"Monte McGuire"...
"Phil Allison"

** 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.

Actually, it's current that gets induced by a changing magnetic field,
not voltage. This current acts upon whatever impedances are present to
then create a voltage.


** Wrong - a voltage is induced. The current that flows depends on the
impedances, conductor cross section etc. The lower they are the more the
current - but this is not relevant to the problem since hum voltages are
what get amplified and heard.


Yeah, it seems like a subtle point, but it completely determines if and
what sort of interference will be present.


** That just compounds the first error.


If the balanced line has
balanced impedances on both sides, then all the interfering voltage
created by the interference current will be common mode and could be
completely cancelled out by an ideal receiver. If there is an impedance
imbalance, then some of the interference will result in a difference
mode signal, one that can never be removed by any balanced receiver.



** Wrong - the loop formed by the two signal carrying lines is *the
circuit* in which the hum voltage is induced. This is in differential
ode - same as the wanted signal.


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.

Yes, twisting makes the loop area effectively zero, so there's no mutual
coupling and thus no induced current and thus no induced voltage.


** A balanced audio line that is **NOT** twisted is just an induction loop.



............... Phil