under what conditions do two single coils humbuck

[Amplexus]

There seems to be conflicting information as to what will humbuck.

I think these are all the variables
magnet polarity
winding direction
series
parallel
phase reversal both active and passive


also is there any condition where two coils with the same magnetic polarity can be humbucking? I have heard this can be done with inverting op amps.

Does anyone want to have a go at all possible combinations along with the switch wiring, common mode (hum) signal path, and guitar signal paths showing current flow etc.
How many dB is the 60 hz rejection and can this be equaled with a high order 60hz notch filter to make a pseudohumbucker?

also comments about sound, inductance, impedance vs frequency etc are welcome.

[Chuck H]

I won't 'have a go at all the possible combinations' but I will explain the principal behind the humbucking operation. Hopefully this will get your head wrapped around it since it is fundamental in this forums field...

Anytime you have two identical AC signals of reverse polarity at the same amplitude they will cancel each other. This is how white noise generators for modern car stereos work.

So now consider the transducer operation of a pickup. To work it needs a coil wrapped around a magnet. The coil acts as an antennea and picks up stray magnetic field (hum) but also any disturbance in the magnets magnetic field (string vibration). We want to get rid of the stray EMF but keep the magnets information about how it's field is dirturbed. To do this we need to introduce an equivalent signal to the stray EMF without also cancelling the magnets magnetic field information (or at least most of it). Any time two coils are out of phase (reverse polarity = winding direction) stray EMF will cancel. Series or parallel, it doesn't matter, because the same information is on each coil but since they are out of phase any AC from stray EMF will be cancelled. By making two coils magnets magnetic field opposite in polarity in each coil the magnetic fields now "correct" the phase of each transducer and there is minimal cancellation of the disruption of the pickup magnets magnetic field. So the disruption in the coils magnetic field created by the pickups magnets is now actually IN phase and doesn't get cancelled while the stray EMF is out of phase and does get cancelled. This is how hum gets "bucked" but not the string vibration that disrupts the pickups magnetic field. Again, series or parallel, it doesn't matter for hum cancellation. Series and parallel will however effect the impedance and output voltage of the intended signal though.

Take your time getting your head wrapped around this concept. Any time an equal number of identical coils are in operation, and half are reverse polarity, there will be hum cancellation. One trick to getting on top of this concept is understanding that the stray EMF you need to cancel is always the same on any and all coils in operation, but the pickup magnets magnetic polarity can be controlled to avoid the same cancellation.

Good luck. Hopefully someone can put it in simpler terms than I could.

Chuck

[Mark Hammer]

I'll give it a quarter twist and see if that helps.

When two signals that are identical but 180 degrees out of phase get combined, they cancel each other out; like matter and anti-matter. If the amplitude of one is greater than the other (and it doesn't matter which one) the cancellation is incomplete. If the composition of one signal is a little different than the other, the cancellation is incomplete.

The coils in a dual-coil pickup respond to distant electro-magnetic interference arising from the 60-hz AC power supplied to all those transformers around us. They also respond to the perturbations in the up-close magnetic field produced by strings. Happily, it is possible to achieve cancellation of the one without having to produce cancellation of the other. More about that later.

If the coils are reversed such that the 60-hz hum they pick up - as "antennas" - is opposite phase at the output of one coil, then the hum will cancel when they are combined.

This will be true whether the two coils are adjacent, stacked, or two differently-located coils on the instrument. Moreover, this will be true even if the two coils don not have any magnets in them. This is the basis for using "dummy" coils, or innovations like the Suhr backplate system (which is essentially a coil). HOWEVER, since they do operate like "antennas", cancellation is greatest when the two "antennas" pick up the same "station" to the same degree. Remember that differences in the detected hum will result in incomplete cancellation. Coils located apart from each other, even if they are absolutely identical in other respects, will pick up the hum differently depending on where the hum source is located, not at all unlike turning a radio or rabbit ears around to improve or decrease reception.

