Do all toroidal transformers hum?

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.

Thanks Gbart, the fact that my B&K MC Pre-amp with external power supply also had a faint hum (again when I put my ear to the power supply), does that indicate a DC presence on the AC line?

Dstandard, where can I find your product catalog?

I made a DC blocker as a favor to an Audiogon member a while back. There are a could of circuits floating around the Internet. Its easy enough for anyone with decent DIY skills to accomplish, just remember you're working with electricity.

http://img.villagephotos.com/p/2003-5/191557/2020464

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Nice work Gbart, unfortunately I absolutely have no knowledge of electrical circuits.