Sound system hums when powered off!!!!

Mike Caldwell

Mike Caldwell

Junior
Jan 13, 2011
532
27
28
62
Covington, Ohio
www.mikecaldwellaudioproductions.com
You read the title right, “sound system hums with the power turned off” it also hums
when powered up, other than the hum everything still works as it should.

Here’s what I got, a local venue where I not only provide production but also maintain
their in house system. The system is various zones of Community R Series speakers powered with five Crown CL series power amps, processing is a Shure P4800, inputs come from various points around the venue into a BiAmp strip mixer. The Community speakers are all low impedance non transformer models. The system is on a Lowell power sequencing system that controls RPC modules for each power amp and the input processing. System power is from six 20 amp circuits that where installed for the sound system.

You can clearly hear a low level hum when the system is completely powered off, this problem started about a month ago.
Here’s what I’ve done and narrowed it down to:

-disconnected all incoming input lines from the mixer (still hums)
-disconnected all outputs from the Shure DSP (still hums)
-disconnected the power cords directly from all the amps (still hums)
-unplugged all power cords from the outlets ( NO HUM)
-plugged power cords back in one at a time starting with processing power,
system started humming when plugging in the first power amp’s power cord.
Keep in mind the power cords to the amps are still disconnected from the amps,
the cords I’m plugging and unplugging from the wall outlets are the power cords
going to the Lowell RPC power control modules.
-unplugged all of the processing power cords at the wall outlets and just left the
one amp’s power cord plugged in (still hums)
-unplugged that one amp’s power cord and tried another amp’s power cord (still hums)
-plugging in any power amp’s power cord to any outlet will make the system hum.
-I then tried just touching the ground pin on the power plug to the metal outlet box
cover and that will cause the system to hum.
-Tested the outlets with a plug in tester and that showed OK.
-Measured the voltage between ground and neutral on the outlets and found 0.3 volts.

I do want to go back with a long extension cord and plug an amp or two into a completely different
outlet that is fed from a different sub panel that hopefully does not have the 0.3 volts between ground and neutral.

Now I’ve saved the best for last……the building is in the middle of a major renovation at this time,
the electrical contractor has no idea what they could have done if anything to cause this problem.


Any ideas?
 
Rob Timmerman said:
If it's humming with no power applied, the noise is getting coupled directly onto the speaker wires. If I had to guess, one or more of the speaker lines got tied to ground somewhere (perhaps by an errant screw?)
Click to expand...

I kind of thinking some thing like as well, in one way or another a speaker line or lines insulation has gotten cut or rubbed through causing the line to come in contact with the buildings frame, pipe, conduit, ect.
I'm going to pull the lines off of the amps to not only see if that stops the hum but also measure between each line and the power ground for both voltage and resistance.
 
I had time today to go back to the venue and dig a little deeper.
Started unhooking speaker lines from the amps, only had to do about half
of them till I found the trouble maker. As was the general consensus along with my
gut feeling the positive lead on one speaker zone was shorted to the building
frame. I measured 0.5 ohms between the positive lead and the electrical/building
ground. Connecting the speaker's negative lead to the amp or any ground reference
in the rack would complete the circuit causing the speaker to hum from the voltage
difference between the ground points.

The building maintenance supervisor found the compromised insulation after I told him
what zone to start closely looking at the speaker cabling.

Mike C.
 
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