"mr c deckard"

** 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.


** Do the test I suggested with a length of insulated wire, XLR and
pre-amp. Then think how a voltage is created in the coil of a dynamic, mic
sent down the cable to the pre-amp and is amplified. Then realise that the
connecting cable is just an extension of that same coil.

Recall that a dynamic mic hums when placed near an AC power transformer (
except for those with effective, internal hum bucking coils).



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