Mixing Desk Picking up Radio

[Sam Mottram]

Good Morning

 

I was doing a gig last night at church and we manged to pick up our local pirate radio station . This doesn't happened too often but we get it every now and then .

 

The gear we are using is a Berenger 32 channel Eurodesk with a 24 channel mutlicore .We generally have 2 singers , keyboard with mike ,electro Acoustic guitar and a drum kit .The guitar and the Keyboard is run through DIs into the core and everything else is direct.

 

Last night the band brought there own amps witch we than ran a line out into the core also the guitarist had four FX pedals. I got Radio off the lead guitar but I changed it from channel 4 - 12 and that sorted out the problem . but then half way through evening I got radio on on Channel 6 which was the bass guitar .

 

Does any know why this happened and what are the best ways to avoid this

 

thanks

Sam

[paulears]

Was the radio picked up by the guitars there without the PA on?

 

RF breakthrough normally is a pain if the unwanted signal is strong. Proper earthing sometimes works, unshielded speaker cables can be a good way for the rf to enter amps. You can buy ferite rings (they are what is n the little lumps found on many power cables to computer and audio kit nowadays) they help de-couple the kit from this kind of thing. A nice big circular magnet with a hole in the centre (as in old loudspeaker magnets) can work ok if you wind 3 or 4 turns of the speaker cable through it. One tip with it comng through the PA. Carefully disconnect each input and output to the desk, one by one and if it suddenly goes, or gets less you have identified a suspect bit of kit. then try different cables to check they aren't the problem - maybe try an extra ground to see if that makes a difference.

 

You should ALWAYS report the pirate station at the first opportunity - you might find the lcoal council can help - they spend a lot of time tracking these down - the government agencies used to do this, but it's a bit low on their priority list nowadays.

[Sam Mottram]

Was the radio picked up by the guitars there without the PA on?

 

No it was coming from the desk but is seemed to be a stronger signal with guitars in but when I move channels on the multicore it solved it for one of them also I can sometimes get it from the desk with out any thing plugged in the mulitcore .

[Network_Josh]

This can sometimes be caused by your cables being wired up the wrong way - ie you send the signal down the earth/common - the channel with the highest gain will pick up radio stations or just a ultrasonic interference - you cannot hear it, but your desk will pick it up

[PoppaDom]

At the risk of sounding unpopular (asides from checking through your kit which other members will detail on here no doubt) it is important you report the pirate radio to the authorities and give as much information as possible (ie: Dates/Times of operation etc)

 

I love music, and have always been torn on the issue of pirate radio; but as a professional user of such systems this can prove to be a real pain in the a*s!

 

Poppadom

[ampcats]

check the continuity of your multicore - you will probable find that you are missing a connection or 2 - a lead with one core / shield lifted (or broken) acts like an antenna - and may end up, as in your case, picking up radio signals - you might find that by adding a short length of cable to the desk end of your multicore on the offending channel(s) you diminish the effect - but conversely, you may also make it stronger.

 

a humbucker that can lift the shield and / or audio ground might also help.

[Andrew C]

Report in this month's Pro Sound News:-

OFCOM "refocusing" efforts to combat non-licenced broadcasters

OFCOM Webpage

[paulears]

lifting a shield, I'm afraid is a brilliant way to get more rf into the system - you have a nice long bit of wire, grounded at one end, open at the other - a capacitor bridging the screen and ground may de-couple it a bit.

[Dmills]

lifting a shield, I'm afraid is a brilliant way to get more rf into the system - you have a nice long bit of wire, grounded at one end, open at the other - a capacitor bridging the screen and ground may de-couple it a bit.

 

A quick fix that can sometimes help is to wind the audio cables a few turns around some convenient metalwork (Table legs, a scaffolding bar, that sort of thing).

 

I am slightly surprised by picking up an (I assume) FM Pirate, it is normally AM that gets into audio gear. Still I can think of a few possible transmitter faults/poor designs that might put AM onto the carrier along with the FM.

 

Those desks are known to be a little problematic wrt RF, it is down to poor design of the mixer but what do you expect for the price?

 

One possible fix for the "pin 1" problem (which is most likely to be what this is), is to make up a load of short jumpers that connect the screen to the connector shell instead of pin one at the desk end (assuming metal XLR sockets), this causes the RF current to flow in the case of the equipment rather then via the PCB tracks inside the desk where it could pollute the reference for the single ended internal circuitry.

 

Do give OFCOM a call, but don't necessarily expect quick action.

 

Regards, Dan.

[ampcats]

lifting a shield, I'm afraid is a brilliant way to get more rf into the system - you have a nice long bit of wire, grounded at one end, open at the other

 

as I said in my prior post...

 

a lead with one core / shield lifted (or broken) acts like an antenna

 

on the problematic channels on the multicore, if shield is lifted at the desk end (maybe I should have mentioned that), it would at worse turn the now 'free' shield into an unpowed re-transmitter of what it picked up

 

while making up custom leads with decoupling capacitors or shorting screen to shell are both viable alternatives, if a humbucker with screen switch is sitting on the shelf, it saves heating up the iron - but if he is heating up the iron, he may as well fix the dodgy cable

but when I move channels on the multicore it solved it for one of them

the wrapping of cable round a scaff bar or similar is a quick and possibly successful easy way to kill an induced unbalanced signal - definately worth a try...

[paulears]

while making up custom leads with decoupling capacitors or shorting screen to shell are both viable alternatives, if a humbucker with screen switch is sitting on the shelf, it saves heating up the iron

 

 

Never heard of a humbucker before - is this some kind of DI using a transformer? I'm assuming it doesn't have reverse wound coils like guitar humbuckers?

[Andrew C]

Never heard of a humbucker before -

Canford sell them, at a hideous price! No idea what is in them though; tx at a guess...

[ampcats]

they are essentially 1:1 isolation transformers, as in DI's, but the audio ground and shield can be independently lifted, or let through via a pair of transformers (one for screen, one for audio ground) that while I do not know how they are wired, I would assume as paulears mentioned above, that they are reverse wired - as their better known namesakes in the guitar world are...