On 7/21/2020 6:50 AM, Phil Allison wrote:
Note: The internal symmetry of a co-ax cable is the secret to its excellent mag field rejection. Equal and opposite currents inject into the core and shield from a nearby AC mag field - so complete self cancellation.
The ONLY real advantage of a 2 wire "balanced" mic line is when used for carrying phantom power to a mic or device that needs it ( eg a DI unit).
With the assumption that the input is differential, what you're losing
is common mode rejection. You're relying on the construction of the coax
to be all you need to keep EMI from getting to the destination through
the cable. A differential input fed from an accurately balanced source
is like "belt and suspenders."
A lot of the "unbalanced hums" netlore is a function of improper
grounding and shielding in the source and destination devices. Much of
this has been sorted out since the AES devoted a whole issue of the
Journal in 1975 to ways to build stuff right so you don't get EMI picked
up by the cable shield to the guts of the destination device via a
current path through what's supposed to be "ground."
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