speaker humming/buzzing revisited.

[S&L]

before I start with my issue, can I direct everyone to Brian's reply in another recent topic complete with sketch, which is my starting point:

http://www.blue-room.org.uk/index.php?showtopic=60451&st=15

 

at the time I was querying based on almost academic interest - I had only seen it happen in theatres where I wasn't responsible for the whole shooting match.

However, as things do, this one has come back at me and appeared in ONE small bar venue. but since I appear to be working with the same band in the same venue and since it's probably a fault in my gear, I would like to find the source of the problem.

 

the gear:

lighting - two NJD spectres, DMX control to each looped in series, operated from small DMX controller. power for controller and each light to local plug sockets (two double gang sockets for sound and light, power distribution via multiboards)

 

sound - mic and DI signals from stage to desk via multicore. digital desk, back down multicore, connection from multicore to crossover via balanced xlr's, ditto from crossover to 2 amps. direct to 3rd amp for 2 monitors. speaker cable from amps to 2 tops and 2 subs (speakon to speakon and speakon to xlr - 2 core cable)

 

the problem - lifting dimmers on the DMX controller and raising level of lights introduces a buzz into the system. it did formerly seem to be on one channel but thursday night it was across FOH.

 

the solution? - based on Brian's reply, above, the solution is balanced audio and unless I'm missing anything (?) then it should just require balanced audio from desk to amp rack - which I should have. the multicore tests fine, so logic would tell me there is a break in the balanced lines somewhere on the way to the amps (inside the rack?) after the multicore. Does this sound right. I'm not back at the venue for a while and it's not a problem elsewhere but the plan at the moment is to strip, test and rebuild the amp rack in a couple of weeks.

[sam.spoons]

If the dimmers are putting interference into the mains which is affecting the source (guitar amp, mixer etc,) then balanced lines won't help, they reject EM interference generated in the cables but not in the source.

 

Try disconnecting the inputs to the amps, does the interference go away? If so, re connect but disconnect at the other end of the cable, does the hum disappear?, if so try t'other end of multicore, any noise now? Keep working back throughout the signal chain and when you get back to the mixer mute/unmute individual channels to see if the problem is on a single channel.

[S&L]

If the dimmers are putting interference into the mains which is affecting the source (guitar amp, mixer etc,) then balanced lines won't help, they reject EM interference generated in the cables but not in the source.

 

Try disconnecting the inputs to the amps, does the interference go away? If so, re connect but disconnect at the other end of the cable, does the hum disappear?, if so try t'other end of multicore, any noise now? Keep working back throughout the signal chain and when you get back to the mixer mute/unmute individual channels to see if the problem is on a single channel.

 

doesn't this contradict Bob's assertion that any leak to mains earth should not effect an audio signal if balanced lines are intact?

 

the problem has been on a single channel but last Thursday was across the main PA outs - which is what I said above - indicating that the problem wasn't generated from a single source but rather after the sources had been returned from the console. but since I can't replicate the problem elsewhere, I'm unlikely to ever get time to track out the problem at the bar in question - so I need a way to track it out or at least eliminate possibilities, away from the venue.

[smalljoshua]

A hum resulting from a leaked current to earth at 50Hz or multiples there-of and a buzz caused by the repeated switching of live to neutral in dimming are two different things.

 

Noise on the neutral coming from the switching is totally different to mains on the earth. Dimmers with good noise suppression are generally the best way to avoid the latter. Whilst the former comes from good earthing practices.

 

Josh

[sam.spoons]

Not necessarily, if the dimmers are causing hum or noise in a source e.g. a guitar/lead/amp setup then the balanced lines in the rest of the signal path (mic-mixer-power amps) can't remove it. What they can do is prevent any electromagnetic interference in the vicinity of the balanced cables being amplified/audible in the PA.

 

Single coil guitar pickups are extremely good receivers of stray EMI so humming/noisy Strat players can't be cured by using balanced cables (not electrically at least http://www.blue-room.org.uk/public/style_emoticons/default/rolleyes.gif), the hum/noise is picked up by the guitar's single coil pickups themselves.

