Dual coils magnetically coupled vs not

[Mark Hammer]

When one has a "classic" humbucking pickup with two RP coils that are magnetically coupled underneath such that their tops are opposite magnetic poles, the "sensing area" is essentially between the tops of the polepieces, correct?

When one has a "classic" single coil with a top-to-bottom magnetic orientation, the sensing area is essentially between the top and bottom, but since the top is closest to the strings there is a "bias" in the frequencies being sensed.

If one has two SC pickups side by side, each with a reverse magnetic polarity, but not coupled magnetically underneath, is the sensing area still on the sides? And if so, just exactly how close can they get to each other without the inside section between the two pickups turning into a kind of dead spot? In other words, if I had two SC pickups wired as a humbucking in a PAF space, is their sensing area essentially the outside of the each coil? Or is it also based on a sort of "through-the-air" magnetic coupling that creates a smaller top-to-top sensing area?

Part of my curiosity in this regard are things like the old Fender humbucker, which WAS effectively two uncoupled SC pickups in a HB format, and the oddball arrangements like Robbie Robertson's old Strat with the two side-by-side SCs in his Strat (as seen in "The Last Waltz").

I'm just curious about the extent to which the sensing (and sonic) properties of a pair of SCs can mimic a PAF-style HB. Of course, the astute will realize that because of the magnetic coupling of the polepieces under the coils, simply cutting one coil of a PAF to achieve a SC sound will change the inductance and such of what is still in circuit, but the sensing area is still between the tops of the polepieces.

[salvarsan]

When one has a "classic" humbucking pickup with two RP coils that are magnetically coupled underneath such that their tops are opposite magnetic poles, the "sensing area" is essentially between the tops of the polepieces, correct?

Mostly, yes, if the static simulutions we did a few years back have any merit. The magnetic "aperture is wider than the magnet but drops off rapidly.

Steve Kersting put up some screen captures from the FEMM simulations.

http://www.skguitar.com/SKGS/sk/Imag.../Magnetics.htm

I'm just curious about the extent to which the sensing (and sonic) properties of a pair of SCs can mimic a PAF-style HB. Of course, the astute will realize that because of the magnetic coupling of the polepieces under the coils, simply cutting one coil of a PAF to achieve a SC sound will change the inductance and such of what is still in circuit, but the sensing area is still between the tops of the polepieces.

The magnetic permeability of the whole magnetic circuit of humbucker screws and pole pieces is what determines inductance. I would guess that pair of skinny P90's would better represent a PAF.

-drh

[David Schwab]

You are really only sensing out the top of a pickup, since there are no strings under it, so having the field extend out the bottom isn't serving any purpose. Leo Fender's last few pickup designs had a U shaped steel plate under the pickup to point the flux field back up towards the strings.

I think the pickup would be more efficient with the magnetic circuit closed at the bottom. If you put two single coils close enough the fields are going to couple anyway. As the coils move farther apart this becomes less practical ... like with a ceramic magnet Music Man bass pickup. But you could add a steel plate between the two ceramic magnets.

The old Shergold/Hayman guitars had a steel plate that connected to the bottoms of the three single coil pickup's magnets.

Obviously you can do it with or without closing the magnet circuit on the bottom, and each will probably sound a bit different.

Seth Lover said he didn't want the screws extending out the bottom of the pickup because that was wasting some of the magnetic flux out the bottom of the pickup, where it doesn't actually sense anything.