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[Robert Walker (dj)]

Large nightclub soundsystem all connection to one phase there are addition non PA items on this circuit, tv's x 8, all lighting and atmospherics, 3 computers, switching packs etc. The system has always been switched  with the master on/off on front of fuse box.

issue: Buzz Buzz - not constant Hum

We get random buzzing when large 500w par cans go on think there's around 20 of those - which we can live with as we don't use them much. but recently the buzz has been more prominent to the point of annoying when silent section of music are being played.
Now putting UV cannons x 4  on gives a buzz then it may go away, martin pro 2000 smoke machine also adds to the buzz when heating up and yet again it may be fine another day.

To try and isolate problem only mixer/CD player and amps/speakers were turned, the sound was fine for a little while then the buzz power noise was just a loud through the system

One of the only major things that have been added to the building is Mitsubishi ceiling Air conditioning units with external fan units. 4 have been added - these are on a different circuit to the pa/av system yet when two of the units are switched off the buzz disappears. I think that phase has the aircon,  bar lighting  + neons and a few external lights.

thanks in advance
thoughts please

[Stephen Swaffer]

Phase shouldn't matter.

If shutting the units off eliminates the buzz, there is your source.  More than likely these units of VFD (variable frequency drives) running the fans.  There is a switching frequency internal to the drives-often in the 4-12 kHz range that could cause a buzz if it was coupled to the line.

Sometimes guys will run line and load to a VFD in the same conduit-not a good idea for a lot of reasons-but I could see that coupling that noise to the line side.

In any case, if they are creating the problem, the problem needs to be addressed there.

[Robert Walker (dj)]

The aircon was added in 3 stages over three weeks. At first them was no issue, but as mentioned switching onthe UV & smoke machine brought the buzz on. (all aircon was also on)
Only when switching off the last two aircon units added does the buzz go away, sometimes just switching the two fuses off and back on helps. then after a while the buzz returns

its like a frequency pulse/buzz.

The aircon guys are in on tuesday - we will await there findings.

[Stephen Swaffer]

The other possibility is a ground/neutral bonding issue-or maybe a loose connection in a neutral.

Guys have been known to run 3 phase to an AC unit then realize they need a neutral for a control transformer so they take a shortcut and use the ground instead-again NOT OK by code-and very likely noise source.

[Mike Sokol]

The other possibility is a ground/neutral bonding issue-or maybe a loose connection in a neutral.

Guys have been known to run 3 phase to an AC unit then realize they need a neutral for a control transformer so they take a shortcut and use the ground instead-again NOT OK by code-and very likely noise source.

That's a strong possibility. Anytime the ground is improperly required to run the return current instead of of the neutral, any changes in the load will cause the voltage on the EGC (Equipment Grounding Conductor) to modulate by a few volts. That will usually case a ground loop hum for a constant load. However, if the load has any sort of switching supply, then that chopped current will modulate the ground voltage. That buzz can sneak into the sound system via the pin-1 problem. I would measure between the EGC of one of the sound system outlets and the service panel ground if possible, then start turning on loads. You should measure essentially zero volts between the grounds, but if that moves up and down as the HVAC switches on or the lighting changes, then you have the neutral currents being cross connected into the grounding system. You can also learn a lot with an oscilloscope between the grounding points. That can show all sorts of high frequency garbage on the grounds.

[ProSoundWeb Community]