OT: Dummy coil - has anyone done this and does it work?

[Rob Strand]

the hum-rejection comes from the two coils sensing the same amount of hum.

Yes, you want to match the sensitivity to external fields but not the physical shape of the pickup.
However, if the do match it physically and locate it close to the pickup, as a Humbucker does, it will gives the best cancellation.   When you play with the coil shape and position, the field that passes through the pickup can be different from that passing through the humbucking coil.

Consider two perfectly matched pickups connected in series but with the coils are connected out of phase.  If the two pickups are close the effect of external magnetic fields cancel.  However take the extreme case where one pickup is placed near a transformer and the other remains 1 meter away: the hum cancelling doesn't work.   The degree of cancelling you need boils down to the relative distances from the source to the two coils.   If you want to cancel a field to -20dB from something 1m away then you probably wouldn't want the coils to be displaced by no more than 0.1m.   This example regards distance but you can also get similar mismatches regarding shape.

I suppose the motivation for adding adding a dummy coil is to provide a humbucking mechanism for single coils.  The motivation for not using humbuckers is they sound different.   

One proposal is to modify humbuckers as follows:   

The idea is to allow the bucking coil to be bypassed at high frequencies without bypassing the lower frequency hum signal.   The idea is to put a R + C series network in parallel with humbucking coil.   The R and C need to be fine tuned.  I'd say an R of about half that of the pickup's resistance, then tweak the C, which will probably end up in the 1n to 10n zone.   You want to use the smallest C possible to ensure the hum cancelling in minimally affected.

[ashcat_lt]

The motivation for not using humbuckers is they sound different.   

My point for much of this thread has been that the dummy coil make sound different in mostly the same way and for mostly the same reasons that a humbucker does.  With a dummy coil, you get the humcancelling and the tonal change, but you don't get the extra output, which technically actually works to reduce the potential benefit in S/N ratio.  A humbucker cancels noise the same, but the desired string signal is still about twice as loud, so the S/N is about 6db better.

Your idea with bypassing one coil of a humbucker will not make it sound more like a single coil.  I have this option on a couple guitars.  It's a different sound, but it's not at all like an S/C.  I mentioned above, though, about bypassing the dummy coil with a cap.  That might come a little closer, but you'd have to try it.

[Rob Strand]

My point for much of this thread has been that the dummy coil make sound different
...
With a dummy coil, you get the humcancelling and the tonal change

That is true!   The impedance of the hum-cancelling coil is the root coase.  The idea of bypassing the hum cancelling coil is to reduce that difference as it bypasses the hum-cancelling coil at high frequencies.  Ideally you would have two single coils one sensing the strings and one as a passive humbucker (ie. hum cancelling but not string sensing.) then bypass the passive one.

in mostly the same way  and for mostly the same reasons that a humbucker does.  ,

The sound of a humbucker also comes from the fact it senses at two points on the string.  It inherently has a wide aperture.  See

http://www.cycfi.com/2014/08/virtual-pickups-part-3/

but you don't get the extra output, which technically actually works to reduce the potential benefit in S/N ratio.  A humbucker cancels noise the same, but the desired string signal is still about twice as loud, so the S/N is about 6db better.

Yep.

Your idea with bypassing one coil of a humbucker will not make it sound more like a single coil.  I have this option on a couple guitars.  It's a different sound, but it's not at all like an S/C.   I mentioned above, though, about bypassing the dummy coil with a cap.  That might come a little closer, but you'd have to try it.

The classical humbucker is fairly symmetrical in that it senses the string more or less equally on the each coil; perhaps a tad higher on the side with the screws.  Like this it has to sound like a humbucker and the cap bypass idea will not work.  The only way is to make one of the coils more string sensing and one more like a passive hum-canceller (passive coil).   Once you get enough asymmetry the bypass cap should work.

There are quite a few "improvements" to humbuckers nearly all of them introduce asymmetry between the two coil halves.  One ends-up being more string sensing and the other ends up being more dummy hum canceller.

I have not tried the idea (as I use single coils!).

[snap]

seems like it has been done before, indeed: http://www.diystompboxes.com/smfforum/index.php?topic=79479.msg655913#msg655913

[Mark Hammer]

When you have a SC pickup that has been designed to sound the way it does, on its own, then there is a reasonable chance that only a very specific type of dummy coil design can reduce hum without deleteriously affecting tone.

Of course, as Chip Kinman has amply demonstrated, there is nothing that stops one from designing a SC pickup in anticipation of an additional hum-sensing coil, such that the second dummy coil does not detract from the pickup's tone.