Potentiometer type for op-amp gain

Discussion in 'General Electronics Chat' started by Lambda, Mar 8, 2011.

  1. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    I need some advice on pot selection for gain adjustment of an op-amp. What I need to know is, should I use a linear taper or an audio (log) taper pot?
    For testing purposes on the breadboard, I have put a liner taper trimpot there. (in the final, it will be a panel-mount pot) I looked at the waveform in the oscilloscope, and the amplitude seems to respond linearly. But this is a subject that I don't know too much about.
    The op-amps in question are in an audio mixer, containing two pre-amps and one for the actual mix. Pictured is a simplified schematic of one of the pre-amps, with the resistor in question circled:
    [​IMG]
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You would want linear taper, and if you wish to increase the gain above 2, you will need to swap it with the fixed 10K resistor to ground.
     
  3. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    0
    Thanks. But an audio-taper pot would still be used for the volume control, correct?
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    896
    Why does your opamp circuit have two 2.2k resistors at the input? They do nothing and can be removed.

    You cannot use an "ordinary" logarithmic audio pot for the gain control because its response will be backwards. It should be linear or reverse-log type.

    If you use a linear pot for a volume control and turn it down to half then the volume will be reduced only a little. If you turn it down from half to 1/4 then the volume will be reduced a little more. When you turn it down almost to zero then the level will change a lot when the pot is moved only a little. A poor volume control.

    When you use a logarithmic audio pot for a volume control and turn it down to half then the volume will be reduced a lot. If you turn it down from half to 1/4 then the volume will be reduced a lot more. When you turn it down almost to zero then the level will change a little when the pot is moved only a little. The way it should be.
     
  5. Lambda

    Thread Starter New Member

    Feb 28, 2011
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    Thanks for the info. I don't know why the two 2.2k resistors are in the circuit; I found the schematic on a website. This is the first time I've worked with audio circuits, and when I built it and it worked, I had no idea that they were extraneous. I've just removed them, and everything works just fine.
    Thanks for the explanation of the volume control.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Regardless of the POT type, one cannot reduce the output level to zero. Either one have amplification or gain=1 when POT resistance is zero.
     
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