Potentiometer Taper Determination

Discussion in 'Luthier's Corner' started by Huntroll, Jun 6, 2019.

  1. Huntroll

    Huntroll Active Member

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    Potentiometer taper determination.

    Please correct me if I'm wrong.

    Facing the pot from the shaft side,

    Measure the resistance of the two outer legs.

    That's the value of the pot.

    Now, with the shaft at exactly halfway,

    Measure between the center leg, (the wiper) and the left leg.

    Now measure between the center leg, (the wiper), and the right leg.

    When set to halfway, a LINEAR (B) taper pot will be exactly half the resistance of the outer two legs.

    i.e., a 100k ohm pot will be 50k ohms from the center leg to each outer leg when set to halfway.

    An AUDIO (A) taper pot will measure its halfway point when its set to 2/3 of the way up, (when viewed from the shaft side).

    The logic behind this is in general, functionally, people want to hear half volume when turned only 1/3 (when viewed from the back side) of the way up.

    The next 2/3 of the pot range, people don't expect much of a change.

    Please post the types of pots that you run across in your experiences .

    Thanks !
     
    Last edited: Jun 7, 2019
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  2. cerebral gasket

    cerebral gasket Well-Known Member

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    Resistance is futile...

    [​IMG]
     
    Last edited: Jun 6, 2019
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  3. Joncaster

    Joncaster Member

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    Last edited: Jun 6, 2019
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  4. donepearce

    donepearce Well-Known Member

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    Equal halves is a linear tape pot. There is no general rule for audio taper - different manufacturers will have different tapers
     
  5. DrBGood

    DrBGood Well-Known Member

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    I beg to taper.
     
  6. papagayo

    papagayo Well-Known Member

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    I think that a taper pot is a logaritm scale pot, we use this scale for the volume control because the sound pressure level scale (décibels) is not proportional to watts but log.

    Am I wrong?
     
  7. donepearce

    donepearce Well-Known Member

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    Sort of wrong. There are no logarithm pots. They are all a pair of linears of different resistance joined end to end. the values are chosen to end up somewhere near log.
     
  8. plankton

    plankton Well-Known Member

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  9. Huntroll

    Huntroll Active Member

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    Thanks for the input, please keep it coming !

    I'm hoping we can standardize our way of testing pots so at least our description of what were looking at jives with each other.

    That's why I stated "Facing the pot from the shaft side" and "set to 2/3 of the way up . . ."

    I dug out a 2016 SG HP PCB assembly and did some measuring:

    Its pots seem to be a 500k audio taper with a mid-point around 2/3 of the way up.

    I remember seeing some LP tone pots once that had 500k on them but measured 300k.

    I'm using a Fluke model "110 plus" to test with, its nice because it has a bar-graph indication along the bottom of the display that I can zero-in on the mid point without looking at the numbers.

    When I get the bar indications the same for each leg, then I look at the actual measured resistance values to verify.

    Maybe they should make a meter with dual inputs & bar-graphs so you could really get a handle on balancing the shafts position setting.

    The BEEP-TONE 2000 w/the in-balance beeper function"
     
    Last edited: Jun 7, 2019
  10. papagayo

    papagayo Well-Known Member

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    I don' t know how they are made but the scale of taper pot is log, used for the volume/output level.


    [​IMG]
     
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  11. donepearce

    donepearce Well-Known Member

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    That would be the ideal. In practice it is impossible to make that way. Split that right hand curve into two intersecting straight lines meeting at about the 30% level, and approximating the curves above and below. That is what an actual pot looks like. The exact point of intersection varies from maker to maker.
     
  12. Semla

    Semla Member

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    There can be quite a big difference in the so called audio- or logarithm pots. Modern day CTS audio taper pots is something like 90/10 (linear being 50/50), while i.e. RS superpots is more around 70/30. The 50s original from Gibson I’ve seen measured around 60/40 in some really nerdy sites, but that means that they are closer to modern day linear- than logarithmic pots!
     

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