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hum-buckers - how they function

Discussion in 'Pickups & Electronics [BG]' started by Killed_by_Death, Apr 12, 2019.

  1. First, electromagnetic induction:

    Electromagnetic induction - Wikipedia

    In the case of noise that we're trying to cancel, EMI is inducing electrical noise into the coils of the pickup.
    Some use the term RFI, but much of the interference falls outside the realm of what the FCC has designated as radio-waves.

    Here's a rudimentary drawing of how a hum-bucker is wired:


    Because one of the coils is reverse-polarity (North vs. South) & reverse-wound from the other, hum-cancelling occurs.

    The same noise is induced into both coils, but one side is reversed, so it cancels the hum from the other coil.
    To work perfectly both coils must be matched in output, so that all the hum is canceled.

    Assuming that both coils are using the same diameter of wire, they'd need to have the same amount of windings & be the same size.

    A split-coil Precision-style pickup operates in the same manner, just with the coils in a different orientation to one another:

  2. megafiddle


    May 25, 2011
    It might be mentioned that the coils appear to be wound in the same direction. And from a fabrication standpoint, they often are. In other words, the two coils are identical. But if you trace the current path starting from ground, it is clockwise through the lower coil, and counterclockwise through the upper coil. So as far as the signal and hum currents are concerned, the coils are in effect reverse wound.

  3. LoveThatBass


    Jun 28, 2004
    I had some "Humbuckers" that were somewhat noisy apparently due to not being wound equally. How do that let that get by them? Nevermind I know they typically due random sampling or maybe not at all as the case "might" be with inexpensive Asian models
  4. sikamikanico


    Mar 17, 2004
    Here's what I think I read somewhere, and I'm not sure it's true: For hum-cancellation, you only need the coils to be reverse wound. The reverse magnetic polarity is not necessary, but it puts the signal back in phase when two coils sense the string (so necessary to sound right, but not for the hum-bucking part). I'm not sure if it's right, but it makes sense to me - dummy coils don't always have magnets, but still work. So I guess technically, split coils (like P) only need to be reverse wound, no?

    Also, I read that the two coils don't have to be identical. I suppose it's a question of "close enough" (surely at some point they don't buck hum), but some even claim that old PAF Gibsons sounded "better" because consistency was so low that most coils were indeed not identical. Supposedly, slight differences sound more musical (or, rather, perfectly identical coils sound less musical). Also, those large-diameter Illitch/Suhr dummy coils work well enough (for hum-cancellation), and are obviously NOT identical. And then there are 5-string split-coils that also work well enough...

    Hope I'm not spreading misinformation. Just things I picked up that I didn't know until rather recently.
    Killed_by_Death likes this.
  5. A coil can buck hum w/o needing reverse-polarity magnet. The reason you need reverse-polarity in a typical hum-bucker is to send the signal from the notes being played.
    The noise is being induced into the coils, which is why a dummy-coil will work, but a dummy or phantom coil isn't going to pick up notes & doesn't need a magnet or poles.

    Surely there's some serious thought put into the amount of windings & diameter of wire used in a dummy-coil to match it up to the noise in the other coil.
  6. Coolhandjjl

    Coolhandjjl Supporting Member

    Oct 13, 2010
    Humbucker pickups are also expected to have a fatter sound because both coils span across the entire string area, and are picking up the same signal.

    Hum-cancelling pickups also have two coils, but with each coil only spaning half the string area and with their associated pole pieces flipped.

    Noiseless pickups use two full width coils also, with the other one called a phantom or dummy coil and not sensing any string vibration.

    Sidewinder pickup is yet another type with two full width coils laying on their side on opposite side of the poles.
    Last edited: Apr 12, 2019
    .:Aidan:. and ThinCrappyTone like this.
  7. sikamikanico


    Mar 17, 2004
    Surely, to some extent, but I think the "close enough" applies. Looking at the Ilitch system for jazz basses, he/they offer three basic models based on how hot the pickups are wound (and resulting resistance). But it comes with a small PCB to fine tune, I think via trimpots. So there's tricks around small differences.

