Potentiometer Values

Discussion in 'General Electronics Chat' started by mapleman555, Jan 22, 2015.

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  1. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    Hello,
    Where can I learn about potentiometer values why I would choose one over the other? For example guitars use 250/500k pots but effects boxes use 10k? Why?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What matters is what kind of circuitry the pot drives, or is driven from. The circuitry surrounding the pot determines if to use a 10K pot or a 1meg pot.
     
  3. #12

    Expert

    Nov 30, 2010
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    Umm...because guitars produce microwatts of power and don't need to be losing their voltage in a low resistance potentiometer and an effects box has a power supply that can run a 10 milliwatt amplifier for a 40 hour week on one 9V battery?
     
  4. JohnInTX

    Moderator

    Jun 26, 2012
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    Seymour Duncan has the answer about guitar pots.
    As far as effects boxes, as Mike and #12 indicate, they are powered circuits and values are chosen to fit the circuit at hand.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Impedance matching.

    The pot has to do two things:
    1) Match the impedance of the source.
    2) Match the impedance of the load.

    And when I say match, I don't mean equal. I mean not be detrimental to the signal.

    Let's look at requirement #1.
    The pot resistance should be greater than that of the source to avoid loading the source. A good rule of thumb is the pot resistance should be at least 10 times that of the source.

    Now let's look at requirement #2.
    The pot, since it is now the source, should have a resistance less than that of the load, i.e. the input resistance of what ever it is feeding. Again, a good rule of thumb is the pot resistance should be 1/10 that of the load or lower.
    (The resistance of the load should be 10 times greater than that of the pot.)

    Now, as you can imagine, in most cases this an impossible situation. So the best you can do is come up with a compromise somewhere in between.

    Here is a simple example. The output impedance of the preamp is 1kΩ. The input impedance of the power amp is 100kΩ. Hence a volume control pot of 10kΩ would be perfect in this example.

    Hope this helps.
     
    Last edited: Jan 23, 2015
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are also a few variations and types, logarithmic, antilog, linear etc.
    Audio systems usually use Log type.
    Max.
     
  7. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    Thanks for all the info so far, it makes sense. I'm just getting into the electronics part of guitars so I have a lot to learn.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    Actually, a lot of the old valve amplifiers have a few stages of amplification ahead of the first pot - the grid leak resistor on the first valve can be pretty high, 6M8 - 8M2 is typical.

    A guitar PU is a hell of a lot of turns of very thin wire so the impedance is pretty high, any loading will attenuate it, especially the high notes due to capacitance in the guitar lead.

    DOD made a basic FET booster to eliminate this attenuation and enhance sustain - it has an input impedance of about 3M9 ish.

    Some guitars have volume pots as low as 50k, the DOD FET circuitry would work very well as active electronics in the guitar, you'd be able to reduce the volume pots and get more treble through the guitar lead capacitance.
     
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