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a big sigh from me too

Jack, your discussion of shielding misses my point that *unshielded* twisted pairs are not nearly as effective as we might hope in an unbalanced system. You are correct that twisting send and return conductors minimizes the loop area, thus minimizing inductive coupling, but it does nothing for capacitive coupling in an *unbalanced* system.

A balanced system is one where both conductors see the same impedance to ground. They both pick up the same noise in the same phase; the noise is all common mode. When those two conductors are applied to a differential amplifier (or a transformer with an ungrounded primary) the common mode noise is rejected or 'canceled.'

If one of the wires is connected directly to ground it picks up virtually no noise while the signal carrying wire picks up plenty. In this case the noise is not common mode. It is differential mode and cannot be distinguished from the 'good' signal. It gets amplified right along with the music.

You are correct that balanced lines have a long history. My understanding (could be wrong) is that it started with telephone lines. They have always been unshielded twisted pairs. They have always been balanced and originally made use of transformers at both ends of the line. When was the last time you heard any 60Hz hum on your telephone even though the signal traveled dozens or hundreds or thousands of miles over unshielded wire sharing the same poles with power lines? If the system were not balanced you would probably hear nothing but hum. (Ok, phone systems are also bandwidth limited; they don't go down to 60Hz anyway, but still.)

"Since you've brought up the link of UTP. All LAN cables, e.g. Cat5e or Cat6 are U are UTPs - Unshielded Twisted Pairs."

Exactly. And the computer networks for which they are intended are balanced systems. If they were unbalanced they would not work without shielding. In fact, they probably wouldn't work very well even with shielding. Certainly they wouldn't work with many lines bundled together; each would require its own shield.

At the risk of being tedious, I'll repeat it one more time: noise suppression with unshielded twisted pairs is seen in balanced systems, but is almost entirely lost in unbalanced systems like a single ended amplifier.

-- Dave

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