Church setup with monitor hum

[Yorkie]

Our church has the following setup:

 

Spirit 8 32ch desk

Aux 1-4 feeding Behringer DI4000 down 4x 3m unbalanced jack cables

DI4000 feeding 20m multicore

multicore feeding 4x monitors; 2x Studiomaster PAX12+ and 2x Studiomaster PAX10+ (all powered and balanced inputs)

 

We have a problem with a slight buzzing in the monitors. We have spent some time today going through various tests, all with the desk completely muted and the desk completely unmuted, but to no avail:

* Continuity testing all the cables and all the lines in the multicore

* Pressing the earth lift switch on the DI4000, this made the buzzing worse

* Removing the DI from the equation and just feeding the monitors directly from the desk from balanced jacks into the multi. Still buzzes

* Removing the DI, feeding the monitors directly from the desk and disconnecting the ground at one end of the multicore. Buzzes worse

* Using a different DI4000. This makes no difference whatsoever

* Using an EMO passive DI box instead. This lessens the hum but does not eliminate it. Using the earth lift switch on the EMO DI makes it slightly better still.

* Powering the monitors from the same power socket as the desk (the desk, main amp rack and the monitors are powered from different phases of the same supply, via different circuits on different distribution boards normally). This did not lessen the hum.

* Turning off the loop amp. This doesn't solve it

 

When we remove the DI and feed the monitors directly from the desk we get another issue. Even with the entire desk muted, the radio mic (Shure UHF) can be heard coming out of the monitors. The mic can be 10m away from the monitors, with the loop amplifier turned off and this is still an issue.

 

We know that the monitors aren't high quality items; they do have a slight power supply buzz within their amplifiers, but of themselves whilst there are no cables connected to them they make no audio hum.

 

The plan at the moment is to acquire a decent 4 channel DI box and install it as close to the desk as possible, but it does seem that this won't solve the buzz problem altogether. Will this system ever work without a DI box?

 

Am I wrong to think that (impedance) balanced desk outputs, balanced monitor inputs and the shortest earth path in the power supply should leave us problem free?

 

The same problems have never affected the FOH speakers.

[paulears]

The first thing you need to do is disconnect every single input and output from the mixer and start from scratch. So first connect the mixer to your main amps, turn them on and see if it hums.

Hopefully you'll hear just a gentle hiss, and no hum. If this first stage produces hum - then you have your first earth loop. If the amps are distant from the mixer, then suspect slightly different earth potential at both ends. A first simple solution to this is to find a long mains cable, plug it in at the amp end to the sockets there, and then power the mixer temporarily from this cable. It probably will solve the hum in one single swoop. If it does, can you make this cable permanent, and run all the foh gear from it?

 

If the first mixer to amp connection was clean, no hum - then start to add each output cable one at a time, testing each actually does something. If at some point, plugging in produces the hum - it shows you where to look - and then maybe you can just repeat the first step again - power the offending bit of kit from your 'quiet' mains cable you know is trouble free.

 

Then move on bit by bit. You MUST be systematic, or the cause gets lost in the ok connections. One at a time, testing each one as you go.

[TAG1960]

Hum problems are ofter caused by differential earthing in your 240 volt mains supplies. If you have a Front of house mixer powered locally, and amps, Foldback and stage equipment powered locally at the stage there coulds be an earth potential mismatch. Sounds to me like the Mixer and front of house equipment may be on a different ring main circuit to your stage sockets. If it is it could have a different earth potential. A quick test to prove this involves running a long lead from one of you stage sockets, to your front of house possition, and use this to power your mixer.Hopefully this will remove the hum!

 

Often different ring mains are earthed locally (usually to copper piping), and different circuits in one installation can have different earth potentials. If you are unlucky enough to spread your gear across two ring mains you can get a 50hz mains hum. Correct mains distribution, and signal earthing should cure the problem, but do not start removing earthing to solve ground noise issues.

 

This can be a time consuming process so good luck

 

Neil

[c.cam108]

Just to add, we have no hum at our church as the amps are all fed from a long permanant extension from the FOH position. It's interesting to see both responses suggesting the opposite.

 

Is there a particular reason why? We find it much more convenient being able to switch everything on and off at FOH rather than running back and forth.

