UPS systems installed at FOH, introducing noise into RF rack

Pip

Active Member
Hey all, I pop in here from time to time and I'm always glad to see this great community still lives on.

I recently implemented 3x UPS systems at front of house, and last night during a rehearsal I was trying to track down why some of my wireless mics had a bunch of disgusting noise. Eventually I kinda recognized the feel of what I was hearing, as it reminded me of the fan noise from the UPSes. So, I killed my FOH power as a test... Lo, the mics became much cleaner.

I'm wondering if anyone else has experienced power noise issues with UPSes, and what they did to solve it.

The system is powered by a 400A company switch with isolated ground. My PD has powercon outputs which power the monitor console and the RF rack, and a 50A Hubbel for the monitor drive rack. It also has 6x 30A twistlock outputs for lunchboxes on stage. When we got the UPSes, I ran a 200' 30A cable to FOH to power the UPSes off of the same PD. The cable and lunchbox were built by an in-house electrician guy who works at our other venue. Before that FOH was powered by the wall power in the cockpit on the same audio ISO.

Would love to see if anyone has any ideas here. Part of the issue was high preamp gain, I solved it for this show with slightly lower gain and gates on the affected channels. So I have a reasonable workaround for the show tonight. However, I'm quite interested in returning to my squeaky clean RF lines as is typical. For fun, I did a scan and found I had degraded backup frequencies but primaries were fine. I deployed a new set of frequencies anyway. Unsurprisingly, this didn't solve the problem although it's interesting to note that one of the channels which was clean picked up some noise after the freq change... I have 8 channels of Axient with Showlink AP, so I'm super flexible with frequency management.

Thanks in advance for any thoughts of solutions!

Warm regards,

Phillip
 
Most likely, you've created some kind of ground loop in the system. The other possibility is that the UPS has square wave inverters instead of a sinusoidal output. Square wave inverters can radiate garbage into the RF spectrum, and the hash can sneak through the filtering of equipment power supplies. A couple hundred feet of cable is a good noise radiator. Transformer windings can can make acoustic noise from square wave power, too.

Two tests to try. Turn off the receivers and see if the noise goes away. If the noise is still there, it is a ground loop. Check again with receiver on, but carrier off and squelched. If it's quiet, the receivers are picking up RF noise.

UPS units range from garbage to great. Sometimes, a cheap UPS is worse than having none at all. I'd be interested to know what model of UPS you have.
 
Hi there,

Thanks for the reply. More or less what I was thinking. The units are true sine wave... But I'm wondering if an On-Line UPS might have been better than Line-Interactive like these ones... Maybe there is still a way I can isolate them.
It's a ridiculous story of why I have them, really. I went through the last three years dealing with a problem with my Midas ProX, talking to MusicTribe and any other Midas expert I could get in contact with across the country. The UPSes were purchased by someone in our org. who was trying to help and probably didn't fully understand the problem. There may have been some reserve capital improvement funds or something too, but that's above my pay grade to know.

I'll try those tests! Thanks for the ideas.

Anyway, they are Trip-Lite SmartPro 3kVA/ 2.25kW Line Interactive Sine Wave units. Three of them... I believe they're wired each to one leg of the three phases in the long supply line. I did have some thoughts about the long cable being some sort of antenna as well...

To be continued...

Cheers
 
Hi there,

Thanks for the reply. More or less what I was thinking. The units are true sine wave... But I'm wondering if an On-Line UPS might have been better than Line-Interactive like these ones... Maybe there is still a way I can isolate them.
It's a ridiculous story of why I have them, really. I went through the last three years dealing with a problem with my Midas ProX, talking to MusicTribe and any other Midas expert I could get in contact with across the country. The UPSes were purchased by someone in our org. who was trying to help and probably didn't fully understand the problem. There may have been some reserve capital improvement funds or something too, but that's above my pay grade to know.

I'll try those tests! Thanks for the ideas.

Anyway, they are Trip-Lite SmartPro 3kVA/ 2.25kW Line Interactive Sine Wave units. Three of them... I believe they're wired each to one leg of the three phases in the long supply line. I did have some thoughts about the long cable being some sort of antenna as well...

To be continued...

Cheers

One on each phase leg? At some point I'd have questions about how this is done and exactly which product... etc. But right now I think @FMEng has broke down the trouble shooting tree. My personal guess is the UPS units are generating spurious RF that is being picked up by the signal lines. We'll see. Let us know what you find.
 
I'm curious why the (3) 30A/200' cables to FOH instead of using wall power on the same IG system?

What's the cable routing out to FOH? Are those cables bundled or in proximity to audio signal cables?

