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Push/pull tone bypass

Discussion in 'Pickups & Electronics [BG]' started by wvbass, Jun 10, 2012.


  1. wvbass

    wvbass Supporting Member

    Mar 1, 2004
    West Virginia
    e4dd49ca.jpg

    683f89db.jpg

    Are both of these correct? Is there any advantage to one over the other?

    Please forgive the sloppy, sideways drawings. I did the on my iPad.
     
  2. You're overcomplicating it and may end up with more switch pop.
    3396717693_59d9a94d36_o.png
     
  3. wvbass

    wvbass Supporting Member

    Mar 1, 2004
    West Virginia
    So just use the switch to disconnect the capacitor from ground? Thanks!
     
  4. As a side note, though it's not an issue in this particular application, it is generally never a good idea to short out a capacitor. At higher voltages and currents, you can damage the capacitor.
     
  5. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    there you go, and more proof that line6man is Da Man when it comes to diagrams!

    you could simplify this still further; skip the switch entirely and just use a higher-value tone pot, like 500k or even 1M, or even a no-load pot, to take the tone out of the equation on "10" and bring it back in when you turn it down.

    one action instead of two to do the same job.
     
  6. wvbass

    wvbass Supporting Member

    Mar 1, 2004
    West Virginia
    Understood. Thanks again!
     
  7. wvbass

    wvbass Supporting Member

    Mar 1, 2004
    West Virginia
    Does it really take it out of the circuit? Even at "10" there is still treble roll off, right? With a .047mfd capacitor, what size resistor pushes the roll off above 20k hz?

    Edit: the no load pot is a good idea. And I was just reading about them the other day. Duh.
     
  8. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    at 500k, the cap's effect is negligible, at 1M it's pretty much non-existent.

    and the higher-value "resistor" (pot) doesn't move the rolloff up at all, it just makes it closer to not even being there.
     
  9. The frequency cutoff point is determined in part by pickup impedance. The pot value just changes the impedance of the capacitor in the circuit. For most pickup impedances, a 500k resistance between the capacitor and signal effectively removes the capacitor from the circuit.

    If you want a different frequency cutoff, that would be done by changing the capacitance of the filter. A higher value will give a lower frequency cutoff, while a lower value will give a higher frequency cutoff.
     
  10. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    Actually you do get some high frequency roll off between the pot resistance and the pickup inductance even when the pot is set to its max value. They form a low pass RL filter. As you make the pot resistance higher and higher that corner frequency moves higher and higher. Certainly by the time that corner exceeds 20kHz you have effectively removed both the capacitor and the pot from the circuit but in a bass guitar I don't think you have to have the corner nearly as high as 20kHz. Especially if you are over 40....

    Ken
     
  11. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    i think the corner frequency stays where it is, but the peak just gets less and less evident as you lower the pot value.

    when you roll it way down (past the point where the peak is shaved off entirely) a resonant hump forms at a lower frequency.
     
  12. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    The RL filter formed by the pickup inductance and tone pot resistance will change cutoff frequency as the pot resistance is changed. You can see this effect in the plot below for three different values of tone pot resistance. However the fourth trace on that graph shows why this may not be worth bothering about. The first three traces ignore the interwinding capacitance of the pickup by setting it to 1 fF. When a more reasonable value like 160 pF is used you get a parallel/self resonant notch just below 10 kHz and the capacitor completely shorts out the coil inductance for frequencies much above the notch so the RL filter formed by the coil inductance and pot resistance has little effect. For this particular pickup (the data comes from some measurements made at the University of Illinois) the RL corner is at or above 20 kHz for all but the 250k pot and its corner is high enough for most people's taste. There may be some pickups where the self resonant notch occurs at a high enough frequency to make the RL corner important. Hard to say for sure.

    Ken
     

    Attached Files:

  13. wvbass

    wvbass Supporting Member

    Mar 1, 2004
    West Virginia
    If I follow you correctly, it is, to some degree, a crap shoot without knowing the pickup's inductance value. But, given what you described regarding the notch frequency, there is the possibility that a 250k pot might sound a little different than the higher values. It looks unlikely that you'd hear much difference between the 500k and the 1meg pots.

    At any rate, the educational value here is huge. Thank you. For my needs, though, the no-load pot is the answer I was looking for, even if I asked the wrong question. It probably took me longer to get the pot out of the bass than it did to convert it to no-load.

