** 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.

Hi Phil, could you explain this is more detail? I always thought that
a magnetic field induced a current (or voltage) into the two wires of
a mic cable (for example) in equal magnitude and angle. If I
understand correctly, this isn't the case, rather, the hum induces a
current/voltage in one of the wires as a postive going signal, and the
other as a negative going signal. That is, they are equal and
opposite in polarity.

Magnetics begins to get a bit beyond me, but I understand its
importance in what I deal with everyday.

Thanks, Phil.

Chris Deckard
Saint Louis, Mo.