If you killed all of you FOH power for a test but still had mic's coming through the system, how is your system set up? Are you running the wireless racks off stage left to a digital stage box or something that dumps into....some console that's not out at FOH?

Trying to understand your layout and system topology.
 
Thanks for the input folks.

One on each phase leg? At some point I'd have questions about how this is done and exactly which product... etc. But right now I think @FMEng has broke down the trouble shooting tree. My personal guess is the UPS units are generating spurious RF that is being picked up by the signal lines. We'll see. Let us know what you find.

Yes, the "electrician" who built the lunchbox did it that way. I know plenty about sound and enough about electricity to be dangerous but I'm not an electrician and if it were up to me I would have done a lot more research before implementing this solution but sometimes they just do things without talking to me first. Too bad, because it more than once has resulted in wasted money and poor solutions to problems that aren't fully understood by the people throwing money at it.

One UPS per leg on three-phase is a no-no. That isn't causing the audio problem, but it shouldn't be done. If they are line-interactive, they are nothing but a pass-through under normal conditions. Since the inverters are not running, that points toward ground loop.

Interesting. As I said above, I'm not an electrician - could you explain why that's a no-no? Cheers

I'm curious why the (3) 30A/200' cables to FOH instead of using wall power on the same IG system?

What's the cable routing out to FOH? Are those cables bundled or in proximity to audio signal cables?

If you killed all of you FOH power for a test but still had mic's coming through the system, how is your system set up? Are you running the wireless racks off stage left to a digital stage box or something that dumps into....some console that's not out at FOH?

Trying to understand your layout and system topology.

Good question - the UPSes do not have regular edison connectors, they are 30A three prong twist lock NEMA of some sort. So, the wall sockets were not an option. Of course, as I said above these were sort of half-assed solutions. After I inspected the breaker in my shop below my mix position, I found that they obviously missed the fact that there were three available breakers in my panel which probably could have been used instead of running a long ass cable to the stage...

Yes, RF rack is SL with the MON console and I was listening at MON. Coming out of the RF rack analogue, going into our analogue snake which has a 3 way transformer isolated splitter going to FOH, MON and an alternate FOH position.

If you're curious I could go into more detail or draw a rough diagram just for fun. Anyway I'm going to remove the UPSes from the system for tonight's show although gates saved my recording last night.

Cheers
 
Interesting. As I said above, I'm not an electrician - could you explain why that's a no-no? Cheers

Three phase circuits have a 120 degree phase relationship between each hot leg. That provides a certain amount of current cancellation on the shared neutral conductor, which generally prevents overcurrent on that wire. With three, separate, UPSes, when they switch to battery power, there is no frequency and phase relationship between them. That could lead to an overcurrent condition on the neutral.

The other issue is that there isn't one circuit breaker to control the supply to that cable. Someone could turn off a UPS, thinking they can work on the equipment, and still have a hot cable.
 
Three phase circuits have a 120 degree phase relationship between each hot leg. That provides a certain amount of current cancellation on the shared neutral conductor, which generally prevents overcurrent on that wire. With three, separate, UPSes, when they switch to battery power, there is no frequency and phase relationship between them. That could lead to an overcurrent condition on the neutral.

The other issue is that there isn't one circuit breaker to control the supply to that cable. Someone could turn off a UPS, thinking they can work on the equipment, and still have a hot cable.
Well, you're assuming Wye. Not completely impossible it could be... oh; it's a company switch, isn't it?

Yeah, that wouldn't be Delta. Nvm.
 
With three, separate, UPSes, when they switch to battery power, there is no frequency and phase relationship between them. That could lead to an overcurrent condition on the neutral.

The other issue is that there isn't one circuit breaker to control the supply to that cable. Someone could turn off a UPS, thinking they can work on the equipment, and still have a hot cable.
Maybe I'm reading it wrong, but I interpreted the layout as having the UPSs at the sound desk, so loads are connected to them individually and the long cable is just between the PD onstage and the UPSs. In that case, when they switch to battery there's no load on the shared neutral and each UPS switch properly controls all the loads attached to that specific UPS. If it's the other way around and the UPSs are at the stage end of a long, 4-conductor+gnd cable with some sort of break-out/break-in combo, that would indeed be very wrong.
 
Maybe I'm reading it wrong, but I interpreted the layout as having the UPSs at the sound desk, so loads are connected to them individually and the long cable is just between the PD onstage and the UPSs. In that case, when they switch to battery there's no load on the shared neutral and each UPS switch properly controls all the loads attached to that specific UPS. If it's the other way around and the UPSs are at the stage end of a long, 4-conductor+gnd cable with some sort of break-out/break-in combo, that would indeed be very wrong.
I had the same question ... so which way is it built?
 

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