Potentiometer question.....

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by lgehrig4, Feb 14, 2006.

  1. lgehrig4

    lgehrig4 Silver Supporting Member

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    I've rewired a few strats and have dabbled in simple eletronic circuits, but I have only followed schematics so I don't completely understand what I have been doing.

    Potentiometers.......

    Ohm is a measurement of resistance so the higher the Ohm rating the greater the resistance?

    If my first statment is true then a 500K pot offers greater resistance than a 250K pot?

    If the first 2 statments are correct, then here is my first question.

    1. Is this resistance (500K or 250K) the resistance when the pot is fully open (volume on 10?) I assume yes because the resistance should be 100% or close to it when you have the volume turned down regardless of 500K or 250K

    2. If I have been on the right track so far, why don't we use 250K pots for everything? If a pickup calls for a 500K and you use a 250K wouldn't you hit 500K somewhere on the dial? I would think that this would offer greater tonal possibilities.

    I know that this probably should have been posted in the technical forum, but there isn't much traffic there and this is very basic.

    Thanks!
    Jeff
     
  2. Jim Collins

    Jim Collins Member

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    The resistance of the pot is measured between the upper end and the lower end. The resistance between the wiper and either the upper or lower end of the pot will be less than or equal to the value of the pot. So, if the pot is 250K, the resistance between the wiper lug and either end will never be greater than 250K ohms.

    The switch or pickup is typically connected to the upper end of the pot, and the pot's wiper is connected to the output jack. When the pot is full on -- fully clockwise -- the wiper is in contact with the upper end of the pot. You might think that this would short out the pot, taking it out of the circuit. The low end of the volume pot, however, is connected to ground. This means you have the full value of the pot in parallel with the pickup(s). This is why a 500K pot sounds different from a 250K pot. As the wiper is backed off from the full position, a portion of the pot is in series with the pickup(s), and the rest of it is still in parallel. When the control is almost at its lowest point, just about the full value of the pot is in series with the pickup(s), giving you minimal volume. When the pot is at its lowest point, the wiper is in contact with the low end lug of the pot, and that low end lug is already connected to ground. This is what causes the guitar to go dead when the volume is all the way off.
     
  3. lgehrig4

    lgehrig4 Silver Supporting Member

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    So, are you saying that when a 250K pot is fully on there's no resistance or 250K resistance?

    My bascic understanding is that a pot is a variable resistor and that resistors do just what their name implies which is resist current. To me that means that as you are turning a pot counter-clockwise the resistance increases which allow less current to pass. I also thought that the higher the Ohm (k) the more resistance?

    I might need a more basic explanation if thats not a problem.

    thanks again
     
  4. Jim Collins

    Jim Collins Member

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    The variation in resistance is measured from the wiper (which is connected to the center lug) to either end of the pot. As the wiper moves from the bottom to the top (in a clockwise direction), the resistance from the bottom to the wiper increases, while the resistance from the wiper to the top decreases. Since the pickup is connected to the top, and the output jack is connected to the wiper, as you turn the volume up, resistance decreases between the wiper and the top.

    When the control is all the way counterclockwise, the resistance from bottom to wiper is zero, and the resistance from wiper to top is 250K (assuming a 250K pot). Maximum resistance between the wiper and the top -- which is between the pickup and the output jack -- means minimum volume. On its own, that does not mean no volume. Because the low end of the pot is connected to ground, and, at its lowest point, the wiper is in contact with the low end, everything goes to ground, which means no sound. That is what causes no volume at the low point, not the mere fact that the resistance between the wiper and high point -- between the pickup and the output jack -- has been maximized.

    When the control is fully clockwise, the wiper is in contact with the upper end of the pot. Since the pickup is connected to the upper end of the pot, the wiper is connected to the pickup. This means there is no resistance between the wiper and the pickup, and the pickup is at maximum output. However, since the bottom end of the pot is connected to ground, the entire pot is in parallel with the pickup. This is why a 250K pot sounds different from a 500K pot, when they are each used with the same pickup, and at full volume.
     
  5. jbro

    jbro Member

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    You have the right idea Jeff.. a pot is just a sliding resistor, however it will never put up more resistance than it is rated for. The pot slides from zero resistance to its rated resistance, so you would not get 500k out of a 250k pot at any point in its sweep.

    For example let's look at the resistance between the signal and the ground in a simple 250k guitar volume pot.

    When the volume is fully UP, there is 250kohms resistance between the signal and the ground.. so most of the signal will go to the jack. When the volume is fully DOWN, there is zero resistance between the signal and ground.. so the signal is shunted to ground.

    A 500k pot is capable of more resistance than a 250k. So let's substitue a 500k in the above example. Now when the volume knob is fully UP, there is more resistance between the signal and ground. As a result, more signal will end up going to the jack.

    High frequencies are the weakest part of the signal and first to bleed to ground, so the less resistance between the signal and ground, the more highs you will lose and the lower your overall volume will be. This is why a 500k pot will sound a bit brighter and louder than a 250k on the same pickup. If you use a 25k pot on a passive circuit, you will have a very dull, weak signal. A 1M pot may not bleed enough highs off, and may be too cold and sterile, depending on the pickup. I find that the rule of thumb "250k for single coils 500k for humbuckers" doesn't always work best, sometimes it's good to experiment to get the sound you want.

    Hope that was clear.
     
  6. lgehrig4

    lgehrig4 Silver Supporting Member

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    First off Guys, thank you for taking the time to write these thorough explanations! I think I have successfully discarded my old knowledge and ideas of the inner working of potentiometers and assimilated this new info.

    This raises a few new questions.

    1. Can standard pots work in reverse? (Clockwise=off, Counter=on)

    2. Would there be a more noticible (sudden) cutoff in sound with a 250K over a 500K pot when turned completely counterclockwise? I never noticed this, but I'm assuming yes b/c you are going from less resistance to complete cutoff to ground.

    3. Why do we use a combination of capacitors and resistors to retain highs in a guitar set up as opposed to just resistors?

    These will be my last questions. I promise ;)

    thanks
    jeff
     
  7. jbro

    jbro Member

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    1. Yes a pot can be wired either way, and actually works both ways at once.. ie when there is no resistance on one side there is full resistance on the other. However, with audio applications there is a non-linear taper or "audio taper" to the sweep of the pot. We don't percieve volume changes in a linear fashion.. ie 20 decibals is nowhere near twice as loud as 10 decibals. So the sweep of an audio taper pot is tailored to give the signal a more smooth percieved increase in volume. If you try wiring up an audio taper pot backwards as a volume control, you will find that the volume jumps in suddenly at the bottom of the sweep and changes very little throughout the rest of the sweep. If you use a linear taper pot you will not have this problem, but it will not respond right in a volume control application.

    2. Not really due to the audio taper, but the maximum volume and frequency output would be different with the 2 pots, so it may sound a little different.

    3. In a "treble bleed" type circuit like you described, a resistor and capacitor are connected in series between the input and output (jack) lugs of the volume pot. The capacitor is used to bleed high frequencies directly to the output jack, and the resistor limits the amount of highs that are bled. The value of the cap will dictate the range of frequencies that are bled through, and the value of the resistor will determine how much of those frequencies will bleed through. There are several other ways of accomplishing this, but this is the most effective method I know of. You can do it without using the resistor, but with nothing to limit the amount of treble bleed it messes with the way the pot responds during its sweep.
     
  8. lgehrig4

    lgehrig4 Silver Supporting Member

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    Awesome! Thank you!
     

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