picking up local BBC radio on FOH pa
Posted 25 September 2008 - 01:38 PM
Abbeydale Picture House Sheffield
Posted 25 September 2008 - 02:06 PM
Firstly, improving the building earth is unlikely to help, and is often surprisingly tricky to do while still complying with the requirements for an electrically safe installation.
Making sure all your kit does NOT have a pin one problem will help, as will common mode chokes at all inputs and outputs. Starquad may help as will passive low pass filters.
Make sure the whole rig is balanced.
Some gear (Mackie and 'B' desks) are notorious for rfi susceptibility and can be a real fight to get to work right (As in it is often easier just to replace with something that works).
Take a divide and conquer approach to fault finding, and remember that the fault may not necessarily lie with the last item you added (which makes this a bit of a pig).
Tony Waldron has some words to live by on his web site, and I would heavily advise that even if you ignore the rest of it, the fitting of a common bonding plate for all rack IO is a good step under these conditions. The all too common 'lift the shield at one end' advice is very likely to make things worse given the proximity of that transmitter.
Good luck, your going to need it (This is pretty much the nightmare scenario).
Posted 25 September 2008 - 02:35 PM
What you need to do is a bit time consuming, but you MUST be methodical. Draw out every audio line going in and all the speaker cables going out.
What you need to do is isolate whee the breakthough is getting in. So use this example. You have a stereo feed to the rack amplifiers, plus a couple of aux feeds. Disconnect them all by physically pulling the connectors out.
Does the interference stop?
IF NO, go to B below
IF YES, read on
A - it stopped. Reinsert the feeds one by one. If you find just one that introduces the breakthrough, then if another spare line is available, try that and take the interference prone one out of service. Could be something very simple like either a broken ground or maybe a good ground that needs to be broken, allowing it to float. If attaching any of them starts the interference/breakthrough it's more difficult. Having some XLR couplers with lifted grounds around is always handy.
B - it didn't stop when the inputs were pulled out.
This next step involves deciding if the interference is being picked up on the loudspeaker cables. To do the test, you will need a local speaker on a short cable that can be used to replace the long runs. Don't forget to check you don't have unused loudspeaker runs connected - some permanent systems have live speaker sockets in useful places - permanently connected, you just plug in to use them. These can be a simple way for RF to get into the system - long unterminated wire doing a great job of acting like an aerial.
So you can replace each of the amp outlets in turn with the local one. If the interference appears on the local speaker, then it's unlikely to have got in that way.
If the problem is on the inputs, it is possible that the audio feed is not the part of the system open to the RF energy - it could be getting in by mic lines, or sometimes even the mixer, and the interference is just superimposed on the wanted audio delivered to the amps.
All this is simpler if you draw it all out as you can tick off which links are trouble free. The idea is to be systematic, and use the clues. Is the breakthrough present on the mixer output? Check the headphones - is it clean, or dirty. If you cannot hear the problem FOH with the mixing and processing kit, then move further along the chain and try to work out where it's getting in. If you have disconnected stuff, start re-assembling at the speakers and work back towards the microphones at the very opposite end.
It's possible strong RF levels are just swamping the electronics, but in kit with metal boxes, this is unlikely, even at a moderate distance like next door. The wavelength of the BBCs transmissions is quite long compared with VHF and UHF, so the most likely entrance route to your system is via long cables. They make pretty decent aerials - especially if one end is floating - maybe ground loops were a problem, so somebody cut the grounds one end to stop hum. It cut the hum, but gave access to strong RF fields? AM broadcasts have the ability to be 'decoded' into analogue audio by very simple means. People might have heard of wartime radio sets called cat's whisker radios? Really simple receivers that were powered from the RF energy coming in - with a long, tuned aerial wire strung down the garden, there was just about enough energy to allow a pair of headphones to hear something.
The output transistors of power amps can do the same job under certain circumstances - and being connected to loudspeakers already means you have a kind of basic radio installed by default. This situation is always talked about, but my experience is that the breakthrough is always VERY quiet. Most problems are with the breakthrough being introduced at mic or line level, and then amplified. Look too for just one device plugged in that is the culprit. I once had terrible problems with AM breakthrough and tracked it down to a cassette recorder. The thing was making a great job of demodulating the unwanted RF and introducing it into the system. It was fed through a mixer, that had an pre-fade aux turned up, and the breakthrough was getting in that way. The fact it was only on the monitors had me doing all sorts at the amp end, when it was coming in he very opposite end of the system.
Once you find the place it's getting in you can move forward. It's a time consuming frustrating process.
If your interference isn't coming from an AM transmitter - then the same basic concepts apply - but FM interference is rarer.
Posted 25 September 2008 - 04:52 PM
Telephone engineers sometimes have this as well and cure it by winding the line pairs around ferrite toriods..but clumsy for PA applications.
K. Edwards Electronics Engineers
Posted 26 September 2008 - 02:26 PM
An easy way to check if the multi is at fault is to use a mic and lead(that has been tested and all 3 pins are there ok), plug it into the stage box and check if you are getting interference. If so take the same mic and plug it into the same channel of the desk. if the problem goes away then its the multi if not then could be desk or mic.
Try out the few things suggested and let us know how you get on.
Posted 26 September 2008 - 09:52 PM
Speaking from bitter experience here - I run an AM radio station, and the transmission electronics are directly underneath the mast...
Posted 26 September 2008 - 11:26 PM
Strangely back when I used to do that stuff for a living, we had very few problems with the transmission and processing kit a dozen meters away from an electrically short vertical radiator, it was the studio stuff a few hundred yards away that always got hit badly, I always put it down to polarisation effects as the aerial was too short to have meaningful directivity.
Ferrite's help a bit common bonding helps a bit and getting rid of pin one problems by fair means or foul helps a lot. Pretty much everything must be balanced or it will be a complete nightmare.
One place that is often overlooked in this hunt is actually the mics themselves, some of them are horrible in this respect (even from expensive German manufacturers that should know better ), there are now some XLR plugs with 360 degree screen bonding and a capacitive connection between pin one and the shell available, see for example the NC3FXX-EMC and related items that may help if you find it is getting into condenser microphone electronics.
 Me? Bitter? It only took 2 days on a fixed price job to figure it out!