Re: Speaker hum and electrical safety
I think you need to be absolutely clear about the source of this problem: is the hum noise generated in the electronic circuit of the amp (or CD or whatever) or is actually an artifact of the mains supply wiring in your room?
Originally Posted by Hu
For example: I have been playing with different power amps at home in my tiny 3 x 4m listening room recently. I noticed that there was a small hum on the left channel of one amp (right channel was clean, just the usual tiny hiss). I could live with that hum, no problem. On the next amp, there was a hum on both channels. Actually, it was not a hum, it was a buzz; I could live with this since it was only audible when I put my ear about 3cms from the bass unit. On the third amp, there was a strong hum on both channels.
It turns out that there were several problems and I had to tease them apart. First point to resolve was that the amp was plugged into a mains wall socket that was on the other side of the room. You may think (as I did) "so what? It must be served by the same electrical ring-main, and what is a distance of only about 5m between wall socket A (powering the CD, and preamp) and wall socket B, (powering the power amp)". Both the preamp and power amp were mains grounded.
But you and I would be wrong. There is a microscopic difference in the potential of the earth pins in wall sockets A and B due to the finite resistance of the bonding earth cable between them. To put this into perspective, that small a voltage is about the same as the voltage from your TV aerial that reaches your TV receiver: very difficult to measure but obviously there! So, the potential difference between socket A and B may only be millionths of a volt - surely insignificant - but it confuses the amplifier which will amplify the difference and then delivered to the speakers as a low-level hum or buzz.
I decided that I'd bundle up all three amps and send them back to their manufacturer's excellent service dept. (total cost ?400 to repair, check and calibrate). I then brought all the equipment mains to ONE point; this killed the ground (hum) loop.
Moral of the story:
1. As I have said here before, amps do age. The big smoothing caps (made with a wet paste) do dry out, and they let mains hum through onto the speakers. Only solution: replace them periodically - say every 10-15 years or so.
2. Always - always - connect all your audio equipment to ONE wall socket (or a pair of sockets if they are together on the same moulding).
3. Never, ever, remove or tamper with safety earths as a way of breaking ground loops. This could be FATAL. If the equipment is double insulated and does not have an earth connection that's fine, but if the equipment has an earth pin it needs one to ensure that the case is safe sould there be a mains leakage inside.
Alan A. Shaw
Harbeth Audio UK