I recently bought a Hegel H400 and have also experienced a slight hum, normally not audible from more than a foot or so away, but very occasionally it's just loud enough to hear a very low-frequency hum from my listening position. A DC blocker hasn't helped. Apparently, this is completely normal for a Hegel when the power supply is sub-optimal. Their own website has the following info:
If you are hearing a mechanical hum from the transformer and this also varies in loudness, it is nothing wrong with your amplifier.
Hegel amplifiers have very large unwrapped toroidal transformers. Unwrapped transformers are more prone to picking up electrical interference. Still, we have chosen not to isolate/wrap our transformers, as this would seriously degrade the sound quality of our amplifiers. The humming noise does not harm the amplifier in any way, but it can be annoying.
What you are hearing
The noise you hear is the vibrations in the windings of the toroidal transformers. Imperfections in the copper wires allow the windings to vibrate. We, of course, use the best quality copper and production methods in our transformers, but on a microscopic level, the wires are still not 100% round. That means that even if the windings are completely tight, they can vibrate, and make this humming sound.
Where does the noise come from?
It's the transformer that picks up other sources of noise on your home's electrical system. Typical culprits are products such as induction stoves, fridges, adjustable lighting, heaters, etc. These create an uneven load and thus generating DC back into the electrical system, something the transformer in the Hegel can pick up.
How to solve it
If you hear this hum try to troubleshoot by removing all other electronic components from the mains circuit the amplifier is connected to, to see if the sound disappears. If you can’t find the culprit you can use a DC filter/blocker between the amplifier and the mains outlet. This will remedy the problem.
