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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
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 )
jea48
2,944 posts
 
It is my understanding you cannot measure DC on the AC mains just by connecting your digital meter, set on DC voltage, across the mains.

Not trying to start a flame but

http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

Eva
.
jea48
2,944 posts
 
Blocking Mains DC Offset
casouza
239 posts
 
Jea48, its easy to measure DC on top of AC if one uses a Fluke RMS meter. My Fluke 77 measures it accurately. I see less than 100 mV at most friend's homes.
I have seen a few noisy power transformers (including toroidal ones) when DC measures more than roughly 200 mV.
Best Regards
liguy
228 posts
 
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.

http://www.musicdirect.com/p-2845-ps-audio-humbuster-3.aspx
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