Slight hum in my amp. Is this normal?

Atmasphere 5-24-2017
DC on the line occurs when 1/2 of the AC line (for example, positive going portion of the AC waveform) is loaded while the other half is not.

Space heaters or hair dryers on half power do just exactly that. So one half of the AC line can have lower amplitude due to the loading as a result.

But its still an AC waveform and so can travel through a transformer. But the output will still be asymmetrical. The asymmetry can be filtered out by a DC blocker which is a simple circuit consisting of 2 or 4 diodes that are bypassed by some large capacity electrolytic caps.

Ralph, I share Jim’s puzzlement about that. If a waveform that is asymmetrical above and below zero volts is sent into a transformer, I would expect that what would emerge from the output winding of the transformer is a waveform that looks similar (assuming that the frequency components other than DC are within the transformer’s bandwidth), but that has been offset up or down such that the zero Hz (DC) component is eliminated.

Best regards,
-- Al

@almarg , that's true, and if you look at my initial post I mentioned that DC on the line can also be interpreted as a 2nd harmonic. Its pretty clear that filtering using a transformer is an inefficient method; a DC blocker is a lot cheaper!

FWIW, most of the time the reason for DC on the line is an issue that is pretty closely associated with the local AC situation. For example, if there is a distribution transformer serving five houses, only one house might be running a space heater at half power, but all five houses would experience DC on the line.

Thanks, Ralph. To be clear, though, am I correct in believing that to the extent the asymmetry of the waveform corresponds to a 2nd harmonic component, neither a DC blocker nor a transformer would eliminate it?

Best regards,
-- Al

^^ Yes- and that is what we are dealing with when the power transformer in most audio equipment is involved. Its best to block the DC prior to a transformer- otherwise you have to use a very sophisticated power conditioner!

Is your system set up for pure stereo?  Or, are you connected to some video theatre stuff too?

If so....disconnect all the inputs from sources leaving only one source at a tine....cd, phono...etc.

I'm thinking you need to rule out groundloop.