    I appreciate everyone's input here. Your knowledge and experience is invaluable.
     
  14. Remember that a tone control's pot is simply being used as a variable resistor, and not a part of any voltage divider circuit in which the resistance across the outer terminals affects the behavior in the circuit. A 500k pot at the 250k setting (Which is typically around four-fifths of the rotation with audio taper pots.) is identical to a 250k pot at its 250k setting, unless you want to get down to things like noise, given the differences in the area of the resistive element, or other such things that would not be of practical relevance in this application. There is no appreciable tonal difference between various pot values when used as tone controls at the same range of resistances.

    As a side note, there is no reason to ever fool around with a 1M pot for a tone control, unless your pickups have a very high output impedance. At such a high value, the usable portion of the rotation will be very small, so the pot will function more like a switch.
     
  15. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    maybe we're talking about different things here;

    my tenuous understanding is that the tone control behaves more like this:

    Tone_Control_Effect_0.jpg
    http://charliehall.proboards.com/index.cgi?board=project&action=print&thread=2409

    each line is the same fixed capacitance, with progressively lower resistance in series with it (a tone knob getting turned down);

    the peak gets progressively shaved off, then the treble range genuinely starts to go away, and then at the last bit of rotation, the cap starts to resonate and brings in another, lower peak.

    at no point does the peak "slide down" in frequency, and the peak won't "go higher", certainly not anywhere near 20k, with higher tone pot resistances. it gets a little taller, but even with the tone pot snipped out entirely it doesn't shift up in frequency.
     
  16. wvbass

    wvbass Supporting Member

    Mar 1, 2004
    West Virginia
    I must not undrstand. The above graph looks to me like every different resistance also has a different cutoff frequency. Isn't the cutoff point where the signal hits -3db?
     
  17. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    yeah, but there's not a peak that stays in place but gets slid down in frequency.

    if there was, working the tone knob would sound like shifting a filter on a fatboy slim record.

    it's almost like a passive tone knob has just 3 settings, 1-treble peak gets shaved off, 2-sound gets genuinely dark, and 3-low mid resonant peak gets reintroduced.
     
  18. bassbenj

    bassbenj

    Aug 11, 2009
    Actually my favorite tone mod is a mini on-off-on switch rather than the push-pull. I usually use a push pull for series/parallel of the pickups.

    My deal is with the three way switch the center position is open so that is NO tone control at all and is as bright as the bass goes. To one side I use the standard .047 mfd. cap which makes the tone circuit factory standard. To the other side my favorite cap value is a .015 mfd which is sort of a milder cut on the higher highs, but you can actually use ANY value there that floats your personal boat.

    The reason I like this better than push-pull and high value or a "no-load" pot is that with just a flip of the switch you can go from NO tone control (max bright) to a PRESET tone setting you've already dialed in. There is no hassle with knob-twiddling to change tones (like say from solo to groove or chorus to verse etc.) And then the icing is you've got a third variation there too that may or may not be useful. I find the whole setup VERY handy!
     
  19. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    Any circuit design is a crap shoot if you don't know the values of the components!

    Ken
     
  20. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    Yes we are. You show the effect of turning the tone control resistance down and of course as you do so the tone capacitor starts to have a serious effect on the frequency response.

    I thought the OP was asking about the effect of the pot resistance itself, in other words the difference between 250k, 500k, and 1 Meg pots when all are set to their max resistances. The tone cap has little to no effect in this case (at any frequency over 135 Hz a 47 nF tone cap has less than 25k Ohms impedance, less than 10% of 250k) but the higher the pot resistance the less effect the RL filter will have, in theory, and that is what I though the OP was concerned about. As the graphs show the interwinding capacitance of the pickup pretty much eliminates the effects of the RL filter -- if you consider the tone pot alone. Throw in a couple of volume pots and the amplifier input resistance and now the RL filter can start to bite you. Increasing the tone pot resistance alone will have limited effect due to the presence of the other load resistors but evidently enough effect that people find value in no load tone pots and tone circuit kill switches.

    BTW, I modeled the circuit with the indicated compnent values with a single pickup and no other load resistances to isolate the effect of the tone pot resistance for illustration. Simulations of more complete circuits and measurements on real circuits will not look much like these results in detail. But since people do put in higher value pots, no load pots, and tone kill switches in a quest for a brighter tone, the effect of the RL filter is large enough to be significant even though it does not stand out in simulations of complete guitar circuits.

    Ken
     

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