Recently, on the computer at home, I noticed that when the lights in the ceiling of the room are dimmed, the speakers buzz. This doesn’t happen when the lights are off or on full brightness. What causes this?
Must be an inexpensive solid-state dimmer. These use an SCR and some sort of resonant circuit to “chop” off the AC waveform at various levels to reduce the average power delivered to the bulb(s). At full brightness, the SCR is on all the time and full power is being drawn. The buzzing is caused by electrical noise being emitted by the inductors in the resonant circuit. You can try plugging the speakers into a different outlet–one which is on a different circuit breaker than the lights. You might also try one or more snap-on ferrite chokes on the power cord for the speakers. A more expensive dimmer might also solve the problem. Better units have suppression circuitry to keep switching noise down.
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On preview, what QED said.
Would plugging the computer and speakers into a “power conditioner” that outputs a clean AC waveform clear up this problem?
The noise can be coupled through the air and picked up by nearby electronic devices via inductive coupling, so I don’t think that cleaning up the source voltage is going to help. Just walk around the room with an electric guitar and you’ll see that the intensity of the buzz increases tremendously as you approach the dimmer switch and/or the light bulb.
Can you check to see if the ground wire was left unattached when the dimmer switch was installed? The light bulb itself also radiates this noise, and you can’t really shield against that.
Yea, a dimmer switch can generate a lot of EMI. As mentioned by Q.E.D., it’s due the harmonics that are generated when the triac kicks on. You know, Nyquist’s Theorem and all that. The signal radiates from the wiring that’s hooked to the dimmer switch.
I’m pretty sure a lot of the EMI consists of a strong (and obviously varying) magnetic field, in which case it’s very difficult to make a shield that will attenuate it. A better solution is to simply “round off” the high di/dt signal. This is done with a filter circuit built in to the dimmer switch. So my advice is to buy a good, expensive dimmer switch that incorporates a built-in EMI filter, like one made by Leviton.