We got into a discussion about buzzing single-coil pickups over in the lipstick tube thread. Reader Matthew Seniff chimed in with some well-informed observations and remedies. I’ve moved them here so more folks will see.
Another reader had written: “Every time my kids turn on my kitchen light my amp starts buzzing. It’s due to the rheostat that controls it. It’s old but I’m too lazy to change it.” To which Matthew replied:
Actually it is most likely a poorly designed dimmer switch where the solid-state switch (usually a triac) is not being turned off and on at the zero crossing point of the AC line. It could also be malfunctioning. A rheostat is a big wire wound variable resistor that is too expensive and large for most modern dimmer applications, but which used to be used commercially a great deal before solid-state switches had all the bugs worked out of them. When the solid-state switch turns off at the wrong time, it creates hash, which radiates much like a radio station thru the electrical wiring. This is the same thing that creates noise onstage in bars from the lighting systems that have problems.
It is possible to reduce this problem on single-coils by using copper tape to line the single-coil cavities, or even wrap the sides and bottom of the coil pickup. Lining the control cavity and pickguard on a Strat-type guitar can help reduce hum and noise. This stops electrical fields, but not magnetic fields, which is good, since that would impede string pickup. But it’s bad if the noise was coming from a magnetic source that could still be a problem.
There are also issues with grounding the controls and pickups I have found that establishing one ground centrally in the control cavity and keeping the ground wires short and all near the same length can drastically reduce hum in a guitar. This called a “star ground” system.
Seymour Duncan P90 Phat Cats are single-coils inside steel shells like old Gibson PAFs. This makes them very quiet for P90s, but does make them a tad more controlled-sounding. (I like my P90s to be as mean and snarly as possible, but I do still like the Phat Kats even if they are a bit too “polite.”)
There is also a material called mu-metal that is an alloy of nickel, copper, iron and molybdenum. This is even better than copper, but it also stops magnetic fields, so it can’t be used at the working side of the pickup or it would kill the string signal. If you can find the stuff, it will work wonders. We used to use it on car stereo installs where there was a lot of engine noise in the dash wiring and nothing else worked.
A pickups works like a generator. A changing magnetic field around a coil produces current in the coil of wire. A coil of wire can also act like an antenna for electrical fields, like those produced by poorly designed dimmer switches and lighting controllers that work by having solid-state switches that turn on and off during the AC cycle. When the switches are designed properly, the solid-state switch turns on and off at the zero crossing of the AC wave, creating very little noise. But when they are poorly designed or defective, they turn on and off while there is a voltage present, and that causes spikes to radiate out of the electrical wiring.
My dad had a dimmer like this in his house for many years that I finally replaced with a new $15 dimmer, which solved the noise on all the AM radios, my guitars, and stereos in the house. Florescent lights used to be a big problem as well since the ballasts (which are essentially half a transformer) could produce a lot of electrical noise. Modern fluorescent lights use electronic ballasts that eliminate virtually all noise from them. Neon lights sometimes have very crappy supplies that can produce a lot of noise as well.
Finally there is the problem of RF radiation causing problems usually thru audio rectification of the RF by some electronic components. I won’t try to explain that one since it would just give you a headache but like you say above a good ground and an AC power conditioner with RF suppression built in will usually handle it.
Thanks, Matthew! Now I’m going to replace that 10-year-old dimmer switch in my studio…