Hum Cancelling Question...

[eclipse666]

The Classic I was the traditional Strat shape with 3 single coil Seymour Duncan Alnico II pickups and a fourth single coil hidden underneath the pickguard purely for hum cancellation.

image and quote from www.vintagekramer.com

Can someone explain how a fourth hidden single coil cancel hum?

Edited October 18, 2005 by eclipse666

[lovekraft]

A reverse wound single coil in series or parallel with a standard single will cancel hum, period, whether the reverse coil senses the strings or not. The closer the two are, the better the rejection will be, but it's not critical that they be in contact or even under the strings.

[eclipse666]

Okay thanks, Now is this hidden pickup a good method of reducing hum?

Edited October 18, 2005 by eclipse666

[lovekraft]

Yes, it's an excellent method, but it does have it's downside - running the two coils in series boosts the output level and lowers the resonant frequency (basically boosting the upper midrange rather than the highs), just as it does in a standard humbucker, and a parallel setup does the exact opposite, boosting the highs while cutting the overall sugnal level, which is often percieved as "thin" or "brittle". Either way, it will alter the sound of your sinlge coils, and not necessarily for the better - if you can find a way to get it to sound good, you've got a winner!

[ansil]

hmm midrange notch filter?? a simple 6db drop per octave would put it down to about the same level again wouldnt' it.

[nollock]

There were a couple of threads about dummy coils over at Ampage recently. The general concensus was that you should use a coil with no magnet or pole pieces as they dont aid hum cancelling and will only increase the inductance. You want as little load / inductance as posible in the dummy coil as the greater they are the more it will efect the tone of the pickup being humcancelled.

The conclusion seemed to be that what matters is that the turns*area product of the 2 coils match. And as area increases squared in respect to circumference, a larger coil will cancell more hum and put less load on your pickup. Eg..

1" diameter coil, 900 turns, area turns product = 706sq", 2827" of wire.

3" diameter coil, 100 turns, area truns product = 706sq", 942" of wire.

so each coil will cancell the same amount of hum but the 2nd one uses only 1/3 the amount of wire and hence has only 1/3 of the resistance/load of the smaller coil.

thats what i remember anyway, not sure how acurate it is.

[Saber]

...running the two coils in series boosts the output level and lowers the resonant frequency (basically boosting the upper midrange rather than the highs), just as it does in a standard humbucker, and a parallel setup does the exact opposite, boosting the highs while cutting the overall sugnal level, which is often percieved as "thin" or "brittle". Either way, it will alter the sound of your sinlge coils, and not necessarily for the better - if you can find a way to get it to sound good, you've got a winner! 

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I've never tried the dummy coil, but how can the output be increased if the series dummy coil doesn't pick up string vibration like in a true humbucker? I would have guessed that the resulting output would be almost equal to the lone single coil pickup but with a loss of highs. In series, you add the voltages so if you add the dummy coil's 0mV to the pickup, you get only the pickup's output minus the losses of the dummy coil. Or am I missing something?

And in parallel, I would assume that the dummy coil would just load the pickup and produce very, very low output. Wouldn't it?

Edited October 19, 2005 by Saber

[lovekraft]

Running a dummy coil in series doesn't add any signal, but it does reduce the loading of the working coil, hence boosting the output a bit.

[Jester700]

Running a dummy coil in series doesn't add any signal, but it does reduce the loading of the working coil, hence boosting the output a bit.

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I've heard that before, but it doesn't work that way in the DiMarzio HS series of stacked humbuckers, which are wired in series. Those have no poles in the lower (dummy) coil, and the lower coil is sheilded from the upper. In those, there is loading and decreased output and treble when the bottom coil is connected. Thus, I disconnect the bottom coil. I always use those in combination anyway, so no loss for me WRT hum.

[Saber]

Running a dummy coil in series doesn't add any signal, but it does reduce the loading of the working coil, hence boosting the output a bit.

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Could you please elaborate. I can see how the signal across the pickup itself would increase since it sees less of a load but that's not the signal you're using in humbucking mode. The pickup (string) signal actually goes across a voltage divider formed by the dummy coil and the volume pot (in addition to low-pass filtering from the dummy coil inductance) so whatever output you gain from the miniscule reduction in load is lost in the voltage divider. What am I missing?

[lovekraft]

The signal "sees" the pickup's resistance and the volume pot's resistance as parallel shunts to ground, and raising the resistance of either diminishes the attenuation of the signal (or at any rate, that's the way it was explained to me many years ago). I do however see your point about the voltage divider, so perhaps it's important to wire in the dummy coil between the pickup's ground and the actual ground plane, to avoid it. But you caught me dead to rights - I was simply parroting what I'd been told, without giving it any serious thought on my own! I guess none of us is immune to that!

[frank falbo]

Also on the stacks, like the HS and IBZUSA, your second coil is directly underneath the main coil. That does introduce phase cancellation in the audio realm, regardless of what it's doing electrically. That's why Dimarzio only sends the magnets through the top coil, and they install that cool metal "U" shield around the top coil. It shields the top coil, but also directs the magnetic field more efficiently around the top coil. But if you've ever wired the stack up incorrectly as a single, you can still hear an audible sound, about 1/3rd to 1/2 the output of the top coil when it's wired alone. The magnetic field is definitely passing through the bottom coil, even though the magnets are not. The effective magnetic field extends about 1/4" around. So perhaps if the Dimarzio stacks were made with a 1/4" spacer between the coils, then you'd only be left with the electrical variation, and not the audio one as well. But then the pickup would be pretty tall.

But the stacks would make a perfect experiment for this one. First, install some stacks. Then, disassemble one and use the bottom coil as the dummy coil somewhere else in the guitar. That way you'd have the perfect mirror image coil for your test. (well, within manufacturing tolerances anyway) You could toggle between them and see how much different it is when the dummy coil is totally out of magnetic range, vs. right under the hot coil.

Edited October 20, 2005 by frank falbo