Persistent ground buzz

Dimmers work by shortening the mains frequency half cycle so that the power to the lamp is reduced. The circuit delays the switch-on by an amount programmed by the control knob -- it delays the gate signal to a bi-directional semiconductor switch called a Triac -- often a Diac, a bi-directional equivalent to a zener diode is placed in series to effect a voltage drop to the gate. Once the Triac gate voltage reaches the threshold set, it's gate "fires" the semiconductor switch, and rather than having the natural sinewave "ramp" over the 0.01 seconds of half cycle, the firing puts the full voltage to the load at a point part way through the half cycle. The light bulb filament is inductive and that tries to oppose the current driven into it. The result is "ringing" which although a snubber circuit is commonly used, still puts some spike onto the mains wiring. The instantaneous spike if repetitive -- i.e. a frequency -- will be at radio frequencies, and therefore highly transmissive. The mains wiring acts as transmitting aerial and the magnetic phono cartridge acts as receiving aerial due to its inductive properties. A series of spikes is received and amplified by the phono stage and corresponding system components. These are not however hum, but spikes which repeat at twice the mains frequency (100 or 120Hz). Some dimmers claim zero-crossing but the Triac inherently switches off at the zero crossing point awaiting its next trigger anyway. True zero crossing would lead to full-on all the time and no dimming. Some expensive dimmers provide a different solution where so many full half cycles are passed whilst some full half cycles are off. However, this type can look like the lamp is flickering, and if the flicker were to occur around 11Hz, it could trigger an epileptic fit in normally healthy people.

The best solution is to use a combination of lower and higher powered lamps and switch to the lowest power when you require to be in dull lighting.

I have had the dimmer buzz problem in the past. In the last 25 years I went for two light fittings (ceiling lights for bright and low wattage wall uplighters or similar), which is similar to Graham's low/high wattage bulb solution.

Guess I'll just have to get rid of the dimmer then. I'm just glad to have found the source of the problem at last