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The effect of volume control

Discussion in 'd.i.y.' started by philiphifi, Oct 12, 2019 at 9:20 AM.

  1. philiphifi

    philiphifi pfm Member

    hi - I noticed something very odd yesterday. On my Audion monoblocs, there is a volume control at the front panel for each device. If i turned that to max (to bypass the resistor) and use the preamp to control the volume adjusted to the same loudness, the sound is very different. (the preamp is used in both cases). More bass and treble. How is this possible? A resistor acts as a filter or is it that the preamp is sounding different at different volume settings?
     
  2. Jim Audiomisc

    Jim Audiomisc pfm Member

    Sounds like your amp has a significant capacitance, etc, right after the controls you mention. The output resistance of the control varies with the setting.

    A shunt capacitance will combine with the output resistance of the control to produce an HF filter. A series capacitance means the LF may change. Thus the frequency response can vary with the control setting. The result should be flattest at the max and min settings, and least flat when (approx) halving the input voltage.
     
  3. philiphifi

    philiphifi pfm Member

    thank you Jim. Just so i understand properly, is the best way then to bypass the power amp resistor and use the preamp to control the volume? I don't understand the halving the input voltage part. I would need to halve the preamp volume but would this lead to the same problem?
     
  4. Jim Audiomisc

    Jim Audiomisc pfm Member

    If you have an 'active' preamp it will have a gain/buffer section after it's volume control. So your power amp won't know anything about the change in output resistance of the preamp's volume control. Only the change in level.

    'Passive' pre-amps can produced the same effect, though. Sometimes made worse by the capacitance of the connecting cable.

    The snag is that your *preamp* might do the same sort of thing! The behaviour is almost impossible to totally prevent, but good design should ensure it is too small to be audible.

    So it is worth setting the power amp control to 'max' and trying just using the preamp volume control. That should be OK. But can't be sure without various details or measurements.
     
  5. darrenyeats

    darrenyeats pfm Member

    This is an advantage of digital volume control, another one is perfect channel balance. (There are disadvantages, but if done properly, nothing I worry about.)
     
  6. philiphifi

    philiphifi pfm Member

    thank you very much. I guess it's back to the old way. Trust your ears and experiment!
     

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