[timsabre]

I'm pretty sure the Spirit desk has balanced jack aux outs, is there another reason for using the DI?

Have you tried moving the DI box around a bit, sometimes they can pick up magnetic fields from power supplies etc which can introduce a hum.

[themadhippy]

Often different ring mains are earthed locally (usually to copper piping),

If your relying on copper pipes for your earth its time for a rewire.

[Simon Lewis]

Is there a particular reason why? We find it much more convenient being able to switch everything on and off at FOH rather than running back and forth.

 

Because the desk doesn't draw much current, so running it on a longish extension shouldn't cause too many problems. Doing it the other way round could be an issue with larger amps.

 

My church has a Spirit 8 (probably mentioned when we were discussing DIs for this problem earlier). I suspect that it suffers from either the Pin 1 problem or some similar power supply / earthing issue. We rewired the system so that everything is run from a single 32A mains supply via an EMO switcher to ensure correct power up/down. Before that point, hum was a major problem, especially on monitors. We plugged littlelites into the back of the desk, and the buzz became unbearable. Soundcraft refused to concede there could be any problem.

Admittedly, several of the main amps were eventually changed and a foldback desk added so the monitor amps are driven by a fairly local desk, but it was running all the power from the same point to all the audio components that solved the hum. Oh, a nice clean 63A feeder to the stage didn't go amiss either...

 

As probably mentioned before, DIs are a sticking plaster over a bigger problem. The issue of earth loops / psu problems /etc. need dealing with first. Can you try different powered wedges or amps & speakers to see if your monitors are partially to blame?

[paulears]

Most people feed foh from the amp end because that's where the current demand is higher - FOH might only need a few 4 or 5 amps, while the amp racks in bigger installations might need many times that. No reason why you can't run amps on a feed from FOH, just that it's less common.

[c.cam108]

That's what I'd figured - current draw.

 

Thanks

 

Colin C

[dan slv-tech]

If the buzz/ hum your hearing in the monitors is the same with or without an audio input, you should be looking at

 

A) integrity of the monitor - mains cable good? has earth on it? is the monitor safe electrically?

 

B) Is the mains supply good? if you have poor earthing/ little/ none then this will be your issue

 

C) As mentioned before you could have different earth potentials between different bits of kit - this can be rectified on the audio side with resistors/ isolation transformers (your behinger DI I believe is electronically balanced, the emo is a TX based DI)

 

1stly id find the problem, then try to address it, DI boxes are ok, but to be honest isolation transformers work better if indeed it is hum from the audio, and in this case it sounds like its not.

 

More than likely

 

1) bad earthing in the building

2) something else on that phase thats upsetting your kit - immersion heaters, heaters, sodium lighting?

3) or simply the integrity of the el cheapo monitors - ive seen very few powered monitors that dont have noise on them!

 

good luck

 

Dan

[paulears]

Really?

ive seen very few powered monitors that dont have noise on them!

I've never noticed a problem at all? A few are a little hissy, when they're capable of being very loud, but in a silent room - but amps and ordinary speakers are the same - but I've never heard any hum with nothing plugged in? Do you have some specific problem brands in mind? handy to know in case we every consider buying them?

[Bobbsy]

The only sure way to hunt down the problem is to go back to Paulears' initial post, disconnect everything, and systematically reconnect things one at a time, starting with the mixer then the problem monitors (and even to these on circuit at a time. Anything else is just flailing around blindly--you may hit a result or you may not.

 

I certainly wouldn't buy any extra gear until you've done Paul's process.

 

Bob

[Yorkie]

Thanks for all the input folks. I realise now that we do need to take a bigger approach to this and be completely systematic in our faultfinding; it may well take a day and two hours was all we had available on this occasion. My biggest source of confusion so far though is the fact that reducing the mains earth path between the mixer and the monitors appeared to make no difference. I think the next step will be to arrange a day when the building and two of us are free. Dismantling and rebuilding the whole setup is on the cards anyway because the control area is getting into a state (the company that installed it didn't label comprehensively, which doesn't help)

 

We can try some other powered speakers instead of the current monitors as part of the process.

 

The system installers clearly didn't think there would be an issue with mains earth path; despite the desk and FOH amp racks only being 5m apart, they are not powered from the same circuit or phase