On Thu, 15 Apr 2004 10:13:39 -0400, "Arny Krueger"
wrote:

Nick Gorham wrote:
Dave Plowman wrote:
In article ,
Mike Gilmour wrote:

What constitutes 'dirty' mains anyway? What about utilities
signalling systems, mains borne baby alarms, intercom systems etc,
not as invasive as Stewarts suggestion of a neighbouring welding
shop but pollution never the less. I'm way out the in country but
'scoping my mains showed evenly spaced spikes... tracked down to an
electric fence a third of a mile away. ;-)


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.

The transformer is the secondary or tertiary line of defense. The power
supply itself carries the major burden.

A typical power supply for audio gear has about 15 VAC on the secondary of
the transformer, which includes a lot of power line and trash. The
rectifier actually ups the trash content by quite a bit so we're still
talking something like 15 volts of power line and other trash at the input
to the filter cap(s). Most audio gear filters the mess with a simple
capacitive filter, which brings the trash content down to a volt or two.
Then, the DC+trash goes through a commodity voltage regulator chip that
costs $0.50 or less. The trash is now under 1 millivolt. Total attenuation
gets us from 15 volts or more of trash to less than one millivolt or trash
or more than 80 dB of attenuation.

On top of the good the power supply does, most audio gear has additional
power supply rejection built into the circuitry itself. There is another 20
or more dB of trash attenuation there, so total trash attenuation is 100 dB
or more.

In every case the power line frequency is the predominate source of trash.
Most of the means used to deal with the trash are very broad band, so the
basic process of keeping power line hum away from the output terminals also
nails the other kinds of trash.

Nice explanation. Keep it up.