Do the coils have to BE identical? No. They just have to detect the hum to the same degree, and generate the same hum amplitude. Certainly identical coils positioned adjacent to each other will sense hum to the same degree, but as things like the ultra-low-impedance Suhr backplate demonstrate, what matters is not the congruence between coils, but congruence in the amount of hum sensed.

Again, there is a universe of variation in hum-cancellation between none and complete. For that reason, many players have found that the tonal advantages they might gain from having mismatched coils outweigh the modest loss of hum-reduction that might occur as a result. Whatever hum still lingers in their signal may well be easily addressable by some other means downstream.

So how do you still get string signal when hum signal is being cancelled? Easy. The strings are closest to opposite poles. If the coils were wired in-phase then what was sensed by the polepieces would be opposite phase and you'd get cancellation of the string signal (not perfect cancellation because even being a half-inch this way or that along the string makes a difference in the composition of the string signal). Since the coils are wired opposite phase, though (whether in parallel or series), we have a sort of double negative, right? Reverse-pole/polarity AND reverse phase gets us an in-phase combination of whatever the polepieces detected from the strings.

I don't know if that makes it clearer for you on its own, but maybe bits and pieces from each post , spliced together, will shed the light you crave.

[David Schwab]

I usually put it this way... you have two coils wired out of phase. Any common signal to both will be canceled out.

Then you have the opposite polarity magnets, which also put the two coils out of phase in regard to the strings.

Because the coils are wired out of phase, the signal from the strings is summed in phase, while the noise, which didn't rely on the magnets to be picked up, is still out of phase and is canceled out.

High frequencies are also canceled to a small extent, which is why it's common to offset the coils slightly.

[Amplexus]

I think part of the problem is the often repeated statment that two coils humbuck when out of phase. This is true but there is more to it which may be glossed over in many explanations although the preceeding posts do a pretty good job. A bit more explanation as to the whys the signals are out of phase would still be helpful.
It related to faraday, lenz,and Helmholtz's laws and the right hand rule for coils. It's been a few years since I audited the mit freshman physics class and my recolection is a bit fuzzy. Does anyone have a scope tracing of the string signal and hum signal in each coil? picture worth a thousand words and all that.
I see no way to make a humbucker without opposite polarity magnets (unless there is some clever way to manipulate only the common mode hum signal via active electronics).
I am curious if there is any difference between fliping phasing with an inverting op amp and doing it conventionally.
Also how does a 60hz notch filter compare to a humbucker, how many dB of rejection.

[Mike Sulzer]

Also how does a 60hz notch filter compare to a humbucker, how many dB of rejection.

60 Hz is really not the whole problem; there are harmonics. You would need a comb filter. I am sure this has been done, but I do not have a reference.

There is a bit more to the cancellation than it might appear. You need to get the harmonics the same. With two identical coils, this is easy, but if you are using as a sensor a different type of coil, you have to make sure that it does.

[Amplexus]

Mark
I'm not sure what the strings being closest to opposite poles has to do with coil phasing.
"So how do you still get string signal when hum signal is being cancelled? Easy. The strings are closest to opposite poles."

[Mike Sulzer]

Mark
I'm not sure what the strings being closest to opposite poles has to do with coil phasing.
"So how do you still get string signal when hum signal is being cancelled? Easy. The strings are closest to opposite poles."

The pole pieces magnetize the strings. If the pole pieces of the two coils magnetize the strings in the opposite sense, then the signals have opposite sign. Then the reversed coil polarity restores them to the same sign. And the reversed coil polarity cancels the hum pickup.

[David King]

Remember that combinations of strings don't have to be in phase so all magnets can be the same polarity in a split coil pickup where one coil is picking up the first three strings and the second is picking up the second three.

One signal is inverted and the noise is canceled. Any crosstalk between the adjacent strings that are out of phase will also be canceled.

To have a really even sounding pickup you would want to have individual coils for each string, all magnets the same pole up and all cross talk canceled in the process.

[Mike Sulzer]

Remember that combinations of strings don't have to be in phase so all magnets can be the same polarity in a split coil pickup where one coil is picking up the first three strings and the second is picking up the second three.

One signal is inverted and the noise is canceled. Any crosstalk between the adjacent strings that are out of phase will also be canceled.

To have a really even sounding pickup you would want to have individual coils for each string, all magnets the same pole up and all cross talk canceled in the process.

There are those who claim that polarity matters. I do not hear it, but who knows?

I do not understand how this cancels the crosstalk between strings.

[David King]

Adjacent string signals are out of phase, mix the strings together and any E string signal picked up by the A string coil gets canceled by some the regular E string signal. There is still an artifact of the crosstalk since we've lost part of the main signal as a result of the cancelation. I don't think anyone cares about crosstalk anyway.

[Chuck H]

Adjacent string signals are out of phase

What???

mix the strings together and any E string signal picked up by the A string coil gets canceled by some the regular E string signal.

Thats true enough, in an inaccurate sense though.

The way you put this implies that each strings pole on the pickup is out of phase with the ones next to it. And that is simply not at all true.

For some reason I also get the impression that you think frequency cancellation between two strings played simultaneously is a result of AC signal phase. And this also isn't at all true.

Any cancellation resulting from two strings (or any two notes) played at the same time is always a matter of the frequencies created by each note being incidentally out of phase. It's a strictly physical occurance and has nothing to do with electrical properties.

There is still an artifact of the crosstalk since we've lost part of the main signal as a result of the cancelation. I don't think anyone cares about crosstalk anyway.

Crosstalk between poles doesn't result in cancellation on a typical pickup. The split coil type you referenced earlier, if the two coils are out of phase (I don't know this to be true), then there would be some crosstalk cancellation between the pole signals closest to each other from each coil.

Chuck

[David King]

Chuck,
I'm afraid you didn't see my earlier post in which I stipulated the necessary conditions. I was talking about a hum-canceling pickup with one coil for each string (i.e. 6 coils) and all magnets with the same pole up but each coil alternately in or out of phase with the previous coil.

The way you put this implies that each strings pole on the pickup is out of phase with the ones next to it. And that is simply not at all true.

You are correct in what I was implying. It's not at all true in the pickup you are imagining but it is absolutely true in the pickup I was describing.

For some reason I also get the impression that you think frequency cancellation between two strings played simultaneously is a result of AC signal phase. And this also isn't at all true.

The two adjacent strings need not be played simultaneously. Any string played will be picked up by the two adjoining coils on either side.

I might be all wrong about this but I don't think so. If I'm all wrong please be so good as to explain why.

Sorry for the confusion.

[Chuck H]

No, your not wrong. For the pickup you describe you are exactly right. I don'y think such a pickup would find application on a typical solid body electric guitar. But there are lots of other applications that may work, like steel guitar, dulcimer, etc. Pretty much any other stringed instrument where strings are not "bent" on the fingerboard. Could be a very tunable pickup with very low noise and low output impedance.

Chuck

[Mark Hammer]

The differential phase of the same string vibrating over two RWRP coils is what permits the "Peter Green Les Paul" to exist. Greenie's famed Les Paul has the magnet in the neck pickup oriented in reverse fashion to what normally occurs in a LP. There is no real difference in tone when the neck alone is used, but when the neck and bridge are combined there is a unique sound produced because what is being sensed by the north side of each pickup is a combination of what would typically be sensed by one north and one south...and vice versa for the south.

The fact of the matter is that dual-coil systems that aim to cancel hum need to be in more or less the same location to be able to sense hum equally. Unfortunately, two coils cannot occupy the exact same physical location in space, so they sense slightly different string signals as well. Sometimes those string signals sum pefectly, and sometimes there are small "micro-cancellations" because the partial being detected over coil A has a wavelength that puts it in complete opposite phase over coil B. That's nto a flaw, but rather a part of what gives the particular tonal character of a pickup. It also what makes the tonal character of pickups different, depending on the spacing of coils/polepieces from each other.

Hamer used to make the Phantom guitar (and one or two other models) that used a 3-coil HB, which one could select pairs of coils from, presumably achieving full hum-rejection but with a different coil-to-coil spacing and resulting tone.