How Hum-Cancelling Works, Part 2

In the last part of this article, we tried flipping the direction of current and the magnetic polarity in pickups to see what effect it had on the signal and hum being picked up by the coil. We found that flipping the direction of current inverted both the hum and the string signal, but flipping the magnet’s polarity only inverted the signal from the strings.
I’m sure you’ve jumped ahead of me here, but what about if we do both these changes at the same time? If we flip the direction of the current, then we invert the hum and the string signal. But then if we also flip the polarity of the pickup’s magnet, we will invert the signal again, meaning it goes back to how it originally was, but this time we don’t flip the hum.

Great – now we have a signal we can combine with our original pickup which will cancel the hum, without affecting the signal! You can see on the graph that the two hum lines are exactly opposite to each other. If we combine the two “total” signals from the two coils, we get all the string signal and none of the hum.

At this point it’s worth pointing out that we can never really achieve a level of cancellation this precise. The only way to do this would have the two coils be exactly physically identical to each other, and for them to be in the exact same physical location at the same time, which is clearly impossible. Therefore the noise and signal picked up by each coil will be slightly different and the there will still be a tiny bit of noise, and the string signal will not quite be exactly the same as if there were only one coil.
The only consideration left is whether to connect these two coils in series or parallel. In series, the waves are summed. In parallel, the waves are averaged. This makes no difference to hum cancellation, because (for example) the sum of 5 and -5 is zero, and the average of 5 and -5 is also zero.
In a normal humbucker, it’s normal to wire the two coils in series. This is because it further increases the signal to noise ratio – while the hum is inverted against itself, the string signal strength is effectively doubled.
On a guitar with single coil pickups, however, when two pickups are connected at the same time (for example in the 2 and 4 positions on a Strat), they are connected in parallel. This makes sense because otherwise the signal strength would be twice as much in the “in-between” positions, when two pickups are connected, as when only one pickup is active.
I hope this has demystified how coils can be combined to cancel hum without cancelling signal. This knowledge helps us when we are planning the wiring of a guitar, and we are trying to use the properties of the various coils available to minimize hum.
If you’ve got any questions over what’s been covered in these last two articles, just post a comment below and I’ll answer as soon as I can.

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  1. hello, thanks for the very interesting basic explanation. But reality is sometimes not entirely following the theory : in my experience hum cancelling pickups never cancel 100% of the hum, some are better than others. And also, when connecting 2 identical pickups in series or parallel, reversing the connections of one of them, the sound is not totally cancelled – what you get is the infamous “out of phase sound” where certain frequencies cancel out and others remain. This sound is generally considered to be “lo-fi” but it certainly does have a distinct character that might work well in certain circumstances/situations.
    So here’s my question: it would be really nice if you wrote an article on how signals sum (in series) and/or average (in parallel) depending on the 2 pickup positions between the bridge and the neck. Most humbuckers are just 2 coils one next to the other: since they are not in exact same position, the guitar signals won’t just sum up exactly: there will be a slight phase shift. More rarely, humbuckers are of the stacked type where there is another coil in series just under the single coil: in this case the coils are one above the other, hum cancels but the guitar signal doesn’t because the 2nd coil is a dummy coil (just like in split coils). Here the purity/precision of the note is better maintained. Oh but I’m sure you will explain all this much better than me ;o) cheers !

    1. Well, a humbucker is generally much louder than a single coil, but the sound isn’t ‘doubled’ like you would hear using a delay pedal. You just have twice the coil sensing the string vibration which translates to a louder sound.

  2. I’m making pups for single strings that are two coils, back
    to back on a single magnetized alnico pole piece. I have achieved hum
    cancellation by winding one coil in the opposite direction and wiring them parallel
    but out of phase. I have tried many combinations of wiring and, to my ear, the
    hum cancelling configuration has a weaker signal than a single coil or in series
    configurations. Is this the best I could expect from a back-to-back-on-one
    pole-piece setup? Is there a particular
    wiring configuration that will yield both power and hum cancellation?

    1. Are you saying that you are making individual pickups for each string? I don’t now if I understand what you are doing here…

      1. Yes, for one string. Picture one magnetized alnico pole piece, with two coils, one at each end of the pole. the south coil is wound opposite the north coil. I realized that I got a stronger signal in series but wasn’t sure if it was hum bucking and if I had the configuration and wiring correct for hum cancellation.

        1. I have actually never tried a setup like that, so I would be interested to hear your results.

  3. Got confused on this part. Can someone explain?
    On a guitar with single coil pickups, however, when two pickups are connected at the same time (for example in the 2 and 4 positions on a Strat), they are connected in parallel. This makes sense because otherwise the signal strength would be twice as much in the “in-between” positions, when two pickups are connected, as when only one pickup is active.

    1. The signal for each coil is not going from one coil to another (in series), but in parallel, where each coil has their own separate path to the switch. This is the normal way 2 pickups are wired when both are on.

      1. So for single coils that are apart, they should be in parallel so that signal strength is equally distributed? Did I understand it correctly?

  4. Hi, I wondered how can you wire the guitar for coup splitting to try to avoid the drop in volume which this article implies will happen when going from series himbucker to single coil?
    You could wire in series the humbucker but you are taking a hit on signal to noise, and potentially having higher hum… is there a way to keep the series wired humbucker and switch to the single coil yet keep the power?
    I was thinking if you had two humbuckers, if you switch to single coil and wire these two such they are cancelling each others hum but do so in series you would maintain a similar output from them as if they were humbuckers in series. Although you still would have a drop off in output when you switch to a single pickup in this configuration..?
    Is he interested to hear any solutions to this conundrum!

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