Low frequency interference in Installed PA system

[blueboxoffroad]

Hi everyone,

 

I have been working for some time with an installed 100V line PA system in a pair of hospitality rooms in a local venue.

 

The system is around 15-20 years old and, in the traditional way of these things, has had a lot of different people decide they know how to upgrade/modify/optimise/reconfigure and generally mess with it. I am the latest, but I'd really like to get it all working a bit better!

 

Most of the system works ok, and one room (the two rooms can be joined into one, but normally run as 2 separate areas) seems to run absolutely fine.

 

Basically, each room has a pair of Sennheiser hand held radio mics, routed via a Cloud matrix amp to one (or 2 if rooms are linked) Bittner power amplifiers to around a dozen unknown ceiling speakers (which sound rough at best!) The mics can, via the Cloud amp, be routed to either room or both.

 

We have recently replaced the previous TOA active aerials with Audio Technica paddle aerials (which I have checked are fully compatible with the Sennheiser receivers, so there are a pair of aerials in each of the 2 rooms (A & B in each) Most of the time the system seems to work perfectly well, BUT...

 

Every so often (and there doesn't seem to be a pattern to it) we get HUGE interference from one mic channel in one room. This comes in with no warning - it's not like feedback, and is seen on both the amp and the associated Induction Loop amp... Needless to say, punters in the rooms are less than impressed by this deafening noise and we (I!) have been on the receiving end of a lot of grief over it.

 

We've retuned the mics, replaced the aerials and Co-ax to them, plotted frequencies, had allegedly clean mains run to the PA rack, swapped mic receivers for hire stock ones, replaced the PSU for the malfunctioning channel... sworn (I've done quite a bit of that!) and still can't identify the cause. I'd like to swap the receivers to the opposite room, but the venue don't want to run the risk of upsetting another room full of paying customers with a fault they've not encountered before!

 

So - does anyone have any idea what might be causing the huge, unpredicatable, largely low frequency interference? I'm currently providing a stand alone system with 3x radios in channel 70 territory, which works perfectly - the installed mics are all new ch-38 compliant units... I'm stumped! HELP please! Thanks in advance

[paulears]

Have you got a recording of the noise? The likely cause is co-channel interference of some sort, and because a venue is RF quiet on ch 70, doesn't mean it is on 38. It would be worth checking that the venue, a hotel? isn't distributing their TV feeds around the building on an old RF system - there used to be quite a few of these that used a channel around 37 (because it was often quiet) to send a hotel produced channel from a computer to the rooms. All this was before today's more clever distribution systems. It's possible that even if they have a new system, the old one is still sending to the old system, and you're picking it up? Worth a search around with an old TV set, maybe - to see if there is video on or around 37/38. One system I remember simply had a multi-turn pot that enabled it to appear anywhere from ch35 to 40.

 

One thing - have you tried changing the operating frequencies - maybe selecting the ones used in the other room when it's not in use and seeing if the interference re-occurs?

[jamesperrett]

Are you sure that the problem is with the mic? Have you tried swapping outputs on the Cloud amp to see if the fault appears in the other room?

[Shez]

I've once encountered a loop that caused significant interference with a handheld radio mic at one particular position in the room. It was fine anywhere else in the room; just that one location was a problem.

 

It was a badly installed loop system put in by a company who didn't appear to be particularly competent so we assumed that perhaps the field strength (which was very uneven anyway) was particularly strong at that point. Switching off the loop amp without changing anything else cured the problem completely.

 

Might not be the most likely problem for you but something else to consider.

[Brian]

I've once encountered a loop that caused significant interference with a handheld radio mic at one particular position in the room.

 

This is usually encountered in mics with a dynamic capsule as opposed to a condenser mic. If the OP wants to eliminate this as a possibility, and he has dynamic mics, then a quick borrow of a condenser radio mic, or a beltpack connected to a condenser, would help.

