Remember that at the transformer outside your house, at the secondary feeding you, is a very low impedance point with extremely low noise. The transformer is a large step down in voltage. As it steps down the voltage, also the disturbances on it are stepped down. As with any transformer, its wound to be most efficent at 60hz and is much less so to higher freq disturbances.
The point being that much less is actually coming into you from outside than from inside! If a neighbor or two is fed from the same transformer, the low impedance of the secondary largely, not totally, keeps you rather uneffected.
So why the noise...why the need for filtering? The noise present on the line is mostly all self induced locally. The big noise makers are of two varieties...the big current hogs like the air conditioner/heater, washer and dryer, kitchen appliances and then the growing problem child...the whacked waveform noise generators! By that, I mean the cruel SCR's light dimmer variety and the noisy power supply devices that are such a growing problem.
You see, the rectifier stage is a very noisy, hash generating device. And it's not our nature to think about the fact the power line isn't a one way device. We always think of the power going in...but the transformer there is not a one-way device!
Take the cheap wall wart power supply brick. It's a step down transformer of cheapiest possible sourcing. Followed by a rectifier stage and filtering. So the attached device is after the filter and advantaged by the physical distance and it's typical input stage additional filtering. But back on the AC side of that wall wart, the clipped nasty generated by the diodes turning on and off is right there at the secondary of the transformer. So if the transformer is a, lets say, a 120v to 12v winding, then the transient generated by the sudden switching of state is now not just coupled back onto the line....but actually stepped up by 10 times! Thank goodness the transformer is at least somewhat inefficient at higher frequencies! But keep in mind that as the cheapest of cheap transformer was dropped into that unit, that its not going to be well designed and likely no effort will have been made at lowering noise coupling. Shielding....???? Ha!!
So now, the noise stepped up is riding on the household AC wiring. Looking for some poor audiophiles equipment to mutilate!! Well, you're thinking that my uber expensive equipment has a marvelous power supply and it's going to render that AC power line into a perfect pool of quiet DC before it goes to the audio side of the unit. Right? Well, now realize that a power supply is quite good at removing low frequency ripple. Typically designed for 120hz (or 100hz) filtering, the problem is that the high frequency noise coming in is many magnitudes higher in frequency!
So when you hear the thing about "how can the last six feet make any difference when you've got miles upon miles of wire feeding you", thats a very basic lack of understanding of the situation. And we really haven't discussed the nature of signals when they get up into the RF region and how they can couple without the direct connection we think of with the AC line.
Hope everybody has and has had a great day!! Enjoy the music!! :rocker:
OldMonolith
