a Nice crackling noise

[PeterD]

Hi everyone,

 

Having a bit of trouble with our Allen and Heath GL4800, we get a nice crackle at random times which seems to be coming from desk end.

 

we have sent it in to get looked at but nothing was found so no repairs taken out and we received it back and its started again.

 

I've manage to isolate the problem by putting my Left and Right through 2 DI units.

 

I was wondering if anyone could give me some advice on what I need to do or what I can purchase to get rid of the problem?

 

thanks in advance

 

Peter Donaghey

[peza2010]

The fact that the di boxes have sorted the problem shows the potential that the problem could be a ground loop issue. I think we need a bit more information.

 

Does the desk move or is it an install?/ have you tested the desk on a different mains circuit/with a different power cable?

 

Could the repair centre even replicate the problem you were having?

 

Does the desk send out random crackles even with nothing plugged into it? This test can rule out anything "further upstream" causing the problem

 

And I think dependent on the desk's uses, if the di boxes sort the problem out, is that not a possible long term solution? Obviously if it goes out on dry hires thats not an acceptable solution, but if it's one venue and it works, I personally would turn a blind eye to it.

[Mixermend]

Much more information is required to make an accurate diagnosis.

Are you saying that you never experience the problem when feeding the L+R outputs via DI's to the power amplifiers?

Is this a low (or high) level crackling noise - or a disconnection of the signal?

Does it affect both L+R channels - do you see anything on the mixer metering?

[Rob_Beech]

I assume that the desk plugs into amps or a Loudspeaker Management System. Or an EQ inline, or something, rather than say, another desk.

 

Rob

[Matt Riley]

I'm sure I'm teaching granny to suck eggs here, but have you completely eliminated the possibility that it could be a cabling issue (possibly from the L/R outputs)?

 

Matt

[PeterD]

The fact that the di boxes have sorted the problem shows the potential that the problem could be a ground loop issue. I think we need a bit more information.

 

Does the desk move or is it an install?/ have you tested the desk on a different mains circuit/with a different power cable? It's an install, I have tried the desk on a different power supply, I have not tried the effects rack on a different power supply though. Crackle is still there.

 

Could the repair centre even replicate the problem you were having? Repair center could not recreate the problem at all, they even lost our bog standard hum we have.

 

Does the desk send out random crackles even with nothing plugged into it? This test can rule out anything "further upstream" causing the problem Yes the crackles are random, from very quiet to extremely loud. I have unplugged everything from the desk other then a pair of headphones and the crackle was still there.

 

And I think dependent on the desk's uses, if the di boxes sort the problem out, is that not a possible long term solution? Obviously if it goes out on dry hires thats not an acceptable solution, but if it's one venue and it works, I personally would turn a blind eye to it. Issue with the DI's was the amount of padding I had to do then caused issue when running mics. Lots of distortion and bad sounds happening. also our DI's would need batterys changed frequently so even if this was a solution I would need to purchase a rack mountable set.

 

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Are you saying that you never experience the problem when feeding the L+R outputs via DI's to the power amplifiers? It did at the time I had them plugged in but the system itself was having issues with peaking etc

Is this a low (or high) level crackling noise - or a disconnection of the signal? The crackling is mainly top + a little mid

Does it affect both L+R channels - do you see anything on the mixer metering? It fires across every output on our sound desk. L, R, M, Aux 1 - 10, Matric 1- 4, AFL and PFL.. When the crackle occurs the meters bounce so the desk is reading it. I have had all the cables unplugged from the back with only headphones and the meters still jumped.

 

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assume that the desk plugs into amps or a Loudspeaker Management System. Or an EQ inline, or something, rather than say, another Desk Yes the system goes from the sound desk to a Graphic EQ downa multicore to a crossover unit and into our amplifiers.

 

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I'm sure I'm teaching granny to suck eggs here, but have you completely eliminated the possibility that it could be a cabling issue (possibly from the L and R Outputs) As I have previously said I have disconnected all cabling from the desk minus the headphones so that I can still hear, and its still there. I have tried several different headphones to rule them out.

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Since I posted I have tried several other things, removing the earth from the desk plug; This worked for a few days and then the issue re occured with a new sound constantly on which sounds like either a fan rotating or a motor sparking. am still getting the random crackles.

 

Tried a line isaolater to see if that would help, have tried it in several locations from desk down to amps and there has been no change.

 

I have also noticed then when we turn all of our effects + desk equipment off and just have the Amps on the crackle and low hum have gone. Someone has mentioned its possibly Dirty power, but I'm not 100% sure what that would mean.

 

 

It's gotten to a point where alot of people are now complaining about it, and I agree that they should, they don't want to hire a venue with a poor system. but I have exhausted everything I can think of other then running a completly new cable from the power to where our control room is.

 

another thing that might be of use is, when the problem got really bad and I sent the desk off to be repaired they had hired us a soundcraft venue 2. while this desk was in the building for the 3 weeks we had no crackle at all. currently we have a allen and heath gl4800. is there a difference in the components?

 

Sorry its a long email I'm so lost now and I feel really bad when all I can say to a customer I... "I'm sorry, I don't know what causes it"

 

Anyway thanks for the replys anymore would be amazing

 

Peter D

[J Pearce]

removing the earth from the desk plug;

 

Erm, WHAT?

 

I might moan about a crackly sound system, but you wouldn't catch me near an unearthed class 1 appliance.

 

Dangerous. Very.

 

You of course realise you'd have full legal responsibility if there was a live to chassis fault and then someone touched the chassis and was killed?

 

Tempted to email Broxbourne Civic Hall and enquire as to their PAT Testing regime and when the desk was last tested <_<

[PeterD]

No I was not aware of that jpearce. I shall be replacing the earth as soon as I can.

 

It was solely done as a test.

Thank you for the harsh but true warning.

 

Peter

[J Pearce]

The earth wire has a purpose, its not there just to create earth loops!

 

It ensures the chassis of the device is always at 0V. If a live wire became loose inside and shorted to the chassis the earth wire carries that current safely away, and in the process of doing so blows the plugtop fuse (and these days hopefully kicks out the RCD too).

 

Without the earth wire there is little protection (though an RCD should still disconnect, but only once someone touches the device, and then they still get shocks for 30ms).

 

Not something to take out.

 

 

Audio 'earth', the screen on your connections, can be removed for balanced connections. For more on audio earthing seek out Tony Waldrons excellent articles available on the web.

[PeterD]

I shall have a look at that. Thank you for the helpful advice :-)

 

Do you have any suggestions on what this issue could be?

[J Pearce]

Dying capacitor or opamp, stray static voltages somewhere in the system, noise on the power supply, any one of a number of things.

 

Be systematic and eliminate variables. Take the desk to a different building, does it still do it; if so try using a different psu etc

[PeterD]

It has been tried in different buildings and the crackle does not arise.

 

Would you recommend trying a new power line run from power outlet to all my equipment ?

[J Pearce]

I'd get a sparky in to assess where the noise is coming from. It could be the supply, it could be another piece of equipment near the desk or on the same or a related circuit.

 

It might be easily filtered or it might need a new power feed to foh or to the noisy equipment.

[PeterD]

Ok well thank you for your input I will get this put into action to see what happens.

 

Thanks again

[KevinE]

could even be a loose wire behind the wall socket faceplate. Some mixers might be more sensitive to this than others.