Ron Hardin wrote:
My telephone line developed an AC hum, which, thinking about it, left
me curious how ground loops are avoided.
If you drive two ground stakes into the ground about 20 feet apart,
you can measure about a quarter volt of AC between them. There's
AC floating around everywhere, and no two pieces of ground are the
same potential.
Correct.
The telephone company would then seem to have an unsolveable AC hum
problem,
Why do you think that ? They don't use ground at the consumer end of the line.
and I wondered how they got around it.
By using balanced connections they avoid hum (and other) pickup..
They do ground the phone line boxes every couple hundred feet or so,
though I don't know if that affects the twisted pair directly;
It doesn't. If you grounded one side of the phone cable regularly it *would* hum
since it wouldn't be balanced any more.
but certainly they have the trouble at the miles-distant termination of
the line and your house, as to what potential the ground is at the
two places.
That's why they don't use ground. It becomes irrelevant.
It can't be just isolation transformers, because they need the DC
circuit to detect off-hook, not to mention powering the phone on
your desk, if it's got any IC's.
You need to learn how balanced circuits work. Note that balanced circuits don't
require a 'balanced signal' i.e. a signal in both 'phases', only the line
impedance needs to be balanced.
http://en.wikipedia.org/wiki/Balanced
Graham