greateastern Posted February 4, 2006 Share Posted February 4, 2006 H all, This is an urgent plea for help! I have just set the PA system for a show at school tonight. I'm using a behrnger desk and putting it through a Sountech amp, connected to 1600W speakers. I tried the 4 radio mics I am using and the CD player etc (basically anything connected to the desk with under 10m of wire) but as soon as I tried putting the wired mics through a stage box connected to the permanently installed box on the wall and ran cable fromt eh control room to the desk, or even connected teh stage box staright to the desk I pick up a radio station. It cuts in when I put the gain at about half its full setting and put the fader up. I have never had this problem in this hall before and I have isolated teh problem to anything running from the stage to the desk, e.g. a wired mic on a 5m+ XLR lead that is next to the desk does not suffer any interference. I am also getting a noticeable hiss over my headphones which I have never experienced before, it gets worse when I remove one mic lead. The only difference to my normal set up is the amp and speakers in use. Not being that experienced I haven't a clue what is causing it and how to resolve it. Cheers in adavance, Andy Link to comment Share on other sites More sharing options...
Andrew C Posted February 4, 2006 Share Posted February 4, 2006 First thoughts would be a broken screen connection somewhere, but as you say the only change is the amp... Do you get interference at the mixer, listening on the cans, with the amp audio cables unplugged? Are you using balanced cables from the desk to the amp? Are they pinned out the same? 1 = Screen, 2 = Hot, 3 = Cold. Some kit breaks even this basic "standard". A few thoughts to get you going. Link to comment Share on other sites More sharing options...
greateastern Posted February 4, 2006 Author Share Posted February 4, 2006 The radio signal shows up the individual mic channel signal lights so unplugging those cables doesn'thelp. I have tried using different sockets as well, but I'll have one more go with a different mic, different cables etc and whil I think about it I'll check those amp cables to get rid of the hiss. Andy Link to comment Share on other sites More sharing options...
Fleeting Posted February 4, 2006 Share Posted February 4, 2006 Hello, When you say radio interference is it commercial radio (radio 1, radio 2 etc), or something else (mobile phone, walkie talkie). I did have problems at a school some years ago where a new mobile phone mast was causing problems with breakthrough onto the sound system. It took a lot of earthing to get rid of the radio chirp off the system. It might be worthwile breaking the signal screen between the amplifier and desk at one end. Does the amplifier have a earth lift switch on the back? Try using shorter or longer leads between the desk and amplifier. If the stage box has been installed on the cheap. Open it and have a look at the screen inside. I have know someone common the earth ups to a lug inside a stage box and then had it fall off. Does it get stronger the more mic cables you plug in or does it stay the same? This will probably keep you busy for a hour..** laughs out loud ** sorry. Let me know how you get on. Fleeting.. Link to comment Share on other sites More sharing options...
Balanced_XLR Posted February 4, 2006 Share Posted February 4, 2006 Hmmm sounds to me like you simply have some dogey connections....happens to me alot. Replace all the XLR and try again. Link to comment Share on other sites More sharing options...
Bobbsy Posted February 4, 2006 Share Posted February 4, 2006 Without doubt the world guru on getting rid of electrical and RF interference in systems is Tony Waldron from Cadac. The suggestions above about checking and fixing screens are very true...but if you want a bit of the theory involved you should have a read through Tony's site at: http://www.tonywaldron.co.uk/ . Bob Link to comment Share on other sites More sharing options...
paulears Posted February 4, 2006 Share Posted February 4, 2006 Hmmm sounds to me like you simply have some dogey connections....happens to me alot. Replace all the XLR and try again. Replace all the XLR? There are a few clues. The fact that the interference cuts in at a certain point on the channel gain is one. It is pretty important to work out what radio station is coming through. AM or FM? If you discover it is AM, the problem may be something simpler. Something, somewhere in the signal chain is demodulating the signal - the fact a 5m cable plugged in direct to the desk doesn't do it rather makes me think the multi is the likely cause. It is also a damn great aerial - with all those screens floating around then you may well have a high level of rf superimposed on your audio. The other culprit is probably the mic preamp (or amps) in the desk. Dry joints, leaky caps etc can all contribute to the demodulation problem. After all, years ago a single semiconductor - the old cats whisker, was the simple demodulating device in old radio sets. Many modern preamp designs actually have a frequency response that goes well above the 20K or so claimed - right up into the rf area. Expensive and/or well designed preamps filter this unwanted response away, I doubt the behringer does, not at its price point. The first semi-conductor jnction it comes to may well be all it needs to take off like this - and, the gain control initiating the problem makes this more likely. Solution wise, I guess adding some HF blocking capacitors may work, but it does mean getting inside with an iron. Other than this, look at the multicore again. Check to see if each core is truly separate, or does each earth wire in the socket end conenct to the others - as in with it disconnected, is there continuity between any of the pin 1's? If every pin 1 is linked, then with unused ways, you have the ultimate aerial. Some cheaper chinese multis I've seen have a connection between each socket pin 1 and the chassis. Snipping these, bar just one of them may help restore normality. My best guess is simply that you have two problem pieces of kit, individually they are fine, just together they combine to make your life hell! Link to comment Share on other sites More sharing options...
