Oppressive Clear-Com Hum

[RadioKnight]

Hello everyone, first post on Control Booth!

Anyway, straight to the business!

Our auditorium has an MS-232 clear-com base station installed in our control booth for lights, sound, and spot headsets (some are old 501 clear-com beltpacks, others have these little off-brand Que-com getups). However, that base station is also hooked up to an ancient ps-10k power supply located on-stage for redundancy (the control booth was given a single 20A circuit for all ten of my power amps, we trip that breaker a lot). Our on-stage headsets are hooked up to the ps-10k (again, another mixed bag of 501's and Que-coms).

Now, the headsets in the booth sound pretty clean, minimal hum, but the on-stage headsets have a lot of hum, so much so that the flyman will miss a cue because he can't understand what's being said. I think the problem has to do with the location of the ps-10k (mounted in a rack which was wall-mounted on top of giant dimmer rack power conduits), and the stage manager's headset (sits next to the same giant conduits while calling a show). Whenever the lights change for a cue, you can hear the hum in the headsets change pitch and/or volume.

I can't move the rack or the power supply, and the stage manager can't relocate either. I don't think an isolator will fix this either but I could be wrong. I'm trying to help out the guys on rail, but I don't know how to fix this problem.

 

[FMEng]

I suspect that you've created a large ground loop by powering the system from two different locations. If the booth circuit trips frequently, that is an unsafe condition that needs to be fixed by adding more circuits to the booth. That circuit is overloaded, and continued use by resetting it may cause a fire. Get a qualified electrician in there to fix that.

The good news is you can solve both problems at once. If you add circuits, then you can move the com power supply to the booth, where it belongs, which will prevent the ground loop.

 

[Jay Ashworth]

Another possibility: PL circuits, like telephone circuits, are usually balanced and floating; if you've inadvertently grounded one side, then you may have unbalanced that loop, and cause it to be susceptible to picking up all the hum in the air.

I'm not familiar with the PS-10k, or how you're wired, but if the booth box is powering its headsets, and either end has an iso transformer, then this might be a symptom pattern you could see. As a test, unplug the long run from the booth panel, and plug a beltbox into each side of that junction. If you have hum from the stage, but not in the booth, that's a pretty good sign.

And *definitely*: if you're popping breakers to your dim rack on a regular basis, ask your ownership: what's cheaper? A commercial electrician for 4 hours and some copper?

Or this?

And remember: copper is cheaper than people -- if the relevant panel has enough headroom, pull 3 20A's, and make sure each is labeled for which leg it's on. If you have 240V, switch to that and put a 240V outlet in; if you have 3ph, switch to that, and put a 3ph outlet in (each in addition to some 120V receptacles spread across the legs and labeled.

Make sure they terminate in sufficient receptacles, too, to avoid zillions of power strips.

 

[RadioKnight]

I should clarify, the 20A circuit is for audio amps and our lighting and soundboard only. The service for our dimmer racks is properly sized. The tripping we experience is mostly nuisance tripping when we power the amps on. I'd like to get more circuits for the booth but that's up to our department head.

 

[Jay Ashworth]

Got it. Moving the non amp stuff to a separate circuit still won't hurt.

 

[soundofsparks]

What is the path of the run from the booth to the deck? What sort of cable?

If the run is parallel with electrical lines you'll start to pick up the noise in that current. See if killing the power to your dimmer rack changes anything. If so, you'll want to rerun the home run so that it is at least 3-4 ft. from your electrical run and/or use cable with a thicker jacket. You can also try traveling at an angle or in a squiggle pattern.

Also +1 to ground loop comments. If you can't get all the power on the same circuit try using a ground loop isolator.

 

[Max Warasila]

Thicker jacket won't make a significant difference to interference from EM fields. Rubber won't conduct electricity, but does nothing to stop the magnetic effects. What you want for this is higher capacity shielding and or better CMR at the base station.

Definitely transformer isolate the signal from the booth to the stage (probably better to do so at the booth where there is less interference).

I'd consider calling clear com, too.

 

[Chris15]

Another possibility: PL circuits, like telephone circuits, are usually balanced and floating; if you've inadvertently grounded one side, then you may have unbalanced that loop, and cause it to be susceptible to picking up all the hum in the air.

Standard 3 pin Clearcom compatible systems are UNBALANCED - common ground, power and audio.
RTS systems are balanced...

 

[RadioKnight]

Hey guys, did some testing.

I disconnected the xlr cable that links the old on-stage ps-10k to the ms-232 in the booth. Now there's zero hum in the booth headsets, just a little static at max volume.
The headsets running off the ps-10k are also much quieter, although there is still a noticeable hum at max volume, but it's not a deal killer like before. I believe that the hum on-stage is being induced by the dimmer rack conduits being inches away from the ps-10k and the stage managers headset. The hum becomes even more noticeable when the lights are anywhere from 1% to 99% intensity, so I guess it's just EMI central over there.

I may be able to move the ps-10k to the other side of the stage but I can't move the sm, so I guess we'll just have to deal with that. I'm guessing that I need to install this between the base station and the ps-10k? http://www.clearcom.com/product/partyline/accessories/mt-1

 

[AudioGreg]

Sorry, RTS is unbalanced too.

The mt-1 should help with the ground loop between the two ps units. But I would still move the stage ps as far away from the dimmer banks as you can.

 

[Jay Ashworth]

Page 11 of Telex's intercom Engineering Handbook suggests that Clearcom is unbalanced, while audio, and RTS are balanced.

http://www.telex.com/gb/intercom/file/i=97123&lg=eng

(Sorry, that link is broken, and the mobile version of Google doesn't seem to provide me with any reasonable way to fix it. But the book's easy to Google.)

 

[AudioGreg]

The typical RTS belt pack is wired as follows…

Pin 1-Ground
Pin2-Channel 1 Audio + 30VDC power
Pin3-Channel 2 Audio

This applies to the BP3xx series of belt packs, well any of their 2W stations, and their IFB boxes (which are listen only).

The only balanced signals I know of RTS using is either audio inputs for PGM on their PS units, or the 4W signals (separate talk and listen) on the ports of their matrix systems.

 

[Chris15]

Yep, my memory failed me this time - I was mixing the dual channel with balanced...
RTS analog matrix systems are balanced - 3 pairs being a pair of audio each way and a data pair...

 

[Jay Ashworth]

RadioKnight might want to check the Chassis grounding on his dimmer rack; if it's throwing that much EMI around, it might not be grounded/bonded properly.

 

[RadioKnight]

Another update.

The boss wouldn't go for the MT-1 due to budget constraints, so I moved the main station down to the stage rack and coupled the beltpack cables together in the booth. I removed the old ps-10k from the system completely and bam, no more ground loop. I also replaced the stage managers older rc-760 headset with a more modern Production Intercom headset and that helped a good deal with the EMI pickup. So far we have very quiet headsets, and now my boss can make announcements with the base station instead of messing with the rack mixer and a handheld microphone.