less hum: switching power supply?

[Haible Juergen]

>    > Switchers may not be the panacea you are looking for. 
> 
Several people pointed out that, and I totally agree. If you cannot get
a 50 (60) Hz supply quiet, you won't succeed with a switcher either.

Taking this as a starting point, I was quite surprised that my favorite
new toy, a Quantec Room Simulator, apparently has a number of
DC/DC converters (which normally are switchers ...) to derive various
analogue supply voltages from the digital supply. (Which is, rated at 
5V / 8A, a switcher, too.)

The reason for the DC/DC converters is the complete isolation
of both analogue input channels, and the output channels, from
each other and from the digital stuff. Supply voltage isolated
by the converters, data from ADCs and to DACs isolated by
opto couplers. they took this approach to emulate the behaviour
of input / output transformers with electronic means, without the
drawbacks than normally come with transformerless symmetrical
connections.

So it's *possible* to build ultra quiet devices even with switchers
(this was the world's best digital reverb in 1983, and some say
it still is today), but if you don't have design goals as these, I'd
say linear regulators are the way better solution. 

In theory, you can filter remnants of a higher frequency easier
than 50Hz: Capcacitors can be much smaller, and inductors
would be of small size as well.
But in practice you're dealing with electromagnetic fields
and these will couple into sensitive parts at higher frequencies
more easily, so the noise has the tendency to just bypass
your electronic filter circuits.

Bottom line: Switchers can be a necessary evil when high powers
are involved, or for special requirements as in the example above,
but if your only goal is to avoid noise, this would be much easier with
a linear regulator.

JH.