On Thu, 15 Jan 2009 10:33:56 -0600, Tim Duffy K3LR wrote:
>I wonder if there is a problem with your AC house/shack wiring? Missing
>ground at the breaker panel or a connection between the ground wire and
>neutral missing or miss wired?
As I observed much earlier in the thread, this problem has the symptoms of
magnetic field coupling because Gary has described it as pure 60 Hz hum, NOT
BUZZ.
If power system wiring is correct, the magnetic fields associated with power
wiring will be contained between the pair that carries the power -- that is,
the phase (hot) wire and the neutral. In other words, there shouldn't be any
of that magnetic field around for our wiring to detect. The most common
error that causes the field to spread out is a connection between the
neutral and the equipment ground. Another common cause is a LOAD connected
between phase (hot) and the equipment ground (green wire).
The other common source of magnetic fields is the leakage flux from big
power transformers, like those in a linear power supply for our rigs. It
could also be from some other big transformer or motor in the home. If you
turn this stuff off and still hear the hum, the power system is the place to
look for problems.
About 20 years ago, one of my broadcast engineer colleagues was moving into
a newly constructed studio complex. He found that a big audio console that
had been problem-free at the existing studio complex had bad hum that
changed as the lighting in distant rooms was turned up and down. After
considerable troubleshooting, he tracked the problem to mis-wired neutrals.
The neutral for the transformers serving different parts of the building
were interchanged, generating a very strong magnetic field over a wide area.
About five years ago, my colleague Neil Muncy was troubleshooting a hum
problem in a studio. He identified the root cause as neutral current from
another building going to ground on plumbing pipe under the floor of the
studio. The power system was the type commonly known as High Leg Delta.
The only other potential cause I can think of is leakage (capacitive or
resistive coupling) from the hot side of the power line to the audio circuit
that has the hum. Tim -- do you know enough about the circuitry to know if
this might be a possibility?
73,
Jim Brown K9YC
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