Intermittent toroidal transformer hum--any ideas?

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I have a Unison Research Unico integrated amp with an intermittently humming toroidal transformer. It's not a ground loop problem, as it hums whether there's anything plugged into the amp or not (and I can see and hear that it's the transformer humming when I take the cover off to look inside). I tried to get rid of the hum by switching to other outlets in the house, as well as using different power cords (stock unit and a Harmonic Tech Pro AC-11.
What's really weird is that as I turned the unit on and off (while switching outlets or PCs), I found that sometimes it hummed and sometimes it didn't, and that the volume of the hum varied, too. In fact, I got all excited because the first time I switched PCs I heard no hum and thought that I had solved the problem. Unfortunately, this was short-lived excitement because further switching on and off resulted in either some quantity of hum or no hum, regardless of which PC or outlet I used. There just seems to be no rhyme or reason as to when or why it hums.
For whatever it's worth, I also tried putting some thin foam under the transformer in case it was a vibration thing causing the hum; this did not seem to change the situation at all.
Any ideas?
Thanks,
Jon

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Topic - Intermittent toroidal transformer hum--any ideas? - Jonathan 13:30:35 02/22/04 (2)

Humming toroids, what to do - Ole Lund Christensen 08:28:26 02/23/04 (0)

Re: Intermittent toroidal transformer hum--any ideas? - Dan Banquer 15:06:15 02/22/04 (0)

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