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[brad smith]

Last night we ran into a problem that we have no idea what created this scenario. Ran sound check for main act and everything was ok with monitors. Total of 8 mixes- one ear mix and the rest are a combination of powered JBL PRX 712 and  EV TX powered with QSC PLX 3602's. Monitor board was x 32 compact. Using an analog split between FOH and monitors.

Everything was fine and working after soundcheck and then switched our split off X32 and plugged into  1st bands Mackie 1606? IPAD controlled board. They are all ears with one wedge. First band played and everything was fine and then we switched the split back and all of a sudden we would hear a loud hum coming out of the ears and some of the wedges. When we unplugged the speaker cord  out of the monitor the sound would stop. This happened on multiple monitors and multiple power amps. We muted each channel and even unplugged each channel split at the board to see if it we could locate the offending channel and no luck. We even powered down the board and we had monitors still making noise with the power amps still on. We would power one amp down and the noise would stop on that monitor and then it would start on another amp and monitor. Very crazy night. At least FOH went off without any problems other than the delayed start time. We had an issue with a wireless mic that was spiking on and off- the display would fade in and out causing spikes. Why would the monitors still have noise in them with the board powered off. Just need to figure this out so it does not happen again. We did not switch out patch cables from the power amps to the x32 because it happened on multiple outputs. I will hook it up later this week when I have time to try to replicate the issue.

We  used the Mackie mixer with IPAD to get through the show and used just powered wedges and it worked. What could have caused this to happen. I'm just very perplexed if the problem is related to the board, amps, or power.The show was delayed because of the monitor issue we had.  I always appreciate all the feedback form the forum.

[Douglas R. Allen]

On your analog split do you have ground lift switches? Any chance a few were lifted or were grounded when they shouldn't buy mistake in the change overs? Gound will effect monitors desk sends  on or off.  Next time unplug source at the splitter not foh desk.

Also any chance the band change overs  put a guitar amp or keyboard amp etc on a different outlet that was on a different phase?

Douglas R. Allen

[Mike Caldwell]

Along with the noise did you have the proper audio in the monitor mixes.

Any chance some of non power monitors got re-patched so one power amp channel ended up feed into another power amp channel via looping through monitors.

[brad smith]

On your analog split do you have ground lift switches? Any chance a few were lifted or were grounded when they shouldn't buy mistake in the change overs? Gound will effect monitors desk sends  on or off.  Next time unplug source at the splitter not foh desk.

Also any chance the band change overs  put a guitar amp or keyboard amp etc on a different outlet that was on a different phase?

Douglas R. Allen

No ground lift switches on snake and all instruments stayed plugged in. I want to think it was related to electrical issues,but it should have done same thing w the Mackie,except they were plugged into different outlets.

[brad smith]

Along with the noise did you have the proper audio in the monitor mixes.

Any chance some of non power monitors got re-patched so one power amp channel ended up feed into another power amp channel via looping through monitors.

The noise we heard did not sound like feedback from instruments. I was not running monitors, but the mon guy is experienced so I'm confident audio was correct. We also did not unplug outputs at all. We only pulled the split going into the mackie

[Tim McCulloch]

Were any of the Mackie's outputs hooked up to any part of the system?

If it all worked at sound check the question is "what changed?"  What is different about NOW compared to 3 hours ago?  Different power outlet/source for any of the mixers (incl FOH)?  What about stage power - anything taken direct that was powered from different outlet than at sound check?

That it seem systemic indicates to me that you acquired a serious ground loop or current on the Neutral of something feeding the mixer or amp rack.  Was the hum present in the headphone jack of the monitor desk?

[Jonathan Woytek]

...
At least FOH went off without any problems other than the delayed start time. We had an issue with a wireless mic that was spiking on and off- the display would fade in and out causing spikes.
...

Can you elaborate on this a bit? Was it the receiver display fading in and out, or the transmitter? If it was the receiver, this makes me wonder about electrical issues.

Jonathan

[Chris Hindle]

No ground lift switches on snake and all instruments stayed plugged in. I want to think it was related to electrical issues,but it should have done same thing w the Mackie,except they were plugged into different outlets.

Ding ding ding, I think we have a winner.....
Chris.

[brad smith]

Were any of the Mackie's outputs hooked up to any part of the system?

If it all worked at sound check the question is "what changed?"  What is different about NOW compared to 3 hours ago?  Different power outlet/source for any of the mixers (incl FOH)?  What about stage power - anything taken direct that was powered from different outlet than at sound check?

That it seem systemic indicates to me that you acquired a serious ground loop or current on the Neutral of something feeding the mixer or amp rack.  Was the hum present in the headphone jack of the monitor desk?

The mackie was not hooked up to the original x 32 at all.  After thinking on this more, I realized we powered down the amps  and powered monitors when the first band played and switched over the split to the mackie. Once the first band was over, we switched the split back to the x32 and powered up the amp rack and powered monitors and the problem started within a couple of minutes.

Also, the hum was present according the the mon. guy on the ears. I'm not sure if it was on just his pack or  on the board. I feel that this was a power related problem that caused the noise in the monitors. Just not 100 % sure

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