[blueboxoffroad]

Wow! Thanks everyone who has chipped in on this - your thoughts are much appreciated!

 

I will try to track down a copy of a video one of our techs recorded on his phone that demonstrates the fault and post a link to it for you to ponder!

 

I'd be very happy to swap the mics to the other room to see if the fault follows - the only problem is that the fault only appears when the sporting venue is full on a match day. Understandably the venue are not too keen on running the risk of annoying another bunch of punters by blasting their ears with noise!

 

The RF distribution is an interesting possibility, as they run an in-house TV system on match days but no other time - the fault is only evident on match days, so it could be that there is a link between the two. Would this overlap of usage simply blast through the system in an unpredictable fashion? Not knowing much about RF distribution etc, I would have expected it to have been a constant bleed through, rather than coming and going... does anyone know if the intermittent nature of the problem sound typical?

 

We have moved the frequency of the offending channel to a clear area of bandwidth (after an outside contractor plotted frequencies throughout a match day) which seems to have made no difference. The receiver pack has been replaced with one from our hire stock to no avail.

 

When the interference is evident, there is a full RF signal displaying on the mic receiver and a vast signal presence on the induction loop amp (you can actually hear the relays in it as the signal hits the absolute maximum indicated level on it)

 

I shall dig into the combined RF knowledge of the Blue Room to see if anyone there knows about the RF system overlap possibilities. It's a safe bet there is vastly more knowledge on here than there is in the venue, where no-one REALLY knows how much of their data is distributed - this applies to TV, Mics, IT and pretty much anything else! Has anyone ever wondered why they decided to take on a project, once they were up to their necks in it?!

 

Thanks for all your help.

[Brian]

...the only problem is that the fault only appears when the sporting venue is full on a match day.

Does the venue have air-con? Or food and drink facilities that only get heavily used on match days? Do they bring in an outside bar?

 

Is this an interference problem from something like compressors cutting in and out?

[Wilflet]

 

We have moved the frequency of the offending channel to a clear area of bandwidth (after an outside contractor plotted frequencies throughout a match day) which seems to have made no difference.

 

What did the RF plotting show?

Any sources you cant identify?

Anything change in his plot around the time of the interference?

[blueboxoffroad]

The venue has a huge increase in use of nice spiky power consumption on match days - air con, fridges (including one housed next to the PA rack!) beer chillers, lifts, dumb waiters (read that as you will!) any of which could be responsible for the interference. I have had allegedly clean mains run to the rack, but I'm not completely convinced there's nothing else on that circuit - I don't believe it's come directly from the incoming feed from the street or their on site distro.

 

Typically, the day we had frequency plotting done, we didn't get the interference! (just like the classic intermittent rattle in your car!) We did find a couple of spikes on the plot from TV distribution and have moved tuning away from those. There is a fair likelihood of some piece of kit slowly degenerating and emitting a broad band of RF interference (I've found untold decomissioned kit and cable throughout the false ceiling void!) - does anyone have any idea how I might track it down or know of a specialist company who might be able to find it for me? I'm running out of ideas and my client is running out of patience!

[blueboxoffroad]

The venue has a huge increase in use of nice spiky power consumption on match days - air con, fridges (including one housed next to the PA rack!) beer chillers, lifts, dumb waiters (read that as you will!) any of which could be responsible for the interference. I have had allegedly clean mains run to the rack, but I'm not completely convinced there's nothing else on that circuit - I don't believe it's come directly from the incoming feed from the street or their on site distro.

 

Typically, the day we had frequency plotting done, we didn't get the interference! (just like the classic intermittent rattle in your car!) We did find a couple of spikes on the plot from TV distribution and have moved tuning away from those. There is a fair likelihood of some piece of kit slowly degenerating and emitting a broad band of RF interference (I've found untold decomissioned kit and cable throughout the false ceiling void!) - does anyone have any idea how I might track it down or know of a specialist company who might be able to find it for me? I'm running out of ideas and my client is running out of patience!