A friend reports if she plugs her amp into her outdoor GFCI outlet she
gets noise, plugged into any other outlet no noise at all.
can a GFCI cause this? I dont mind replacing it, but havent seen this
trouble before. will plug my oputlet tester in to check it but the home
is recently purchased and passed home inspection, so i doubt its miss
Not for sure ,but it sounds like a possiableground problem or the amp
is picking up some interfernce from antoher device that is on the same
circut. Try unplugging motorized devices one at a time and see if the
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GFCIs inject a half-wave ac signal on the neutral and ground lines, in order
to detect a N-G fault downstream. This is probably causing the noise. The
solution is to use a non-GFCI receptacle for sensitive electronics.
That was my thought too, that the injected signal from the GFCI may be
the source of the problem. But one would think that the signal would
be designed so that it would not cause problems with properly
functioning power supplies in electrical eqpt. So, it seems possible
the real problem may be with the amp.
Manufacturers always try to cheap out on manufacturing costs. Removing/
reducing filtering (that most people won't notice the lack of) is one
way to reduce manufacturing costs.
Eg: "cheap" dimmers make more electrical noise than the premium ones.
This will be the same with GFCIs.
Theoretically, there are certain limits on how far they can go (eg:
FCC limits on EMI), but some manufacturers will skim as close as
they can, and other manufacturers will aim higher.
So, a GFCI from a different "line" or manufacturer _might_ help.
But I tend to agree - sounds more like an inadequately filtered amp.
A line filter from Radio Shack or such like may be the best approach.
It is _remotely_ possible that there's a grounding problem on the
outlet (which in most cases the GFCI would catch, but....). A outlet
test might be a good idea.
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
I actually was a bit inaccurate in my original response. The signal is
typically full-wave pulses (120 hz) zero-referenced by a capacitor. It is
therefore a 120 hz ac signal with sharp points, which produce harmonics. The
higher frequencies will find their way into the amp more readily than the 60
hz power. A marginal amplifier design certainly could make the problem
worse, while a good design might be unaffected. There could also conceivably
be differences in the injected signal from one GFCI manufacturer to another.
Of course, I could be wrong, and one of the other responses could be
correct. The proof would seem to be in the fact that plugging it into
another outlet resolves the problem.
Once upon a time I took an old GFI receptacle apart, and if my memory
is accurate, I remember seeing NO electronics, only magnetic coupling
-- enough to trip the contacts when an off balance current existed.
On the other hand, there WAS a resistor for the test function to
puposely provide an off balance current.
Now I'm sure someone who actually knows will correct me if I'm wrong,
and I will see if I still have the taken-apart GFI at home to look at
My vote on the noise would be position of the amp, maybe there is a
mercury vapor light near the outside plug, or on that same circuit, or
maybe the ground is not properly connected as someone else suggested.
Phil Munro Dept of Electrical & Computer Engin
mailto: firstname.lastname@example.org Youngstown State University
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