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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
atmasphere's avatar
atmasphere
11,615 posts
 
Hmm. So, with the AC line having one side at ground, there's supposed to be a DC level? The idea that a hair dryer can shift the ground potential to a DC level seem far fetched- it would require that the neutral side of the AC line be not connected to ground.

Distortion, OTOH, is the sort of thing that both Nelson and Eva were describing.
jea48
4,310 posts
 
What does one AC line conductor, (neutral conductor), of the electrical service intentionally connected to earth have to do with it.

The main reasons for connecting the service neutral conductor to earth is for lightning protection and to establish one common ground plane.

Distortion, OTOH, is the sort of thing that both Nelson and Eva were describing.
How does the DC blocker filter block out the distortion?

Eva has many follow-up posts throughout the running of the thread. Like I said in my earlier post she ran hands on experiments and tests.

If I get a chance tomorrow I will visit the archives on AA. I remember John Curl posting about DC voltage on the mains.
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Jim
atmasphere's avatar
atmasphere
11,615 posts
 
Well Jim, I'm certainly open to finding out I've been wrong about things. Its how we learn :)

As far as the earth connection, what I am talking about here is that the hot side of the line is measured with respect to the neutral (ground). So if there is a DC component present, the AC voltage will be shifted with respect to the neutral. *That*, since the neutral is tied to the earth connection, will take some current and will likely heat the wiring up in the process.

Now if you have as an example Eva's hair dryer, running on half power so it its only conducting when the diode conducts, I can see that that might be interpreted as DC. But, it can also be interpreted as harmonic distortion. I'm not arguing, IOW, that the phenomena does not exist, just the explanation. One could as easily argue that a 2nd harmonic in the output of an amplifier is a DC phenomena as well.

I would like to see Eva's scope readings, but the images don't load off of DIYaudio.
jea48
4,310 posts
 
Ralph,

I am not dismissing what you are saying about odd harmonics. They can and do play havoc on AC power systems.

They can cause the secondaries of power distribution transformers to run hotter than normal. And yes be mechanically noisier than normal.
sbrownnw
686 posts
 
Gbart, could you make a dc line filter / blocker for me? If so, can you email me at: white 78 911 at hotmail dot com
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