Make sure the Astron power supply is under the desk or otherwise remote
from the rig by at least a couple of feet. Be sure to use the same
gauge wire as provided by the rig manufacurer for power leads and twist
them (or use large gauge wire twisted).
You are not changing the mike stand, but just changing a cartridge in
the same stand? Clearly when you roll off at 200 Hz you are excluding
60 Hz and its harmonic at 180. Of course for best SSB comms
intelligibility, you should restrict the voice passband to 300 Hz to 3
kHz. That is ALL that is needed for punch thru audio. (If it was good
enough for the phone company for 123 years, it should be good enough for
hams.)
Some hams ground the negative terminal of an Astron to its case, but
that gurantees a ground loop with your antenna coax shields, and will
lead to lightning damage to the power supply. (Been there saw that with
a club station grounded by an ex power plant guy who "wanted a good
ground on everything).
The lightning surge came down the coax shield and arced across a rig to
the negative power supply lead, and then returned to ground thru the
Astron case and AC ground pin of power outlet----what a mess! However,
if that cures your hum, you might do that until you track down the rest
of the ground loop. Be sure your mike cable shield is securely
connected, and a bad connection is not the cause of the hum
introduction. Are you using a Heil mike stand, or some other brand
converted to the Heil element?
Change one thing at a time when looking for ground loops and hum problems.
GL,
Stuart Rohre
K5KVH
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