In article ,
"Phil Allison" wrote:
** 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.

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

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.



Regards,

Monte McGuire