Radio 5 loud and clear

[brush2]

This is my first post and I am gaining new experiences as a retirement project with our local musicals club. My slow learning process has been helped by the 'sound' lads in the club. We are running a show at the moment and we are picking up Radio 5 live on just one speaker array of 3. They are all separatly 'amped' ..... can someone please advise how we can stop the live football interfering with our 'love scenes'??

[pritch]

How do you have this arranged? Do you have the speakers and the amps together with short lengths of speaker cable, or do you have longer lengths of speaker cable and the amps distant from the speakers?

 

How have you got the desk (or whatever source you're using) connected to the amps? Balanced or unbalanced line? How long is the run?

 

Are the cable runs for the speaker in question taking a noticably different route to the others? Is there any chance you could re-route the cables?

[p.k.roberts]

There are a number of ways the interference could be getting into your system; so the first thing to try and do is narrow down the possibilities. Try turning every thing down and off (amp and mixer) and then disconnect mixer output/amp input cable. Then systematically reverse the process. Turn on the amp, turn up the input level, turn it down again and connect the input cable, turn up the amp input again etc. etc. until the interference appears. This will hopefully give us some better info to work with. Actually, simply going through this process might cure it - dirty or corroded connectors could be the problem.

[Simon MFR]

Regardless of the lengths of the cable, I have an idea that you are using 'doorbell' type cable (unshielded) that allows interfering signals to, well, interfere...use a shielded cable (similar to below) that will totally eliminate the likes of Murray & Seaman et al ruining your hot lurve scenes...I can also recommend that all the cable is the same length, despite how far/close the different speakers are.

 

http://www.vhaudio.com/images/shieldedtwistedpairbulk.jpg

[paulears]

Why would anybody be using doorbell type cable? What arouses your suspicions?

 

Assuming that you don't actually live next door to the offending transmitter, when the solutions can be a little more radical - the probable cause can be found by working out what is happening. RF breakthrough, in this case from 693 or 909KHz AM transmitters can be a pain. Your profile gives no clue as to where you are, so I can't check your location vs transmitter location. The two main areas to look at are where in the signal chain the problem enters. You say you have multiple amps for the multiple speakers, and the breakthrough is on just one amp? Or even just one of the two(?) outputs from it. This suggest that the problem isn't with the feeds to the amps, but from the offending amp onwards. The most common, in my experience, is rectification by the power output (not input) stage. So what you should try is to disconnect the loudspeaker feeder, and try a short cable, local to the amp and see if the breakthrough vanishes. If it does, then your speaker feeder is acting like an aerial, and the output stage of the amp is performing as a basic rectifier/demodulator. Common solutions are using screened speaker cable, or winding a few tuens of the cable you already have installed through a large ferrite ring, near the amp. This may well block out the rf that's getting in. The input stage could be the culprit, but if the amps are identical, then the problem is more likely to be a fault, rather than a design flaw - so checking shielding, grounding, balanced to unblanced conversion problems, floating amplifiers, rear ground switches if fitted. A very strong signal can simply swamp the system, but if it is this strong, it's less likely to be just on one speaker.

 

If the amps are all together, then swapping the speaker cables between them will help no end. Does the problem move, or stay the same. Bit by bit, you should be able to isolate exactly where the issue is - and that's the first step.

[j_b]

Agreed 100%

 

I watched some "experts" trace a problem with suspected AM radio breakthrough onto talkback once.

 

In the end, it surned out that the fault was the show relay mic being left switched on, next to a radio - tuned to the station in question.

 

No amount of expensive cable would have solved the problem here.

 

Equally, I know of cases where people got perfect Radio 4 (198kHz) only when their HiFi was switched off. They did live down the road from the transmitter though.

[Simon MFR]

Why would anybody be using doorbell type cable? What arouses your suspicions?

 

Door bell type cable, you'd be surprised. To some, cable is just cable.

[brush2]

Thanks chaps.... the AM station is about 6 miles away .... and the cables are are all balanced ... so I'll go and make a few adjustments ...Off to the Hall now so will report on actions later.... Thanks again

[brush2]

Further investigation has isolated the problem to three mikes hanging from a lighting bar. Cables have been rerouted - no difference. By insulating the clamps from the bar an improvement was made. The lighting bar itself is about 9 metres long - the local 5 live transmitter is 909 - could this be the cause of our problem??

[pritch]

It's fairly unlikely that the length of the bar would have anything to do with it - at 909kHz, the wavelength is about 330m, although that said in a strong enough RF environment, pretty much any large enough metal object will act as an aerial.

 

If you've had the microphones in direct metallic contact with the bar, then you've probably managed to form a dipole, with the earthed lighting bar (and depending on how the bar is rigged, possibly the metal roof truss as well) acting as the ground plane.

[p.k.roberts]

So just so I can get this clear in my head, three mics plugged into individual cables straight into the mixer - no intermediate wiring via multicores etc. You previously said that the system is all balanced, but have you actually checked the cables in question? Just because they have XLRs in the end doesn't necessarily mean that someone hasn't tried to save a few quid in the past and wired them with single core cable. I would also check that, even if the cables are using twin core screened (or even star quad!) that they are wired correctly (pin 1 - screen etc.). I think I'd also start checking how good the earthing system is - especiallly on the mixer.

[brush2]

Thanks again for your help. All now derigged so will note all your advice when we next use the hall. The cables have all been checked and are wired correctly.