    But yeah, I imagine Alembic and others that have been using dummy coils are probably more precisely matching the coils...
    MattZilla and Killed_by_Death like this.
  8. I notice badly balanced hum-buckers quickly, because I play with a lot of gain. A typical bass player probably wouldn't even notice.
  9. sikamikanico


    Mar 17, 2004
    I think this vocabulary is not universally applied like that. Except maybe for "humbucker," which most people use to refer to the pickup with two coils, side-by-side, oriented upwards. But I learned to always pay attention to the context to know what people mean. Like with coil splits and coil taps, which I know is another "favorite" of KBD :D
    BlueTalon likes this.
  10. Coolhandjjl

    Coolhandjjl Supporting Member

    Oct 13, 2010
    People just need to know what they are buying.

    If one was expecting a humbucker with its associated thick tone, and wound up with a hum-cancelling or other design, they would be disappointed
    Last edited: Apr 12, 2019
  11. What you're calling hum-canceling is traditionally known as a split-coil design, like the photo of p-style pickups above.
    They're actually a good compromise between single-coil sound & having hum-canceling pickups.

    Hum-Bucker & Hum-Canceling are synonymous.
    Last edited: Apr 13, 2019
    BlueTalon and ctmullins like this.
  12. bfields


    Apr 9, 2015
    I've never quite understood exactly what people mean by "fatter" (I guess I should go listen to some samples side-by-side), and how that's determined by the pickup.

    I guess that as frequency gets higher, wavelength gets smaller, till it eventually approaches the size of the pickup. At some point a significant portion of the wave is over the pickup simultaneously, so there's some sort of cancellation. So you'd expect to lose some higher frequencies. And the more of a string the pickup covers, the lower the frequency at which that starts to happen. So a wider pickup should lose more of the higher frequencies. Is that right? I wonder exactly how you measure the length of string that a pickup senses, and how that determines exactly how the higher frequencies are attenuated?
    sikamikanico likes this.
  13. nnnnnn


    Oct 27, 2018
    Not the ITU?

    Which frequencies are you talking about?
  14. byacey


    May 16, 2008
    Alberta, Canada
    The coils don't need to be reverse wound, just the signal lead wires reversed to flip the phase, and the magnetic pole has to be reversed to put the string signals back in phase when a string(s) pass over both coils, as in the case of a P bass configuration.

    Omitting the pole pieces on one coil would change the induction of that coil, creating an imbalance between the two coils. Both coils should be identical in all respects for high common mode rejection against noise.
    megafiddle and RobbieK like this.
  15. Yes, it's the reason that noiseless pickups with the phantom coil sound anemic.
  16. nnnnnn


    Oct 27, 2018
    Don't P bass strings pass over only one coil?
  17. byacey


    May 16, 2008
    Alberta, Canada
    No, have a look at any precision bass setup. The A and D string is often straddling pole pieces on different pickups.
  18. sikamikanico


    Mar 17, 2004
    My sense is that the wider the pickup (i.e. distance between the two coils), the lower the phase cancellation (in frequency). If you think of a jazz bass, with both pickups on, as a giant parallel humbucker, it's the mids that get cancelled out. If you space them out even further, cancellation occurs more towards lower mids.

    Edit: you can play around with this app to model some of these changes. I'm not sure how accurate it is, but it sort of corresponds to what I'm hearing: Guitar Pickup Response Demonstration

    As far as I can tell why humbuckers (dual coil, side-by-side pickups) tend to sound "fatter" is basically a couple different things. For starters, as you described, some highs are cancelled out. I think technically some low or low-mids are reinforced too. Finally, many humbuckers are wired in series, which raises the output and shifts the resonant peak down, further cutting highs and "boosting" mids. Parallel-wired humbuckers don't necessarily sound fatter than single coils, perhaps rather thinner than fatter.

    But it all depends on specific designs of specific pickups. There are fat-sounding single coils, and thin-sounding humbuckers... And of course, all other specifics of instrument design (like where the pickups are mounted) and other components in the signal chain that can either emphasize or mask differences...
    mikewalker, bfields and Wasnex like this.
  19. Wasnex


    Dec 25, 2011

    The fatness and high frequency rolloff is related to inductance. Humbuckers typically have more windings thus, higher inductance, which modifies the frequency response by lowering the resonance and forming a natural LPF.

    I believe, the size and spacing of the pole piece(s) has something to do with how focused the sound is.
  20. Usidore T Blue

    Usidore T Blue

    Jun 28, 2017
    How was this thread not called "F***n humbuckers--how do they work?"
    JoshS, mikewalker and InternetAlias like this.

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