[S&L]

Noise on the neutral coming from the switching is totally different to mains on the earth. Dimmers with good noise suppression are generally the best way to avoid the latter. Whilst the former comes from good earthing practices.

 

so are we saying that with regard to noise from dimming lights that Bob's original answer is INCORRECT?

it would be helpful if there was consistency here so I could at least understand the problem.

 

and IF noise suppression of dimmers IS the way to go, how does one do that for a simple set of spectres? are we suggesting that another controller is the way to go?

[smalljoshua]

Brian's answer is correct. Which is the case for Hum. Where the noise is superimposed on the earth or radiated, what Brian said is exactly the case. Where poor filtering of noise (at source) causes the noise to be induced into the mains wiring (not the earth) it can cause interference issues on the supply side of OP-Amps which in turn is superimposed onto the signal. Poor noise suppression in the audio equipment (very poor, not something I'd expect in anything in use today) can cause this. There could be a noise suppression cap in something which is loose. It wouldn't affect any of the operational characteristics of the device apart from greater susceptibility to noise.

 

The advice to systematically work through the signal chain until the point where the noise is introduced into the system is found still stands and is the best way to find the source of the problem.

 

Josh

[S&L]

The advice to systematically work through the signal chain until the point where the noise is introduced into the system is found still stands and is the best way to find the source of the problem.

 

except that as I keep saying, the buzz/hum cannot be reproduced anywhere other than this venue, even though the lights, speakers and amps are all set up in similar proximity and identical configuration in a number of other venues. If the buzz isn't there when I get it back to bas, I can't do much to find it.

 

if the noise cap(acitor?) is an issue in one venue, wouldn't it be an issue in most venues? How would I test it back at base? and do I assume correctly noise caps only appear in amps? is there a way of testing these amps for a faulty noise cap when said noise can't be reproduced?

 

 

[david.elsbury]

Uh... so... the problem only occurs at one venue. Therefore the easiest and best way to trouble shoot is at that venue with the kit set up.

 

If you can't do that, and if it doesn't fault anywhere else, you really are chasing your tail.

 

Also you are pretty much wasting your time ( and ours) because troubleshooting 101 is to get the system while it is playing up and to systematically eliminate one piece at a time till you find the fault. If it does not play up anywhere else... how on earth will you find the fault? ?

 

Tbqh it looks like you are clutching at straws trying to speculate on some amazing sounding technical reason for your fault. You refer above to "changing the controller" - a DMX controller will not be the cause of the issues. The DIMMER may well be however fault finding as above is the only way to sort this out.

[S&L]

Uh... so... the problem only occurs at one venue. Therefore the easiest and best way to trouble shoot is at that venue with the kit set up.

 

If you can't do that, and if it doesn't fault anywhere else, you really are chasing your tail.

 

Also you are pretty much wasting your time ( and ours) because troubleshooting 101 is to get the system while it is playing up and to systematically eliminate one piece at a time till you find the fault. If it does not play up anywhere else... how on earth will you find the fault? ?

 

sorry I thought that was implied by saying it was ONE VENUE in my original post and it was a BAR JOB...i.e. fast set up and tear down - hence the conundrum of how to eliminate a problem that doesn't exist elsewhere. I really didn't think I was wasting anyone's time. just trying to work through a problem given real world restrictions.

 

with regard to the DIMMER (not being a lighting tech) are we saying that the dimmer is on board the spectre?

[dbuckley]

Hums and buzzes in the PA is pretty much always the fault of the PA. I say "pretty much" as one day maybe I will see a case when it isn't true, but it hasn't happened - yet.

 

And this is about PA; yes, electrical noise can destroy single coil guitar pickup signals, but that isn't the PA, thats backline.

 

There may be problems, and indeed even faults in the lighting system, but even so, a correctly configured PA will be immune to such things. So any troubleshooting or improvements caused by changes to the lighting system is at best a band-aid, and the underlying problem has not been solved and may occur again at any time.

 

The two things that have to be right, are, in order, all balanced connections within the PA, and single point earthing. Without getting complicated, the easiest way to achieve single point earthing is to run the entire audio system off one plug.

