Probably it´s a good idea to put a noise filter before a switched power supply, but, as always,it´s hard to generalize; it depends of the characteristic of the filter and the frequences of the noise that the supply sends back to the net.
I use several ferrite rings:toroids, of different sorts
(materials) and different sizes and on different locations,on most of the signalcarrying cables, as well as on power cords, and that to a very good result(don´t use them on speakercables).Be warned, this is a science that takes a lot of time to get it righ and to manage.Therefore; I can just see that some people doesn´t recommend it! It´s a lot of thinking and trials involved in it, to get it right.
But, for me it has lifted my system;-along with other tweaks; as RC-links across the speaker terminals and decoupling the speakers from the floor,by using special soft feets,to a level that can compete with far more expensive systems!
I use several ferrite rings:toroids, of different sorts
(materials) and different sizes and on different locations,on most of the signalcarrying cables, as well as on power cords, and that to a very good result(don´t use them on speakercables).Be warned, this is a science that takes a lot of time to get it righ and to manage.Therefore; I can just see that some people doesn´t recommend it! It´s a lot of thinking and trials involved in it, to get it right.
But, for me it has lifted my system;-along with other tweaks; as RC-links across the speaker terminals and decoupling the speakers from the floor,by using special soft feets,to a level that can compete with far more expensive systems!