DC noise come from live or neutral?

[cent88]

Normally DC noise injected through live, neutral or both?

[richie00boy]

Both, current flows in a loop through live and neutral and can be in either direction.

[DF96]

Neither? DC is not noise, although both are unwanted in a mains supply.

DC, if present, must involve both conductors as richie00boy says.

Noise may come on both or just one, so mains filters handle common-mode and differential-mode noise.

[cent88]

my speakers producing the same buzzing sound as transformer when DC present.

good high current DC blocker too expensive to DIY.

[Michael Bean]

Are you sure you have a problem with DC on your mains? That usually causes the trasformer itself to buzz, not through the speakers. Buzzing from your speakers is more likely to be caused by a ground loop or possibly a bad ground connection between components in the system. If you do have a DC problem, it's not that difficult or expensive to fix it, check out Rod Elliot,s site or do a search right here, lots of good info.

[cent88]

My transformer buzz itself for sometimes(don't know what is the source of DC) and on water heater confirm will cause it to buzz. my speaker only buzz when the transformer buzz, put my ear 3-5cm near to speaker only can hear it.

Yes. I found a lot of DC blockers. This is the one I trusted.
Mains DC and Transformers
most of the high current application DC blocker required high capacitance capacitor equal to high cost and Rod Elliot,s required high inductance, which is very costly isn't it?
I have two audio system here, my pc(750watts)+LM3886(225watts) and hometheater 1000-1500watts.

is there any alternative way to do to the mains instead of DC blocker?

[peranders]

I think you have misunderstood Rod's article. You can never get away with other than high capacitance caps. The inductance circuit Rod has drawn is the equivalent circuit for a transformer.

My solution looks like this.

[DF96]

If DC is the problem then only a DC blocker is the solution.

If noise is the problem then a mains filter should help.

[zigzagflux]

DC is extremely rare, and often misdiagnosed. The presence of 120Hz will make a meter read some DC residual; I can measure this on the secondary of my power transformer (definitely no DC getting through there) but the meter says there is DC present. It can be ignored, and is likely not the source of noise you are describing.

DC should not be present in a power system unless there is equipment connected that does not belong, or a system that is lacking. A DC blocker is a band aid to a larger problem.

You say the noise increases when your water heater turns on ?

[cent88]

@zigzag
most of the time I can't hear any little buzz from the trans.
the noise occur when heater turns on. some one told me before that the heater injecting harmonic 120hz, 180hz, 240hz, 300hz or...into the line.