 

Lots of folk go "yeah, I'm all balanced", and forget the unbalanced insert leads to the outboard rack, and the unbalanced leads in the amp rack between the crossover and the amps. Racks of equipment are probably the hardest thing to get right. Most PAs are not all balanced, and thus attention to earthing becomes critical.

 

A common cause of problem is metal-on-metal touch causing undesirable multiple earthing. DI box that has lost its rubber feet on steeldeck, for example. This one is a right pain once the bouncing starts as the connection then becomes intermittent and linked to the beat. Easy to check though; with the fully configured rig, unplug the single mains plug that supplies the rig, and using a testmeter measure the resistance from the socket earth pin on the venue wall to the earth pin on the plug in your hand. If it isn't infinite then you have an earth-to-earth short which needs to be located using divide and conquer.

 

Another common cause, once one has a clean audio system, is allowing the lighting desk to be plugged in to the sound desk supply. This then connects the lighting and sound earths together through the audio earth, which is a very good way of getting noise into the audio system. Some lighting desks are better than oithers in this respect, Avo (at least on their big desks) have opto-isolated outputs keeping the earths separate, if only every manufacturer of DMX equipment were this conciencious.

 

In a bar situation where time is short, then change the order things are rigged to try to troubleshoot; so lighting rig all up first, sound desk with nothing plugged in switched on, check with headphones for noise, desk off, then amps and speakers, check for noise, multicore to amp rack, check for noise, connect multi to desk, desk powered off, check for noise, desk on, check for noise, start wiring things up into desks and multicore, checking for nosie as you go. Report back.

[S&L]

Hums and buzzes in the PA is pretty much always the fault of the PA. I say "pretty much" as one day maybe I will see a case when it isn't true, but it hasn't happened - yet.

 

And this is about PA; yes, electrical noise can destroy single coil guitar pickup signals, but that isn't the PA, thats backline.

 

There may be problems, and indeed even faults in the lighting system, but even so, a correctly configured PA will be immune to such things. So any troubleshooting or improvements caused by changes to the lighting system is at best a band-aid, and the underlying problem has not been solved and may occur again at any time.

 

The two things that have to be right, are, in order, all balanced connections within the PA, and single point earthing. Without getting complicated, the easiest way to achieve single point earthing is to run the entire audio system off one plug.

 

Lots of folk go "yeah, I'm all balanced", and forget the unbalanced insert leads to the outboard rack, and the unbalanced leads in the amp rack between the crossover and the amps. Racks of equipment are probably the hardest thing to get right. Most PAs are not all balanced, and thus attention to earthing becomes critical.

 

A common cause of problem is metal-on-metal touch causing undesirable multiple earthing. DI box that has lost its rubber feet on steeldeck, for example. This one is a right pain once the bouncing starts as the connection then becomes intermittent and linked to the beat. Easy to check though; with the fully configured rig, unplug the single mains plug that supplies the rig, and using a testmeter measure the resistance from the socket earth pin on the venue wall to the earth pin on the plug in your hand. If it isn't infinite then you have an earth-to-earth short which needs to be located using divide and conquer.

 

Another common cause, once one has a clean audio system, is allowing the lighting desk to be plugged in to the sound desk supply. This then connects the lighting and sound earths together through the audio earth, which is a very good way of getting noise into the audio system. Some lighting desks are better than oithers in this respect, Avo (at least on their big desks) have opto-isolated outputs keeping the earths separate, if only every manufacturer of DMX equipment were this conciencious.

 

In a bar situation where time is short, then change the order things are rigged to try to troubleshoot; so lighting rig all up first, sound desk with nothing plugged in switched on, check with headphones for noise, desk off, then amps and speakers, check for noise, multicore to amp rack, check for noise, connect multi to desk, desk powered off, check for noise, desk on, check for noise, start wiring things up into desks and multicore, checking for nosie as you go. Report back.

 

by far the most useful thing I have read on anything (not just my issue) this month - thank you so much for this David - exactly what I was looking for.

I can take another pair of hands out to that next gig so I can try those suggestions as we go but it's June at the moment before we are back there (waiting to hear about a May date)

Meanwhile, a few other things have occurred to me, reading through your reply.

