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In article om writes:

I tried the pickup through the pendulum preamp yesterday. I had the
sm58 and the pickup going through the preamp on the 2 separate
channels. I then monitored with headphones plugged into the preamp.

When I was listening through the headphones I got a strange buzz.
Strangely I could avoid some buzz simply by turning the guitar or
swiveling my position/facing.

This is typical of electromagnetic pickups. Where were you when you
tried this experiment? If you were near a computer, turn off the
monitor. If you were in a room with a light dimmer, turn off the
lights.

I faced north I got lots of buzz. I
faced west no buzz.??? It's too bad I have to face north to present
myself to the audience. I moved a little there or a little here I
could change the volume and amount of buzz.

There's nothing sacred about facing North, it's just where the
hum-producing fields are most strongly aligned with your pickup in the
particular place where you tried it. It's quite likely to be a
different direction in another location, or it may not occur at all.

Any ideas on what could be causing this buzz? It was so distracting
I couldn't monitor with headphones in the end. Turning down the pickup
gain revealed it was to do with the pickup and not the vocal mic.

Piezoelectric pickups are less susceptible to this, as are humbucking
pickups (hence the name). But the problem is that an electromagnetic
field is present near your pickup. You need to find the source of that
field and either remove it (like turning off a light on a dimmer,
turning off a CRT monitor, turning off your cell phone that's lying on
the table) or move to a location where that electromagnetic field
doesn't reach. Building a shielded room in which to practice your
guitar while listening on headphones is possible, but not practical.

maybe some magnetic fields caused by something in the restaurant
ceiling was the culprit. I have no idea what it is.

Ah, so you were in the location where you actually have to use this
rig. Well, you'll just have to do some detective work. Try turning
things off, or walking around.

Oddly enough, I got no buzz at all through the house speakers. Those
are also made by Crate Audio by the way.

Well, if you were to plug the output of the preamp into whatever is
feeding those speakers, you'd get teh buzz. The reason why you don't
get the buzz there is because there's no pickup to pick up the
interfering electromagnetic field.

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