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In article writes:

** 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 is absolutely correct. If you connect the secondary of a
transformer to the input of a mic preamp and connect the primary to an
alternating **current** you will indeed find that AC signal at the
output of the amplifier. If you plug the primary into an AC outlet,
you'll hear hum. If you plug it into a microphone, you'll hear what
the microphone is picking up.

Isn't that the experiment you were describing? The secondary of the
transformer is the loop of wire connected between the input terminals
of the mic preamp, and the primary is the AC power transformer (the
source of an interfering electromagnetic field)?

A transformer doesn't have to have metal laminations and a neat case.
Any two wires sufficiently close together to couple magnetically can
become a transformer.

--
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