@markhammer643

3 years ago

Nice and concise.
Let's extend this a wee bit and explain dummy coils. A dummy coil ALSO senses EMI out of phase, resulting in cancellation of hum. However, lacking any magnet/s it does not sense string signal, only EMI. Combined with an actual-string-sensing pickup, you still get hum-cancellation. The degree of hum-cancellation depends on the relative sensitivity to EMI of dummy and string-sensing coil. Humbucking pickups stick two identical coils beside (or sometimes on top of) each other to maximize cancellation. But dummy pickups can work in locations other than jammed up beside a sensing pickup, and don't need to be "just like" the pickup. I installed a dummy coil in the control cavity of a buddy's Telecaster, and it reduced hum substantially. Not perfectly but enough to make life liveable. The Ilitch/Suhr backplate for Strats runs a dummy coil along the perimeter of the plastic cover on the back, over top of the vibrato springs. It has to be "tuned" to achieve maximum cancellation in conjunction with the actual pickups.

Note that if one uses a push-pull switch to cancel a coil in both the neck and bridge pickups, you need to be careful/mindful of which coil is cancelled in order to retain hum-cancellation when both pickups are used. Naturally, one loses hum-cancellation if using only one pickup in coil-cancelled mode. And while each dual-coil pickup may be nicely hum-rejecting on its own, if using one coil from each of two humbuckers, hum-rejection may be less than perfect (but still good enough) if the two pickups are somewhat dissimilar.

Finally, combining a single-coil PU with a humbucker may let in a wee bit of hum. One of the HB coils will sense hum just like the single-coil PU does, while only one of the HB's coils senses it opposite phase. Again, any time the selected pickups/coils pick up more EMI in this phase than that phase, you get less-than-perfect hum-cancellation. As long as the resulting hum is far enough in the background, though, I think that's okay. If one was making a recording, it might not be.

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