vgizzi, steakster has a great overview above. The only quibble I have is the main improvement we get with a dedicated line is nothing to do with isolation, it is all from the wire running continuous. Most circuits are run outlet to outlet, with each outlet adding a number of connections and each connection adding a lot of micro-arcing and noise. So that is the main benefit, and they should be called direct instead of dedicated. But whatever.
People talk about noise in terms of obvious stuff like a blow dryer. The number one thing to keep in mind is every wire is an antenna. RFI is everywhere. So every wire is an antenna bringing RFI into the system. The AC can be somehow perfectly clean, yet the first inch of wire coming out of it starts picking up RFI. This is probably one reason power cords make such a difference, but whatever. Not the point. Point is noise is everywhere, so the fixes must be everywhere, there is no one silver bullet.
Transformers work primarily because of the way they are designed. The alternating current in the primary creates a rising and falling magnetic field that induces a current in the secondary. There is no electrical connection, just the alternating field. The same happens by the way with the output transformers in an amplifier, or a SUT for MC cartridges.
Ever notice the difference high quality transformers make in these cases? It is huge. Why? Because it is hard to design one to work across a wide range of frequency. The alternating field in a AC isolation transformer is designed to very efficiently pass 60 Hz AC. But higher frequencies not so much. Most of the line noise we are trying to eliminate is RFI, radio frequency, very high. Any old transformer will filter RFI just fine. It is just not that hard. Once you understand the principles on which transformers work it is pretty easy to understand why they are used all over the place.
That answers your transformer questions. And your dedicated circuit question.
As for the stuff you tried, the proof is in the pudding. It does no good to ask what if. Who cares what the transformer at the end of your street is doing? To know you would have to move your stereo to the house up the street. Or run a line from before that transformer. To what end? Why? Such questions are academic.
What really works is to deal with what really works. Sorry, but if you try something and hear no difference then by definition it doesn't really work. All the stuff I am talking about, it really works. You will hear it, and not just barely either.
People talk about noise in terms of obvious stuff like a blow dryer. The number one thing to keep in mind is every wire is an antenna. RFI is everywhere. So every wire is an antenna bringing RFI into the system. The AC can be somehow perfectly clean, yet the first inch of wire coming out of it starts picking up RFI. This is probably one reason power cords make such a difference, but whatever. Not the point. Point is noise is everywhere, so the fixes must be everywhere, there is no one silver bullet.
Transformers work primarily because of the way they are designed. The alternating current in the primary creates a rising and falling magnetic field that induces a current in the secondary. There is no electrical connection, just the alternating field. The same happens by the way with the output transformers in an amplifier, or a SUT for MC cartridges.
Ever notice the difference high quality transformers make in these cases? It is huge. Why? Because it is hard to design one to work across a wide range of frequency. The alternating field in a AC isolation transformer is designed to very efficiently pass 60 Hz AC. But higher frequencies not so much. Most of the line noise we are trying to eliminate is RFI, radio frequency, very high. Any old transformer will filter RFI just fine. It is just not that hard. Once you understand the principles on which transformers work it is pretty easy to understand why they are used all over the place.
That answers your transformer questions. And your dedicated circuit question.
As for the stuff you tried, the proof is in the pudding. It does no good to ask what if. Who cares what the transformer at the end of your street is doing? To know you would have to move your stereo to the house up the street. Or run a line from before that transformer. To what end? Why? Such questions are academic.
What really works is to deal with what really works. Sorry, but if you try something and hear no difference then by definition it doesn't really work. All the stuff I am talking about, it really works. You will hear it, and not just barely either.