The other night my Classe amplifiers started producing a substantial mechanical hum. Classe told me that it was likely from DC on the power line. The hum was there even when the preamp was switched to standby, and even when I plugged the amps into different sockets.
My questions:
1) how much DC on the powerline does it take to cause problems with audio equipment?
2) How does DC get into the AC signal on the power line?
3) Do the power companies have any spec they need to acheive for maximum DC?
4) Or is it more likely appliances within my house causing the DC.
Most hum on power transformers is caused by DC on the power line. The source could be anywhere. It could be caused by a poorly designed switching power supply in a Television or a light dimmer. You can DIY a DC filter to take the DC off the power line before the power transformer or purchase something like PS Audio's Humbuster 3.
It matters if you don't want your power supply transformer to hum loudly. If you are trying to be a smart ass, all you have done is made a ass out of yourself.
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