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Question about analogue potentiometers

Discussion in 'd.i.y.' started by ToTo Man, Feb 7, 2019.

  1. John Phillips

    John Phillips pfm Member

    Yes, in part. Near zero the empirical formula above (and the actual performance of a real-world potentiometer) will certainly be much less accurate / more variable. However the formula is not a pure log function and does not suffer from the absence of "walk-off to zero" in the "perfect logarithmic pot" formula used in the URL. It does hit 0% gain at 0% rotation rather than bottoming out at the -40 dB minimum used by TI.

    Anyway, calculating approximate attenuation values of a pot from specified taper curves at 25%, 50% and 75% rotation, if I have interpreted correctly what you are looking to do, looks quite straightforward.
    ToTo Man likes this.
  2. demotivated

    demotivated pfm Member

  3. martin clark

    martin clark pinko bodger

    Nice link!

    (while quietly smug the old preamp uses a Penny&Giles rotary fader...)
  4. ToTo Man

    ToTo Man the band not the dog

    What would be the formulae for a pot with resistance that's not 10%, but other percentages like the Bournes and ALPS pots you linked to?
  5. John Phillips

    John Phillips pfm Member

    The two occurrences of "81" in the main formula are for 10% (i.e. 0.1) mid-position-gain as you say, and come from the additional formula N = (1/mid-gain - 1)^2.

    For 10%: N = (1/mid-gain - 1)^2 = (1/0.10 - 1)^2 = 9^2 = 81.

    For 15%: change both occurrences of "81" to (1/0.15 - 1)^2 = 32.1. That is, closely enough, gain (linear units) = (32^rotation - 1) / (32 - 1).​

    And so on for other mid-position gain values. The main formula is just an approximation, both to a pure log taper and to real two-part tapers, but may be useful enough.
    ToTo Man likes this.
  6. John Phillips

    John Phillips pfm Member

    Remember, BTW, if you want to calculate the gain through the potentiometer, that the potentiometer track is typically only 270 degrees, not 360 degrees. So the 9 o'clock position of the volume knob is 17% into the track. 12 o'clock is 50% and 3 o'clock is 83%.
  7. ToTo Man

    ToTo Man the band not the dog

    Good point, I hadn't thought about that!

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