Large toroidal transformers are particularly susceptible to being 'mechanically' noisy when the mains supply is asymmetric and/or has a DC content whereas many EI transformers appear less effected. The 3000VA transformer in my last power amplifier buzzed like an angry wasp at times. I'm amazed at the cavalier attitude of some amplifier manufacturers to these occurrences i.e. Its nothing to do with us attitude. Whereas some other manufacturers embrace the problem and build DC filters into their amplifiers, i.e Bryston, Crown, Krell (I believe) whilst some others do provide a DC filter option to be either retro-fitted or on request with selected amplifiers from Classe etc. Considering almost all advertise low signal/noise ratios, quiet cooling fans (where fitted) and seem to ignore this very concerning and potentially noisy problem. At the elevated price point we pay for high-end amplification I'd have expected the amplifier designers to have covered all of the bases. Perhaps I'm being a bit hard in my attitude as it doesn't occur in all cases, however it is still a problem and remains so!
A point to ponder, does DC filtering of the mains have a negative impact or not?
Thats my rant over with for the time being.
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