Is a dimmer noisy when it's turned OFF?

Tim (Tmsorosk) and Pops, in addition to verifying that your dimmers produce no audible noise through your systems (as you have done), you may also want to compare sonics between the situations where the dimmers are dimming say 50% or thereabouts, and when they are switched off.

See pages 36 and 37 of this paper by Bill Whitlock of Jensen Transformers, in which it is indicated that many commonly used dimmers generate "very strong" power line harmonics up to 70 kHz. It therefore seems conceivable that sonic effects may result from intermodulation of those harmonics with the audio signal, which would manifest as low level distortion rather than noise.

I can't cite relevant experience, though, as I have no dimmers in my house.

Regards,
-- Al

06-05-13: Sonofjim
Wouldn't the key be to have lighting and dimmer on a separate circuit or even a different sub panel?

That certainly can't hurt, and certainly may be helpful. But I wouldn't count on it as being any kind of guarantee. There are too many variables involved that have essentially no predictability, including the possibility that radiated RFI may couple through the air to sensitive circuit points within the components and intermodulate with the audio signal, the run lengths and physical routing of the house wiring, the sensitivity of the particular components to distortion of the AC waveforms they receive, and of course the characteristics of the particular dimmers.

I mentioned earlier that I have no dimmers in my house. I do have, however, a floor-standing fluorescent lamp that has a built-in continuously variable dimmer (which is not in my listening room, and is almost always turned off). I just did an interesting experiment with it. I have a battery-powered portable transistor radio that can receive long-wave frequencies (below the AM broadcast band) down to about 140 kHz. I set it to that frequency, and to a volume control setting in the lower part of its range, and positioned it about 10 feet from the lamp. As I varied the setting of the dimmer on the lamp I heard nothing through the radio at most settings. However at a small range of settings corresponding to a moderate amount of dimming, the radio produced noise approaching the volume levels at which I would normally listen to radio stations on that set.

Regards,
-- Al