It doesn't sound normal to me, at least I have never tried it. The bigger question is "do you often listen this way". If not I wouldn't worry about it.
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The question is, does the slight hum have any practical effect. While the best transformers may not hum having one that makes the slight noise yours does does not mean that it is a bad transformer or that performance is necessarily bad. There are components where everything has been selected with cost no object and they are very expensive. Good designers select components that will do a good job at a reasonable price, if you are happy with the performance otherwise don't worry.
Thank you Stanwal and Tbg, that was informative. The funny thing is that I had Denon POA 2400 (200W/channel) not a sign of hum, but the DK does have a faint hum. I guess I have to live with it as it's not audible 3 inches away from the amp. The Denon had a cheap 2-pront (no earth) power cable, do you think the grounding might be an issue? I had a B&K MC 101 Pre-amp with a IEC power cable and that had a hum too.
How did you get from "can't hear any hum more than four inches away"... to.... "All toroids are inferior?".
If you aren't wearing the amps as headphones the hum issue should pose no significant issues.
Certainly, if you have toroids in your amps and don't like your amps... perhaps only those toroids in your own amps are inferior.
Relax. Don't sweat the petty stuff, but do remember to pet the sweaty stuff now and then.
Stan outlined it pretty well already.
Toroid trans usually hum way less than a standard shape. One point might be to tighten the bolt holding it down. The usual deal for toroids is a bolt down the middle.
By the way, the fact they put in a toroid means they spent MORE MONEY than they had to: the toroid cost more than an ordinary transformer. (for the same voltage/ etc) Of course, maybe they got a deal an a couple tens of thousands of crappy toroids??
The reason for the hum is what is called magnetostriction. You can read about it here.
It may also be sensitive to the tightness of the transformer mounting screws, and to the design of the mounting arrangement, which might influence the degree to which the vibrations are amplified or damped by the surrounding structure.
You may also help by using either coupling or isolating devices under your amp. Although they work on opposing principals I have found both to improve the sound to a surprising degree. I have been using Audio Points from Star Sound but am now using FIM isolation devices [modified] under all components. But there are many others.
A friend who suffered with this problem, only to a much greater degree, told me of this thread. Most of what has been said is fairly accurate. DC on an AC line in excess of 1 volt makes a toroidal transformer hum. Also, as has been pointed out, most transformers hum - magnetic flux in action. However, when it is audible it is excessive. It really has to do with how the transformer was wound. In addition to power conditioners with huge toroidal transformers, we make a DC blocking power cord that has thus far been effective in eliminating hum in the excess DC situations. I am not here to hawk my products, but if you have an interest, just ask.
Amplifier hum problem SOLVED!!!. Gbart was kind enough to make me a simple DC blocker (with high quality Caps and resistors). I received it last night, I plugged the amp into it and VOILA NO MORE HUM. Not even a hint of it. I pressed my ear against the amp. to hear the faintest hum, but nothing, absolute SILENCE. You won't believe how much the DC blocker from Gbart costed... $72. Yes it's just Seventy Two dollats.