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External Active Dummy Coil to Eliminate Single Coil Hum?

Discussion in 'Pickups & Electronics [BG]' started by Matthew_84, Dec 29, 2015.

  1. This could arguably go into the Effect subforum, but I thought it was more useful here.

    I was toying with the idea of getting (or making) an active pedal with a dummy coil that would eliminate the single coil hum of a passive single coil bass. I believe if there were two single coil pickups, they would need to be in the same direction, and the pedal's dummy coil would need to be RWRP to them... I think a switch could also be added on the pedal to switch polarity, but I'm not too sure (it may require a second dummy coil altogether that has a completely opposite wound and polarity, and the switch selects between either coil).

    Ideally, it wouldn't alter the tone, dynamic response, or output of the passive pickups - it would just eliminate the single coil hum.

    I got inspiration from this thread: Extra Dummy Coil Pickup In A Jazz Bass With Two Single Coil Pickups. In this thread, it seems like if I were to copy the Suhr Strat system with a very large coil with little wire in it for low resistance, it would work... But, I'm too sure if this would work external to the bass though, after the multiple pickups have already been blended and sent to the output jack.

    Audere has created a preamp that has this feature (LINK), and I believe old Alembics had a 3rd dummy coil in the control cavity to do this as well... I'm looking for something external, so it could be used with different basses and doesn't need extra routing in an existing bass to fit.

    I hate to bug you guys, but you've always been the most knowledgeable and helpful to me and hopefully you could throw in some advice? @line6man @khutch @walterw
  2. Anything external is not going to work. It needs to be close to the audible pickups, and oriented in the same fashion, so that it picks up the exact same hum. If you tried having an external pickup, your noise level would vary wildly, as you move the bass around.
    Last edited: Dec 29, 2015
    mech and Matthew_84 like this.
  3. If it were a feasible idea, there would be multiple commercially produced pedals available.
    Matthew_84 likes this.
  4. Ahh, that makes sense.... Darn. K, thanks for your response Line6man.

    Yeah, I kind of thought so too. Thanks for the confimation though.
  5. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    Yep, would be awesome but even a dummy coil on the instrument has to be angled exactly right with the pickup or it won't cancel.
    iiipopes and Matthew_84 like this.
  6. Damn, good to know Walter.

    Is there any other way to accomplish something like this? (IE, not using dummy coils).

    I know there are noise gate filters (which are probably the only to get close, externally), but I hate how the hum/hiss comes back when you start playing again.
  7. Noise gates simply turn off the signal when the signal gets below a certain level which is why the hum/hiss comes back when you start playing.

    There's always active noise cancellation but I have yet to see that applied in any onboard circuitry in a guitar or bass guitar. The mathematics behind active (feed forward and feed back) noise cancellation were pioneered in the 50's by one of the gods of electrical engineering (Kalman).
  8. Crater


    Oct 12, 2011
    Dallas, TX area
    Well OF COURSE there is, through the modern magic that is digital signal processing. The Electro-Harmonix company (commonly just referred to as EHX) makes an outboard effect pedal called the Hum Debugger and it digitally removes the noise. It's not a noise gate.
  9. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    isn't that the idea behind the ernie ball "silent circuit"? it actively mixes in the "anti-hum" to cancel the original hum.
    meh, i heard these when they first came out, big weird tone change when you turn it on.
    Matthew_84 likes this.
  10. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    It might be possible to wear a dummy coil that would pick up the hum signal close enough to the bass pickups to work. Hmmm, should I apply for a patent on the dummy coil cummerbund?? In theory it could work but I can't imagine many bassists being interested. Something embedded in a guitar strap might sell but I don't think it would work well. You might put something in a box on the guitar end of your amp cable but you would have to have some way to keep the orientation fixed.

    It is certainly possible to notch out the 60 Hz component of a fast Fourier transform of the bass signal with a DSP, which may be what the EHX pedal does, but I don't know how easy it is is to avoid making it sound artificial. From Walter's comment I suspect EHX hasn't succeeded -- yet.
    Matthew_84 likes this.
  11. The G2 (another name for intell) on the web says the 'Silent Circuit' is based on a dummy coil connected in a clever way to an analog circuit which would cancel 60hz hum.

    Circuits involve active electronics and a dummy coil.

    Active noise cancellation uses feed forward or feed back sensors, some fancy math, and some computers to reduce (statistically random) noise. Not sure it would work with 60 cycle hum - that's not random. That's why noise cancellation headphones still allow voice to get through - voice isn't random, it's deterministic and not cancelled by the headphones.
    Matthew_84 likes this.
  12. You'd sell more if you put the coil in an aluminum foil hat . . . or a fedora.
    Matthew_84 likes this.
  13. The principle in noise cancelling headphones is simply creating a phase inverted signal (of the 'noise') and mixing it in the headphones with the outside, original noise. Its just like phase cancellation in mis-wired bass pickups - the result is essentially extremely reduced low end but high end is still present.

    (Also the manufacturer may additionally limit the range of inverted frequencies to ensure that voices get through for safety, liability considerations.)

    And voice is as random as it gets.
  14. Incorrect in both cases. I'll see if I can get some references to back up what I'm saying. Do you have a background in stochastics?

    The problem with noise is that it's random so you can't measure it then cancel it because any time delay will prevent you from generating the opposite signal - the signal that's present at the time you combine the actual and the generated signal will never match. The math behind random signals does show that we can reduce the expected value of the resulting combined signal but never fully cancel it.

    I withdraw my comment - the OP was interested in eliminating 60 hz hum. Stick to dummy coils and humbuckers. ANC only works for statistically random (stochastic) signals.
    Last edited: Dec 29, 2015
  15. Yeah, I heard a clip on Youtube... The tone is certainly altered when the EHX Hum Debugger is active.
  16. Re: noise cancelling headphones here's an excerpt from the paper linked below :

    "All we need is a way to capture the noise from the outside, invert the signal and output it in time such that we can match the
    crests of the original wave to the troughs of the inverted wave. For this, we require a
    microphone that can convert the noise outside into the form of an audio signal
    and circuitry that will invert the signal and output it through the headphones, illustrated in
    Figure 4"


    or this link among others: How Noise-canceling Headphones Work

    What is the scientific principle behind noise canceling in headphones/earphones? - Quora
    Last edited: Dec 30, 2015
  17. Hmm, it would be interesting if someone found a way to isolate the hiss/hum only, and created an equal output inverted signal of it.

    I suspect this is what the EHX Hum Debugger attempts to do, but likely picks up some of the instrument's signal as well, so that as it cancels out the hiss/hum, it also cancels out some of the instrument's signal, altering the instrument's tone.
    Last edited: Dec 30, 2015
  18. I would imagine it creates severe notches at 60 hz and at the harmonics of 60.

    Its not an expensive pedal so it can't be very sophisticated.
    Last edited: Dec 30, 2015
    Matthew_84 likes this.
  19. RichSnyder

    RichSnyder Gold Supporting Member

    Jun 19, 2003
    Columbia, Md
  20. Aah, that sounds like a simpler way of doing it, and would certainly alter the instrument's signal. Thanks.

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