While I am busy picking on Class D power amplifiers and their noisy power supplies polluting our AC lines we should not let traditional power supplies off the hook either.
No, in fact, traditional power supplies can be just as nasty as Class D, just different. A conventional power supply gets its power mostly at the peak of each sine wave. For the reason I mentioned earlier about power factor, where the voltage and current do not match. Right at the very peak of each incoming AC sine wave, 100 to 120 times each second, a burst of energy is consumed by your equipment as its supply is topped off. When this occurs there's a related burst of noise generated at the same time. This noise goes right back onto the AC mains. Worse, if there are enough power supplies sharing the same line, from your home or the home of your neighbors, the peak of the sine wave gets mashed down as well. This is referred to as 'flat topping' and the sharp edges of the flat top AC sine wave produce their own type of noise on top of everything else.
So what we know is that regardless of power supply type, switching or conventional, each can generate a lot of noise that pollutes the associated equipment in your home as well as that of your neighbors. That extra noise isn't what you want for best performance. And so one question you might ask, does a standard power filter help remove this noise? The answer's never so simple. Traditional power filters can help with switching power supply noise injected on the AC line because it's typically very high. These filters kick in around 100kHz and start attenuating noise above that frequency. Many switching power supplies run at higher frequencies than that. But these same power filters cannot help with the noise generated by flattoped sine waves and conventional power supply generated noises. For that you'd need a true regenerator.