I bit the bullet and removed L4 and L7 this morning. It is not difficult;
they are both on the back of the front panel board. I jumpered the pads
with wire. I am now getting superb audio reports from critical local
stations that were guiding me in reducing RF on my audio. My MC-60 mic is
back in good graces.
It works.
Monty K2DLJ
Even in balanced audio systems, the same rules apply. Had the K3 been
designed with a truly balanced, 3-stage instrumentation input for its mic
pre-amp, the inclusion of L4 and L7 on the shielded return paths would
have the same effect. The saving grace in an instrumentation-input
circuit (or in the alternative, an audio transformer input) is the
inherently large common-mode rejection ratio (CMRR) across a very broad
frequency span that limits the presence of RF on a twisted-pair audio
line, even in the total absence of the cable shielding. For nearly 100
years, the Bell System and its progeny have used unshielded twisted-pair
balanced audio systems in the presence of outrageously-high RF fields with
no measurable detriment to performance in many instances.
Paul, W9AC
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