Re: 60hz hum

On Sat, 15 Aug 2009 16:00:29 -0700 (PDT), Dookie
<abstract.dissonance@xxxxxxxxx> wrote:

Is all, at least 95%+, 60hz hum from ground loops or is any
significant portion to due some type of radiation? I have a
significant amount of hum in my amplifier even though I have turned
off almost all electronic devices without any reduction in um. The
florescent light is significant if the amp or guitar is near it and it
is on.

Oh, Electric guitar. An electric guitar and associated amplifier,
with its magnetic pickup and high-impedance unbalanced electrical
connection, is suceptible to just about ALL possible sources of hum.

But if you have nothing connected to the amp but your guitar (and
maybe some battery-powered stomp boxes in between), it's NOT a ground
loop problem. If you've got anything else connected to the amp,
disconnect it and see if that improves the situation.

The flourescent light could be generating it by induction from the
ballast coil to the guitar pickup, or by radiation to either the
guitar, the amp, or the cable between them.


All I can think of is that there is some relation between the guitar
and amp as the closer the guitar is to the amp the louder the hum.

Exactly. If it changes with position or distance between guitar and
amp, it's the power transformer generating a magnetic field that gets
picked up by the guitar pickup.

With the guitar unplugged the amp is very quiet. All I can think of is
that the pickups are picking up radiation from the power transformer.
Any other possibilities?

Yes, other things can cause problems but from your description the
magnetic "radiation" is surely the main culprit.

As for other possibilities, does the amp have a three-terminal
power plug with a ground pin? (this is for USA, not sure what the
outlets are like in other countries) Does the ground pin actually plug
into a three-prong outlet with the ground pin properly wired up?

Get one of these (not neccesarily this exact device from Harbor
Freight, but any equivalent outlet tester):
http://search.harborfreight.com/cpisearch/web/search.do?keyword=outlet+tester&Submit=Go
and plug it into the outlet to help answer that last question.

If the answer to either of the questions above is "No" then not
only could it add to the hum you hear, it could be dangerous. Have
that fixed.


What can be done about this?

As others said, humbucking pickups will help, but humbuckers give a
different tone and some guitarists may insist on the single-coil
sound. I've seen a way to put a "dummy" coil with no magnet in a Strat
(or any guitar with single-coil pickups) to cancel stray magnetic
fields without changing the tone - this search URL looks promising:
http://www.google.com/search?q=stratocaster+dummy+coil

The transformer has a metal case which
should act as a shield?

Yes and no. A steel case will shield some of the magnetic
radiation, but not a lot of it. To do that, you need a shield of
mu-metal which is quite expensive.

Replacing the transformer with an equivalently rated toroid
transformer would help a lot, but that would also be expensive.

Also, what would cause the amp to radiate 60hz
hum? The only non-audio ac that exists is before the rectification and
at the heater filaments. How does the 60hz get radiated? I do realize
that wires can emit radiation through changing electric fields but
surely the strength should be quite low?

You're right, the magnetic fields from the wiring is very low. If
it gets louder as you move the guitar closer, it's surely the power
transformer.

The guitar still picks up the
hum pretty loud 5 feet away.

That sounds like a lot further effect than I've seen. Equipment
(guitar amps, older synths, anything with a 60 Hz power transformer in
it) generate 60Hz magnetic fields that tend to drop off rapidly
between 2 and 4 feet or so. If you rotate the guitar (so the neck
points at the amp or away from it), does the hum reduce? There may be
an angle where it "cancels" fairly effectively.

Is the hum still there at much further distances from the amp, such
as 10 feet away? If so, it's picking up 60 Hz magnetic hum from some
other device's transformer (probably indicated by the level changing
when you face the guitar different directions), or it's picking up
radiated magnetic or electrostatic hum from somewhere (like a
fluorescent light), instead of or in addition to the magnetic field
from the amp.


What can I use to shield the hum? A thick copper or steel case around
the amp?

In general, it's not practical to shield the amp, but perhaps it
has a badly designed transformer. Have you tried a different amp just
to see if it's any better?

.

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