 

1. the amp rack is all SUPPOSEDLY balanced ins, outs and betweens. everything is xlr - but it occurs to me that something may be loose and touching something else, or a line may be otherwise broken. it's been a while since I was in the back of that rack so high time I at least had the back off, tested the cables and checked everything is secure.

 

2. without going into to much detail, I mix from a 3/4 height balcony near the stage but all the power goes back to two double gang sockets on that stage. looking at their positioning they should be off the same ring main but when has common sense had much to do with wiring in pubs?!

I can't remember which was plugged in where previously but last thursday the console and amp rack were definately connected to different double gang sockets. this is easy to rectify.

 

3. this also makes me think about our power distribution. for shows like that we have a box of 10m multiboards and some longer power cables. all SHOULD be fine - but none are individually identified (other than by pat labels which often fall off) and it would be worth in any case testing these and looking for earth shorts - unlikely but worth testing along with the amp rack.

 

4. the lighting controller is powered via a voltage adapter as many of these small budget things are and has always been connected to the same power source as the desk wherever we go for these small bar shows. I'm having to recall but I'm sure it is low voltage and therefore will have a dummy earth pin. the only earth points for the lighting would be on the iec leads to each spectre. the spectres only come out on these small bar jobs (we work with others for lighting on larger jobs) and so it is likely they will always be connected to the same power source (ring main) as the amp rack. so the amp earths and the spectre earths will always have a connection. is there anything I can otherwise to control the noise from one reaching the other?

[dbuckley]

As you have downtime, test and/or take apart all your mains cables and check they are good.

 

I'd also build a 13A plug to 4 x 13A double metalclad sockets on a plank, and use that to run the entire PA and the band backline. In a pub, 13A will be plenty for whatever kit you and the band are running. You might want to make the sockets RCD sockets for the band outlets so as to add a bit of safety for that "just in case" moment.

 

If the light desk is off a wall wort with dummy earth thats fine. There isn't a cable from the audio desk to the sound to light in on the light desk, is there...? The lighting system power arranements dont matter, as long as stuff isn't off the sound distribution, its the sound supply arrangement that is key; single earth point for the entire sound rig. You are running a long mains cable from stage to FOH for the desk, yes? (assuming analoge desk)

[S&L]

As you have downtime, test and/or take apart all your mains cables and check they are good.

 

I'd also build a 13A plug to 4 x 13A double metalclad sockets on a plank, and use that to run the entire PA and the band backline. In a pub, 13A will be plenty for whatever kit you and the band are running. You might want to make the sockets RCD sockets for the band outlets so as to add a bit of safety for that "just in case" moment.

 

If the light desk is off a wall wort with dummy earth thats fine. There isn't a cable from the audio desk to the sound to light in on the light desk, is there...? The lighting system power arranements dont matter, as long as stuff isn't off the sound distribution, its the sound supply arrangement that is key; single earth point for the entire sound rig. You are running a long mains cable from stage to FOH for the desk, yes? (assuming analoge desk)

 

re building sockets - sounds like a fine idea but what does that give me that an MK multiboard doesn't? (aside from rcd's) - the bands and venues vary so I'm probably going to need at least a couple of these to make it work - at the moment we usually run a couple of multiboards out on 10m cables for the band's backline and often either our amp rack or at least one of the lights goes into this. bands are anything from 4 piece to 10 piece.

 

re lighting controller - no audio line, all worked manually by an assistant with occasional timed sequencing.

 

re lighting power source - most places we do this sort of work it will likely be one ring main supplying lighting and amps. in some cases the same double socket. I COULD split lighting and amps/backline into different wall sockets (cables would cross each other back and forth) if we are saying that will make a difference?

 

FOH console - digital x32. in most bars, it's from the same point as the amps. this particular problem venue is from the same ring main but on the last occasion wasn't on the same socket. I was always taught to take the power for the console from the same place as the backline (guy holding guitar and touching mic has both points back to same earth) - just curious but your comment hinted that this need not be the case for digital?

[Shez]

As this issue only occurs in the one venue, are you absolutely sure that the mains supply in that venue is up to scratch? Have you asked them for a copy of the paperwork from the last time the installation was tested? I've been in plenty of pubs where I wouldn't want to plug in a phone charger let alone a full PA...