My church's PA system has a nasty 60 cycle hum caused by a ground loop.

As I have attempted to find the source, I have discovered that by
lowering and raising the masters on the mixer, the volume of the hum is
affected.

Turning off the mixer will shut down the hum

Also shutting off one of our two power amps will stop the loop.

I don't know where to go from here.

Please offer suggestions on finding the ultimate source and fixing it.

Thank you.

Ludwig77 wrote:
My church's PA system has a nasty 60 cycle hum caused by a ground
loop.

As I have attempted to find the source, I have discovered that by
lowering and raising the masters on the mixer, the volume of the hum
is
affected.

Turning off the mixer will shut down the hum

Also shutting off one of our two power amps will stop the loop.

I don't know where to go from here.

Please offer suggestions on finding the ultimate source and fixing
it.

Thank you.

It may not be a ground loop.

Is the power amp that stops the hum near the mic mixer?

If you turn down all the mic levles, but leave the master up does the
hum stop?

Can you isolate it to one mic?

What kind of mixer and what kind of amps?

If the power amp is right next to the mixer, the mixer may be picking
up magnetic hum from the power amps power transormers. Can you move
them about 1-2 feet apart and see if that helps.

Mark

In article .com writes:

My church's PA system has a nasty 60 cycle hum caused by a ground loop.

As I have attempted to find the source, I have discovered that by
lowering and raising the masters on the mixer, the volume of the hum is
affected.

This suggests that either the hum is coming from the mixer internally
(and not necessarily because of a ground loop) or from something
connected to the mixer.

Please offer suggestions on finding the ultimate source and fixing it.

Simplify the system and start building it up again. Connect the
speakers to one power amplifier and listen for hum. Then connect the
mixer to that power amplifier and listen for hum. Then connect things
to the mixer inputs and listen for hum. If at any stage, you hear hum,
troubleshoot the connection you've just made.

How is the second power amplifier connected, and what is its function?
Does turning off either of the two power amplifiers stop the hum
completely, or only in the channel fed by the power amplifier that you
turn off? (obviously turning off a power amplifier will remove hum,
and any sound, from the speaker to which it's connected)




--
I'm really Mike Rivers - )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

The power amp is not near the mixer at all. The power amp is in a
cabinet above the platform, while the mixer, a Carvin DX1642, is
located in the back of the room where you would normally envision the
soundbooth.

No channel muting stops the hum. I have noticed that muting the vocal
mics reduces the hum very slightly.

The master faders on the board do affect the volume of the hum.

Only turning off one particular power amp stops the hum. If I shut down
the other power amp, the hum remains. I have not yet determined if the
power amp where the possible loop resides is powering the mains or the
monitors. I can determine that in my further troubleshooting this
evening.

well if the amp you turn off is feeding the spearkers, of course the
hum stops, as all the sound stops as Mike pointed out....

you need to figure out:

1) which amp is which
2) does turning it off stop the hum in the main speakers and the
monitors?
3) can you monitor the output of the mixer directly with headphones for
example
4) is the hum always present at the output of the mixer but its just
that you can't hear it with the amp turned off

It is possible that the mixer is defective and creating a hum,
If you turn down and remove all the inputs to the mixer but leave the
master up and you get a hum and the hum goes away when you turn down
the master, I think that may mean a problem in the mixer.

Anything else near (within 1 or 2 feet) of the mixer that could be
creating interference into it? Like a clock motor, fan motor,
florescent light ballast etc?

Does the mixer have a 3 prong or a 2 prong plug?

If two prong, can you reverse the way you plug it in?

If three prong, AS A TEST ONLY, try an adapter that isolates the
ground prong.
DO NOT LEAVE IT LIKE THIS AS IT CAN BE DANGEROUS!


Mark