Clean mains supply for the audio equipment

[Kiriakos GR]

Clean mains supply for the audio equipment this is a common request in our days.

Back in time (40 years ago), when I was using sound mixer console:
The low end products producing..
a)  An portion of hum.
b) at EQ mode, a portion of Hiss.

In out times, lots of people makes the mistake, to get sound equipment these powered by a switching power supply. 
 
Quick answer and suggestion.

For sound mixers and pre-amplifier stages, they might be helped due in-line isolation transformer.
Low End Hi-Fi gear this is poorly designed, an poorly designed sound amplifier this cannot be improved with small corrections.