In message , Dave Plowman
writes
In article ,
Nick Gorham wrote:
A well designed mains transformer will attenuate anything outside
about 50 Hz by a considerable amount - if it didn't, decent audio
transformers wouldn't be so expensive. ;-)

I have seen normal mains torroids used as valve output transformers.
Thay do a adiquate job, so they must let through (in that situation
anyway) 50hz.

I must admit to not having tested a toroidal - it's ages since I looked at
this. But if it's letting through significant amounts of 'interference'
that the normal smoothing and decoupling doesn't stop within the audio
band, I'd assume you'd hear it.


Don't forget that the toroids used in audio amps tend not to have
inter-winding screens. The inter-winding capacitance also tends to be
higher than that of E-I core transformers. They therefore let through
pretty much all the common-mode noise on the mains.

I know that neutral is bonded to ground at the sub-station, but in many
places the common-mode on the mains is horrible (not helped by all the
CM crap coming out of SMPSUs, even those with power-factor correction).
A good common-mode filter will clean things up a lot.


--
Chris Morriss