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Problem with volume cutting out too quickly

Discussion in 'Luthier's Corner' started by stoutman, Nov 14, 2004.

  1. stoutman


    Mar 30, 2004
    Hello -

    I am replacing the vol/vol/tone controls in my P/J, Dimarzio humbucker-equipped Frankenfender with a blend control (500K) and a stacked tone/volume dual concentric pot (500K). So far the changes work except that the volume goes from full throttle to zero when I turn the volume control @ 1/8 a turn. Not much subtle volume control here.

    I sure would appreciate any suggestion/guidance before I start throwing some solder at this problem.


  2. luknfur


    Jan 14, 2004

    you can put a meter on the pot and see if it's a pot problem.

    If it's a linear pot you may get wierd volume increase but it shouldn't be that radical - volume technically should be an audio pot.

    Sounds like pups may be out of phase - especially if it sounds like it - but you'll get volume jumps much like that if the pups are out of phase: corrected by swapping the connections to the leads of one of the pups.

    About all that comes to mind except for a possible cross up in wiring or a short in wiring (especially if the control bay is shielded).
  3. A9X


    Dec 27, 2003
    Sinny, Oztraya
    Sounds like the vol control is a linear. Tone controls are usually linear, and a stacked lin/log pot is extremely rare (custom order part).
  4. luknfur


    Jan 14, 2004

    to my knowledge, the only difference between a linear and audio pot is the taper. You're supposed be able to tell an irregularity in a linear pot used in place of an audio, although I've done it on a number of occassions and couldn't. You're not supposed to be able to tell an audio in place of a linear. I've also done that and couldn't tell the difference. As far as manufacturers go, the low end Ibanez GSR 200 comes with all linear pots with vol/vol/tone setup. I've also seen several EMG harnesses that were vol/vol/tone and they were all 25 audio pots.

    Stacked audio pots are available but the problem may be getting the size you want - especially if you're looking for different ohm ratings. You should be able to offset the difference with a capacitor if desired. There's very little difference between a 250k and 500k in terms of tone, it may or may not make a difference to you. Going from like a 25K to 500k is major.
  5. A9X


    Dec 27, 2003
    Sinny, Oztraya
    Yes. A linear taper varies directly with rotation, eg for a 100k pot, each 30* of rotation represents about 10k of resistance change. A log or audio taper pot varies with a logarithmic relationship between the rotation and the resistance.
    Not so. If I took the log volume control out of your hifi and replaced it with a log, you'd be able to tell next time you used it, and the situation would be similar to what Stoutman is describing. Where the difficulty can arise is in MI equiptment that uses 'unusual' arrangements of pots and fixed resistors, that what you expect to be one might be the other because the designer has had something specific in mind
    The EMGs are lower value pots because the output impedance of the active pickups is much lower than a passive pickup. But the ratio of the EMG pups output Z to the pots they specify, and the ratio between may common pups and the 250k or 500k pots they us is about the same.
    True, but all passive pup systems I've ever seen have been 100k/250k/500k or 1M which are quite common. Stacked pots 2 or 4 layers deep are common stock lines for resistor manufacturers, provided all are log or all are linear. Having a stacked pot where one is 100k linear and another is 100k log are very rare and custom orders. Still not hard for a manufacturer to source, but usually require large batches in the hundreds ot thousands to get them at a sensible price. Even a 100k lin and a 25k log would be possible, you just don't see them often in practice, and even for a manufacturer of the size of Ibanez, an oddball custom pot can be a significant parts cost addition, so designs tend to use as many standard off the shelf parts as possible.
    No. A cap is a different animal to a resistor both in construction and how it works. A 100k resistor will have a resistance of 100k at all (audio) frequencies. A cap that has a resistance (actually called reactance) of say 100k at 41Hz, will be 50k at 82Hz, 25k at 164Hz etc, ie it's resistance will decrease by a factor of two every octave you increase the frequency. That's how a passive tone control works, by using this property of a cap, in conjunction with a pot, to remove a variable amount of higher frequencies.

    However, if you needed a 500k pot, and you only had a 250k, you could put a 250k resistor in series with the pot, and the load (as it appears to the pickup) would still be 500k, so you'd get the same tone. But, depending on which side of the pot you placed it you'd either lose half your signal or not be able to turn that pickup all the way off. In parctice, this might not be an issue, or it might, especially if you have lower output pickups and/or are in an electrically noisy environment. That missing 6dB might be the difference between acceptable and unusable performance.
    True, usually it doesn't seem to make much difference, but it will depend on the output impedance of the pickup, how the controls are wired and of course, the expectations of the user. That little bit extra of top end that's lost on you bass by substituting a 250k for a 500k might not be much to you, but it may be a big difference in tone in some examples for me.

    For example, a theoretical 10k pickup, with no pot, ie looking into an infinite impedance puts out 100mV. Into a 500k pot 98mV, 250k 96mV, 100k 91mV, 50k 83mV and 25k 71mV. Even with a substantial change in the value of the load, there's not a huge difference in signal, and between a 250k and a 500k it's very small. But, a pickup is just a big coil of wire which electrically is an inductor. The resistance of an inductor varies with frequency just like a cap, but the other way; as the frequency rises by a factor of two (an octave), so does the resistance. The above example for outputs with pots were considering the low frequencies only to keep it simple. When you factor in the effect of the inductor's resistance, and there's a lot of inductance in pups, especially bass ones, too low a pot value can effectively roll off the top end further and change the tone significantly. Higher impedance pickups generally have more inductance, so they're (usually) even more sensitive to changes in load. Factor in the capacitance of the pickup itself, the reactance of the tone control, the effect of a long, high capacitance signal lead and the input resistance and capacitance of your amp, and it can all change in ways that might seem counterintuitive, unless you can analse the network of components electrically.

    Sorry for the long post, but this is often explained in ways that are really odd to me, and full of assumptions, myths and repeated errors that it needs a sort of explanation/FAQ page, done right.
  6. luknfur


    Jan 14, 2004

    Thanks for the clarification/corrections Dharma.

    Tried to go in through your post but it was easier this way.

    I run straight to jacks now so it's been months since messing with preamps, pots, etc. I dug the Ibanez pots out and apparently they were all audio pots not linear as I'd recalled (have alpha stamped on the back). So apparently it was audio pots used as tone pots I couldn't distinguish a difference. It was apparent to me you were saying a linear pot shouldn't replace an audio but didn't see anything referring to an audio not replacing a linear.

    Thought occurred to me that a resistor may be more appropriate than a capacitor but I'd already punched the shpeal to send it and was headed out.

    Between the two of us - maybe something of use was transmitted ;-)
  7. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Thanks for the detailed post, dharmabass.

    Just a thought - isn't it true that the "steep cliff" effect that you get when using a linear where a log pot is called for, would be gotten even moreso if a log pot is used, but wired in reverse (causing a reverse log taper)?