Tech Tips: All about hum-cancelling
15th April 2020
All about hum-cancelling
You may know that some pickup designs work to reduce background hum and general interference from external sources (the common ones being mains hum and radio frequency interference). Also, some pickup sets are designed so that several pickups can work together as a team to create the same effect. This is achieved by using the design of the coils in the pickups to your advantage.
Each pickup coil works like an antenna to pick up interference from the outside world, so one coil on its own is pretty defenseless against this noise, no matter how you design it. However, when you have 2 coils which you can pair together, you can design the coils so that one coil is wound in the opposite direction to the other. This kind of creates one "positive" antenna and one "negative" antenna, so, when presented with a single source of external noise, the hum created in one coil will be an exact mirror image of that from the other coil, so when you merge the signals from those coils together, the two opposing hum signals will cancel each other out.
So, humbuckers are designed with one coil connected in reverse, to create that mirrored pair and Strat pickup sets are available with the option for a reverse wound middle coil, so that the bridge+middle and middle+neck positions on the pickup selector will also result in that mirrored pair and the hum-cancelling effect.
So, why doesn't it cancel out the sound from the strings too?
Although the hum created from external interference only concerns an interaction with the coils of the pickup, when we pick our strings, there is another stage involved in creating a signal in the coils: the magnet.
By giving each of the coils an opposite magnetic polarity, the disturbance felt by the pickup when you pick a string is opposite in the south coil compared to in the north coil, so, rather than mirroring the signal, like it does with external noise, the opposing pair of coils actually takes the signal which is already mirrored by the magnet and brings it back into unison.
I hope that gives you some useful info about what's going on under the hood! As always, if you have any questions about choosing the right pickups, please drop us a line.