the volume control pot used to control bias and gain on an opamp will make great changes in output dc level. instead of just a crackle as it goes over, it will go rail to rail output.
     
  5. dannybeckett

    Thread Starter Active Member

    Dec 9, 2009
    163
    0
    Cheers for the replies guys. Good point alfacliff although I am tempted to use non-volatile digital potentiometers and muting functions to eliminate all the pops and clicks normally associated with transient power-on voltage spikes and dirty potentiometers. And yes, input noise #12. I agree about getting the advantage when you need it least, but it's an advantage nonetheless.
     
Loading...