Balanced_XLR Posted February 4, 2006 Share Posted February 4, 2006 Hmm...dont forget that XLR is supposed to get rid of interference... and it works.....I think you could have a dogey multi - test each channel. A quick fix for the problem could be to turn the gain right down on the amplifier, and up the gain on the desk....be sure it dosent clip though! But in the long run, you will have to test every single cable, ie. test every pin on the multi, make sure that none of the pins are crossed anywhere. Link to comment Share on other sites More sharing options...
paulears Posted February 4, 2006 Share Posted February 4, 2006 Balanced XLR - with the greatest respect, you have no idea what you are talking about. XLR doesn't get rid of anything. If you read my previous post I believe we have already been there. I posted that after you mentioned dodgy connections. The OP has already stated he tried various sensible things. You are talking about balanced vs unbalanced circuits, and yes, noise reduction is a factor - but this is interference - different concept altogether. Turning the gain up on the desk makes the problem worse the OP has already said. I suspect you may need a little more background knowledge to help out on this one.paul ps - why would pins being crossed have anything to do with this - any particular crossing you have in mind? Link to comment Share on other sites More sharing options...
Balanced_XLR Posted February 4, 2006 Share Posted February 4, 2006 I am just saying what I think is wrong, and I do know what I am talking about. I understand that XLR will not distroy inferference on its own, There needs to be a tri-signal sent between both ends, but I am suggesting that perhaps in the multi, for what ever reason, some wires have been crossed. Link to comment Share on other sites More sharing options...
Andrew C Posted February 4, 2006 Share Posted February 4, 2006 I am just saying what I think is wrong, and I do know what I am talking about. I understand that XLR will not distroy inferference on its own, There needs to be a tri-signal sent between both ends,XLR will not 'distroy' anything; it is a type of connector. Tri-signal? No such thing. I think you are referring to a balanced audio system. Crossed wires may make a system more prone to interference but the OP makes it clear that there is a new problem, which tends to exonerate the installed wiring. Link to comment Share on other sites More sharing options...
Balanced_XLR Posted February 4, 2006 Share Posted February 4, 2006 Ok forget it...you arn't under standing me...check out this website....you may find your answer there http://www.dplay.com/dv/balance/balance.html Link to comment Share on other sites More sharing options...
Bobbsy Posted February 4, 2006 Share Posted February 4, 2006 Balanced XLR, I think you'll find that the majority of posters here are well aware of the difference between a balanced and an unbalanced circuit and the use of common mode rejection to reduce noise and interference. However, a balanced circuit doesn't "get rid of" interference, it just provides a means of resisting it in the first place. Beyond this, what is being talked about here is not standard electrical interference...it's an RF signal being picked up by the system. The causes and cures of RF interference can be harder to track down than electrical noise. There have been some good suggestions made...but turning down the amp and over driving the mixer is not one of them. Bob Link to comment Share on other sites More sharing options...
Lamplighter Posted February 4, 2006 Share Posted February 4, 2006 Greateastern This is a problem that requires step by step analysis. To suffer Radio breakthrough requires two elements, something to act as an aerial and a non-linear device to rectify or turn the radio signal into audio. I would start by disconnecting the snake from the mixer and then reconnect one complete stage circuit with microphone. Set the gain to normal operating level, is this clean, if yes add stage circuits until the breakthrough occurs. Check whether it is a particular circuit or just the sheer number of circuits that cause the problem. If it is only one or two circuits then disconnect them until you have time to check the wiring carefully. If the breakthrough is present when only one circuit is present or the sheer number of circuits seems to cause the problem then you need to find the detector. Try PFL on an affected channel, if it is present on PFL then it is either the microphone or the mixer input stage. If the mic is using phantom power eg condenser or electret type then try a dynamic instead, if the breakthrough is still present then it is a problem with the mixer. If the PFL is clean, monitor the output to the amplifier. Try disconnecting the amplifier and check the mixer output. If this has found the answer let us know otherwise tell us how far you got, I'm sure we can help further. Brian Link to comment Share on other sites More sharing options...
gtm Posted February 4, 2006 Share Posted February 4, 2006 Does your multicore run close to any sources of heat? Had a problem with an installed multicore where it had been run right beside some heating pipes inside the multi the cores in one of the lines melted and 'fused' together causing radio stations to be picked up on that line and that line only however if you had a multi with all pin1 bonded something like this happening to one line could have a knock on effect on the reat of the multi. Therefore I wouldn't necessarly dismiss the installed wiring as the problem these problems usually only occur after some time and just because its installed doesn't mean it been done correctly. How long has this multicore been installed for?? Link to comment Share on other sites More sharing options...
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