There are many ways that noise can be added to the power lines. Large industrial motors, generators on the system, etc. However, when in your home, it can come from florescent lighting, refrigerator motors, microwaves, etc. You can actually scope the power line and see noise on it. A very good piece of equipment will have inherent internal power supply filtering that will remove the vast majority of this noise when converting to DC. Some have large coils in the power supply circuitry also that help with this. Your friendly neighborhood power company will acutally test your home system (if you complain and ask) and see if noise is present, or low voltage, or flickering (yes voltage flickering). Here is how you fix this. 1) remove all bad lighting,2) get a decent power conditioner for your low level electrics to plug into as I mentioned previously. I wouldn't swap around circuitry in the home. This is really a non-issue to me. If one is careful, run dedicated lines (with separate and unshared neutrals and ground conductors), proper interconnect cabling helps. I'm the first house off the power transformer for my area, so I have great voltage. Four dedicated lines for my system (which includes separate ground and neutrals for each dedicated line), CD, TT, Tuner, Pre-amp, Phono stage, DAC, electronic crossover all plugged into a very nice power conditioner and that is plugged into a dedicated circuit. Two stereo amps each plugged into its own dedicated circuit. Result, dead quiet.No ground loop, no refrigerator noise, no microwave noise, no lighting noise, no flickering, no voltage sagging, just music. However, I do believe it is important to not share neutrals or grounds on lines for the music system.