On Tue, 24 Nov 2009 19:14:57 -0500, John Molenda wrote:
>Hello all , I am getting a hum on my audio when I go to the SSB
>menu and turn my l.f. rolloff below 200 Hz at 200 and above the
>hum is gone . I am using the Orion II with a Heil heritage mic
>and the mic with the 5 cartridge
Forget about grounding and rewiring things. I suspect that your
problem is nothing more than simple magnetic coupling of 60 Hz into
the mic. When you set the rolloff to 200 Hz, you knock it down a
lot. Any dynamic mic can pick up magnetic fields, either in the mic
coil itself or in its output transformer (if it has one). The
better dynamic mics have humbucking coils to prevent this.
There are two likely sources of that magnetic field. The most
likely is the power transformer inside the Astron. Try rotating the
Astron chassis and see if the hum changes. If it does, that's the
source (and the fix). The other likely source is a wiring error in
your power system that establishes a magnetic field. One common
error is a double-bonded neutral. The neutral MUST be bonded to
ground at the main panel (where it enters your home), but it should
NOT be bonded anywhere else. Such an extra bond could produce that
field (and is also unsafe). Another common wiring error is reversal
of ground and neutral at outlet, or reversal of neutral and phase
at an outlet. These errors can also produce a magnetic field.
If the source of the field is an AC wiring problem, you SHOULD find
it and fix it (with the help of an electrician), just because it
creates an unsafe condition. If the source of the field is the
power supply, I'd either rotate the supply or let the 200 Hz filter
kill it. Besides -- there is NO useful speech content below 200 Hz,
so you're not losing anything by using the filter, and it's good
engineering practice to use it. Professional sound engineers learn
early on that a good low cut around 200 Hz is important for speech
clarity. That's why Ten Tec includes it in their rigs.
73,
Jim K9YC
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