How Much DC is OK on a Power Line?


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.

Thanks, Peter
peter_s
Thanks Gbart. I'm using Classe Omega amplifiers. I just measured my outlet for DC, and am seeing about 200 mv. I had an air compressor plugged in that night, and maybe it went on and the motor increased my outlet DC?
Power regenerators do NOT remove DC. I use a PS Audio Premier Power Plant in conjunction with the Humbuster III to eliminate the DC on the line that is not addressed by a regenerator.
I've been told that a buzz/hum from an amp can be caused by "odd harmonics on your home AC". Is this the same as 'DC on the line' as Peter in first post was told ? And if different what sort of conditioner is best to 'cure' this?
By the way, I'm using an Environmental Potentials home surge protector / conditioner as discussed inthis thread. As it is on a breaker, I can take it out of the system.
Oh - and the question still remains - how much DC offset is problematic on an AC line for equipment?