Slight hum in my amp. Is this normal?

I explained again that it was loudest when you first power it up then the hum diminishes after a few seconds but is still audible. He wants to keep power applied to it and observe.
Another thing I just remembered and haven't mentioned, since the hum began, it has been running a bit warm even at low listening levels when it was in my system.

Its 100% normal for any toroid to make a bit of noise on startup. This is because the core of the transformer can become magnetized when the unit is turned off. The next time its turned on, the magnetic field causes saturation in the core and the transformer rattles until the magnetic field is degaussed by the alternating current.

The second paragraph above suggests that the bias of the amplifier is high- which could account for the additional mechanical noise.

^^ 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!

@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.

@jea48 , I thought I did. One more try:

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.

You can have DC on the line and oddly enough, it can be passed through transformers. It could also be considered a 2nd harmonic as basically its caused by loading on 1/2 of the AC line and so the AC waveform can be offset slightly just as if it were riding on a slight amount of DC. The resulting asymmetry could be interpreted as a 2nd harmonic.

Its easy to correct with a DC blocker which is a simple and inexpensive device. DC on the line can cause the core of the transformer to saturate, causing it to rattle or hum. It can vary from one transformer to the next even with parts from the same construction run.

Grounding in the house has nothing to do with this problem even if its not DC on the line. So the age of the wiring isn’t the problem.

Transformers can be mechanically noisy on their own despite the best of intentions. Tightening a noisy part down won’t help and might make it worse. But sometime the part is causing something else in the chassis (like a cover) to resonate and it might be possible to damp that part. If you can only hear the problem when you are within 6 inches of the amp its not a problem. IME, most audiophiles listen to their gear considerably further from the amp they are playing :)

Getting quiet transformers is always an on-going challenge. Transformer manufacturers often don’t understand how important it is for the part to be silent and might sometimes skimp on a few core laminations or the like thinking they can save a buck. In the process they might loose the entire run as reject parts!

Good Luck!