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Speakers Humming


JCC1996

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Hi all,

 

I'm involved in a variety concert just now, the opening night was last night and its on again tonight. We have a slight problem in that there is a very noticeable buzzing coming from the foldbacks on stage.

 

For the sake of completeness I'll describe the whole system:

 

3x SM58 DSL, DSC, DSR

2x Rode NT5 DSL and DSR for choral reinforcement

 

These are fed through the venue's built in multi to a soundcraft desk at the back of the auditorium. The output of the Soundcraft is fed into the venue's mixer amp to power FoH. The aux (1/4 jack) is converted to XLR (just a jack to XLR cable) to be fed through a multi return into the foldbacks (DJ Tech Stage Visa 200).

 

There are 8 par cans being run from the same ring mains. The buzzing changes when the pars are being dimmed, so is this likely to be the main cause? Or are there any other ideas for solutions I can implement before tonight?

 

Thanks,

 

Joe

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The aux (1/4 jack) is converted to XLR (just a jack to XLR cable) to be fed through a multi return into the foldbacks (DJ Tech Stage Visa 200).

 

That's your problem.

 

The AUX out is almost certainly unbalanced which is not a good way to send a signal any distance. Do you have any access to something like a DI box? Or even a 'ground loop isolator'? Is there a Maplin nearby?

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You should always try where possible to have lights and sound on a separate power supply (I know this isn't always possible) as running dimmers on same supply as sound can cause a nasty buzz!

 

This is a bit of a myth really. If dimmers make the sound system buzz then something is wrong somewhere. Unless you have two independent generators, using a different socket in the building is not going to fix it.

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This is a bit of a myth really. If dimmers make the sound system buzz then something is wrong somewhere.

I was about to say the same. Moving onto a different mains supply is simply hiding your dodgy sound systems's faults.

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If you can, it usually helps to run the mixer from the same mains outlet as the amps - this is why many people have a mains feed taped to their audio multi. Plugging in to whatever wall outlet happens to be nearest can make grounding problems worse, which manifest themselves as buzzing in the audio.

 

I'd be slightly wary of the DI route as these often attenuate the signal to mic level, which will be no good for your amp. Which Soundcraft desk is it?

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Which Soundcraft desk is it? Most of their desks have balanced outputs on Auxs, so it may be that you just need to change this bit:

The aux (1/4 jack) is converted to XLR (just a jack to XLR cable)
for a TRS-XLR cable.
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If you can, it usually helps to run the mixer from the same mains outlet as the amps - this is why many people have a mains feed taped to their audio multi. Plugging in to whatever wall outlet happens to be nearest can make grounding problems worse, which manifest themselves as buzzing in the audio.

 

I'd be slightly wary of the DI route as these often attenuate the signal to mic level, which will be no good for your amp. Which Soundcraft desk is it?

 

The foldback system the OP is using is a portable PA with mic inputs so, if Mark's cable is not an option (available from Maplin too) a DI would work.

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I don't have the mixer in front of me right now, but from the website, I think it is an MFXi 12... the jack-XLR was provided by the hire company and I haven't even unplugged it to see if it is TRS... there's every chance it is!

 

As has been said, the foldback system will accept a mic level input, so that's not a problem. FoH and the mixer are from the same socket, but its not going to be possible to have the foldbacks powered by the same socket unfortunately.

 

Thanks,

 

Joe

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I'd be slightly wary of the DI route as these often attenuate the signal to mic level, which will be no good for your amp. Which Soundcraft desk is it?

 

The foldback system the OP is using is a portable PA with mic inputs so, if Mark's cable is not an option (available from Maplin too) a DI would work.

 

Looking at it, the only XLR inputs on that system are at mic level so this could be a problem with the output level of the Soundcraft being set really low and the mic inputs being set at normal levels. Any noise on the output of the Soundcraft would be much more apparent than it would be if the levels were correct. A DI box next to the Soundcraft would probably be best here

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...an MFXi 12...

 

So 'impedance balanced' AUX outputs then. Which are only 'balanced' when fed into balanced inputs, no good at all over a long cable into an unbalanced input.

 

True in that the long run may be no good into an unbalanced input, however the impedance balanced output will behave the same as a differential balanced output, or a transformer balanced output under the same conditions. The noise is removed at the balanced input, not the output. Unbalanced input means no common mode noise rejection.

 

Mac

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...an MFXi 12...

 

So 'impedance balanced' AUX outputs then. Which are only 'balanced' when fed into balanced inputs, no good at all over a long cable into an unbalanced input.

 

If he's feeding an XLR then the input is almost certainly balanced - just at the wrong level in this case.

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