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It is most likely distortion rather than dc on the line. There could be dc current on the line if you have some half-wave rectifier-powered stuff, but it should be low power. The pole transformer would largely remove any dc voltage appearing on the line.

Power-factor correction is basically of two forms and addresses two different issues. Traditionally, power-factor correction involves placing additional reactive components on the power-distribution grid to alter phase to compensate for (generally) inductive loads such as motors. They usually use switched capacitors for this. This is less of a distortion issue than a phase issue.

The second issue for power factor is electronic loads. Diode rectification into capacitors causes current to be drawn in pulses, and this puts a lot of harmonic distortion on the line. This causes a lot of additional power loss, even to the point of severely reducing the amount of power that can be drawn from a (line) circuit. Line harmonics are of such a concern to the power company that end-use power-factor correction is now required in electronic equipment that draws above a certain threshold. This was 75 watts in the US some time ago, I don't know if that number has been made even more stringent recently. Most if not all of the new LED lamps include PFC even though they use much less than 75 watts. If you have a bunch of them, this is a big deal. I have been including it as standard in my amplifiers for about 10 years now. This type of PFC uses electronic means to reduce the distortion by causing the current to be drawn in phase with the voltage and to cause the current waveform to be reasonably sinusoidal rather than pulsating.

I put a significant effort to figure out how to do PFC, even though as a small manufacturer I don't think anybody would really challenge me for not having it. To me it makes sense not to put this distortion in the line where your audio system is because it is likely to affect other equipment in your system. At the very least you could expect buzzing transformers as noted above.

David

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Walt

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Walt

I know you mean that in the nicest possible way.

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I have too much stuff - https://www.pleasebuymystuff.com

While on the subject of DC in AC power line, what DC voltage is considered problematic to torroids?

Just yesterday, I was thinking of building a DC filter. Unfortunately, I dont have an accurate digital multimeter like Fluke yet. $20 auto ranging multi meter reads .1 VDC.

I am getting a faint hum from my preamp. Are EI-Core power transformers also susceptible to DC voltage? I am suspecting its either DC or the EMI from power transfomer to the inductors. They are physically too close.

Shall try turning the inductors and see if that helps.
Any suggestions of EMI shield material?

At line frequencies or other low frequencies the best shielding is distance. That is why I try to put transformers in a separate housing away from the audio circuits. In with the transformer is the rectification and one stage of filtering so I am sending DC over the interconnect which will reduce radiated energy. If you can't do that then placement and orientation of the transformer can help. If none of those work, high permeability materials such as mu metal can be used. Here is a useful link:

https://en.wikipedia.org/wiki/Electromagnetic_shielding

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Walt

Noise of any sort in a system is one of the biggest PITA. As Walt says you just have to gut it out. Last weekend I had a friend put a new piece of equipment into his system and he had a noise problem that was not there when he had it on the test bench. Imagine that! Something worked differently when introduced into the system. I told him that he would just have to look at all the possible points that noise could enter but there was no way I could diagnose it remotely.

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Walt

In my house in Takoma Park in the middle 90's I had noisy backgrounds. When I was checking that all my equipment was properly wired to the mains, I discovered a potential between house ground and neutral.
Not wanting to fool with the mains wiring I just sunk another ground rod and ran a low ga wire from the new ground to my system ground. This cleaned up the noise and gave me beautifully quiet background.
I have since then become braver in working around the mains wiring and one of the first things I did when I moved in this house in 2003 was to check the quality of all connections in the mains breaker box. I found many loose and dirty connections. I feel this should be SOP, especially for anyone concerned with thier music system.