Nick Gorham wrote:
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.

Agreed, I was just pointing out that transformers on their own did
little, and as you say a cap input PSU with cheap rectifiers may well
create move crud than gets in from he mains.

It's almost a certainty! Basically the rectifiers take a sine wave with a
few percent crud or less, and turn it into something that has tens of
percent of THD when meausred.

Not sure however just how many power output stages are driven from a
regulated supply,

Typically there is no formal regulation in the power supply related to the
feed for the output stages..

Under full load, there are usually several volts of AC across the power
supply caps. It's easy enough to measure in most power amps.

The first line of defense is the power supply noise rejection of the power
amp itself. That that this is sufficient to build power amps with all noise
and distortion 90 or more dB down should be noted.

I would have expected the pre stages to be fed from a regulated supply.

That is usually the case. The early stages of the main part of the power amp
circuit often run off the same source as the output devices. In other cases
there is some built-in extra filtering.

http://www.citycom.gr/electronics/pr...l/100ampl5.gif shows a
fairly typical design. Everthing runs off the same voltage source as the
output devices.

BTW, RIP Doub Self's "Amplifier Institute". A sad loss.