[Puma Cat]

Tony is 100% correct. While noise can enter a system from the AC "mains", most of the noise comes from the full-width bridge rectifiers in the power supplies of the components themselves, most notably the amps and preamps....and since it is alternating current, the current as well as the noise, goes BOTH WAYS: INTO the component and OUT of the component*

So, the noise from your amps can enter into the power supplies of much lower current source components e.g. phono stages, DACs, and disc players. Which can add fuzz, buzz, blur and slur as well as significantly impact dynamics of the presentation.

This is why a good power conditioner/distributor is key as well as using noise-reducing power cords for powering the components: to keep the noise from both the AC mains and the noise from the full-width bridge rectifiers in the power supplies themselves from getting out of the component power supplies, only to be distributed to the rest of the components in the system.

*– I'm always surprised at the no. of audiophile friends I have that think that AC "flows" in only one direction: from the wall receptacle into the component's power supply, like water from a hose. Many of these folks also think that signal is carried in the signal cable like water flowing through a hose, or like marbles (i.e., electrons) rolling through a tube. This latter perception is also completely inaccurate, the signal propogates along the conductor as an electromagnetic wave.