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In article writes:

** That not a correct example.

A bad penny always returns. Welcome back, Phil.

RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce such
signals at the inputs of the balanced pre-amp.

However, what doesn't get caught by those crude mechanical methods
will be reduced by common mode rejection. That was a bad rebuttal. Try
harder next time you want to get theoretical in a practical world.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the main
aim is to eliminate ground hum from audio systems. A ground hum voltage will
appear equally on the two wires and so be rejected.

In theory it can operate in any frequency range, even DC. Did anyone
say anything about audio here?

** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a hum
signal in differential mode that the pre-amp *will* amplify - its CMRR has
no effect.

Whoa! This is EXACTLY where common mode rejection is useful in audio
circuits. It's what lets us connect microphones with zip cord. The
reason why it doesn't work as well as we'd like it to is that it's
rare that the noise voltage at both inputs is rarely exactly the same,
so there will always be some difference, which will be amplified. The
difference is most often due to imbalance in the source, or different
loop area of the two wires in the cable. We work hard (sometimes) to
make these as good as we can, but it's only perfect in theory.

This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked
up every inch or so along the line and hence cancels it out. Where multiple
twisted pairs are used in the same cable the twisting reduces crosstalk in
the same way as above.

A very twisted explanation. I suggest that anyone really interested in
the theory behind this statement read the book about cable written by
Steve Lampen of Belden. It's pretty easy to understand.

"Star Quad" cable uses four twisted wires instead of two to enhance the
effect of the twisting and virtually eliminates induced hum problems even
when used near to high current AC cabling.

This is correct.

Knowing who I'm talking to, I feel compelled to make this statement.
I've offered the correct answer, explained to other readers why your
response isn't quite correct, and tried really hard not to make you
look like a jerk this time around. That's all I have to say on the
subject until someone changes it.

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo