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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

Showing 1 response by rayooo

rayooo
70 posts
 
I struggled with this issue for quite some time a couple years back. IMO Gbart has summed the situation up nicely.

'being an electronics hacker, I constructed a DC offset blocker for AC line, and all was fine.

I have noticed for instance that in the case of Bryston, at least the amps I have looked at in fact have this DC block built in. ( I don't own Bryston, it's just that they publish schematics on their web site, and the circuitry is shown )
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