Mutual Inductance in Sidewinders

[Joe Gwinn]

Joe never specified whether "series aiding" is the way that the pickup is supposed to be used in the guitar, though.

At the time, I assumed that M would be positive, and computed accordingly. That said, depending on circuit and physical arrangement, the mutual inductance M may enter the circuit equations with a negative sign. In transformer theory, the sign of M is how one encodes the polarity of a winding.

As for the requirements of side-by-side humbuckers, one generally wires the coils series aiding, which means that the overall inductance will exceed the sum of the inductances of the individual coils by twice the mutual inductance. This is true for both music and hum signals. What differs is the orientation of the DC magnetic fields through the two coils: string motion is detected by variations in the DC field, but hum fields are independent of the DC fields.


In another posting it is suggested that stacked humbuckers are different, that the coils must be connected series opposing to ensure cancellation of the hum field. I'll have to think about this. It's a plausible first-order theory, but stacked humbuckers have all manner of complexities with flux-directing shields and conductors, so it may not be all that clean. For instance, in some designs, the DC fields go through upper and lower coils in opposite directions, while others use iron plates to divert hum fields away from the upper coil.

[Mike Sulzer]

At the time, I assumed that M would be positive, and computed accordingly. That said, depending on circuit and physical arrangement, the mutual inductance M may enter the circuit equations with a negative sign. In transformer theory, the sign of M is how one encodes the polarity of a winding.

As for the requirements of side-by-side humbuckers, one generally wires the coils series aiding, which means that the overall inductance will exceed the sum of the inductances of the individual coils by twice the mutual inductance. This is true for both music and hum signals. What differs is the orientation of the DC magnetic fields through the two coils: string motion is detected by variations in the DC field, but hum fields are independent of the DC fields.


In another posting it is suggested that stacked humbuckers are different, that the coils must be connected series opposing to ensure cancellation of the hum field. I'll have to think about this. It's a plausible first-order theory, but stacked humbuckers have all manner of complexities with flux-directing shields and conductors, so it may not be all that clean. For instance, in some designs, the DC fields go through upper and lower coils in opposite directions, while others use iron plates to divert hum fields away from the upper coil.

It is useful to think in terms of linear superposition to cut through the complexity. The dc field can be a factor in the permeability the cores and it magnetizes the string. Then the ac field, small compared to the dc, affects the system independently of the dc. Thus the dc field through the bottom coil of a stacked humbucker is not an issue for determining polarities.

[David Schwab]

It makes sense that a sidewinder would have more high end per turn, but the EB-0 wasn't exactly noted for its high end. It sounded somewhat like a fart at the bottom of a 200 foot well.

Well that had a ton of wire on it! I find I have to wind my sidewinders hotter than my regular humbuckers to get the output level and low end to match. Otherwise they are very thin sounding.

[David Schwab]

In another posting it is suggested that stacked humbuckers are different, that the coils must be connected series opposing to ensure cancellation of the hum field. I'll have to think about this. It's a plausible first-order theory, but stacked humbuckers have all manner of complexities with flux-directing shields and conductors, so it may not be all that clean. For instance, in some designs, the DC fields go through upper and lower coils in opposite directions, while others use iron plates to divert hum fields away from the upper coil.

There are very simple stacked pickups as well. The EMG SA is still made as two coils stacked with a common bar magnet running through them.

[fieldwrangler]

The EMG SA is still made as two coils stacked with a common bar magnet running through them.

Sounds like it, too...

[fieldwrangler]

So, do we think that in general the voltage of AC signals generated in sidewinder-format pickups add up to less than the expected sum of the voltages that would have been generated if just one of them had been present under the vibrating string with the same magnetic elements? If there is such an effect, is it frequency-dependant (let's assume equivalent coils on a common core here.) I feel like this shouldn't follow directly from the less-than-sum inductance effects we've been discussing, but this talk of interactive transformer-type effects has me thinking beyond my level of comprehension.

Bob Palmieri

[fieldwrangler]

Plus, do we now think that sidewinders will exhibit less variation in tone between series & parallel (in-phase) wiring modes?

[David Schwab]

I haven't used the sidewinders in parallel much. I tried it, and from what I remember they sounded kind of thin. In single coil mode they sound very similar to in series.

When I get a chance I'll do some recordings.

[fieldwrangler]

Haven't been able to get over to my bench to look into this but the reason I speculated about the less difference between series & parallel modes in these designs is that I'm thinking there's likely less difference in total inductance between the two modes.

Bob Palmieri