Buzz buzz and arrggghhhhh.....

[gazzer1uk]

Hi guys,

 

I am very amateur at this and for our band we now have 3 x showtec multidim units, par 56 x 8and 4 x 64 and a DMX showtec desk. All works fine and were happy save for the interference over the PA.

 

I am sure your familiar with that horrible buzz you get when the channels are being switched on and off or if your increasing/decreasing the dimming effect.

 

We've found that the buzz occurs on pre programmed sequencing and between full off and full on, at the extremities it does not seem to happen.

 

We always try to put the packs on a seperate supply from the amps etc, and to keep lighting cables away from other cable types.

 

What faults could we look for, would poor earths show this up for example, or is there a filter system that can be employed to reduce the problem?

 

Any hints and tips would be most appreciated, and sorry if this is repetition!!!!

 

Gazzer

[greenalien]

If your venue has 3-phase power, try using different phases for the PA and lighting, it's the easiest way to reduce interference.

Unfortunately, cheap dimmer packs are notoriously noisy, your next alternative may be to buy higher-spec dimmers - the kind of price you would be looking at would be around £400 for a six-channel unit.

[Ike]

It could be any number of things. Realistically the fault is most likely to be with your PA, make sure you're using correctly balanced cables throughout and all the screens are earthed. In my experiance putting a PA on a separate phase rarely makes any noticeable difference and usually isn't worth the effort. Earth and neutral are common to all phases and there are plenty of opportunity for capacitive coupling and radiated emissions to cause a problem.

[paulears]

Moderation:

I am very amateur at this

This is not the right way to use this word. Here e have a mix of professionals (who make their living from this kind of stuff) and amateurs (who have other jobs and do this for interest). So using "amateur" to mean not too good, less knowledgeable, or poor is a bit offensive. Some amateurs are highly qualified and we try to not use this word at all in this sense

 

As has already been said, dimmer buzz from poorly filtered kit is quite common, and although the books say keep power and audio cables away from each other, and try not to have the sound kt on the same phase - it rarely is as simple as this. The actual item making the problem worse can be quite remote. I don't usually have this problem in my venue, but the other day, I did. In the end I started to look for kit that had changed. Tracked it down to an old CD player, from the store, with a figure 8 mains lead. Disconnecting it brought about silence from the dimmers. It was at the rear of the auditorium, and for convenience sake, had bee plugged into a 4 way from the lighting box. Plugging it into the sound kit supply cured the problem.

 

Maybe you can try disconnecting various items of the sound kit, and plugging them in one by one - you could be lucky and find the culprit - on the other hand?

[musht]

Dimmer buzz will always be worst at around 50% intensity as the mains waveform is getting chopped in half at this point generating all sorts of electrical noise at 50Hz and upwards into radio frequency.

 

More expensive dimmers pay more attention to layout, quality of components and hence RFI reduction but mainly they use a larger choke which slows rise time every time the triac switches on through the half cycle. Unless you go to sine wave dimmers, very very expensive, no dimmer is silent.

 

Your on the right track with looking for poor earths , DO NOT CUT EARTHS, sorry for shouting and sure you know this already, but just in case someone came accross topic and thinks because it worked its OK....

 

Personally always refer to the mains Protective Earth, the yellow green one in the plug, as Earth and Signal Grounds, like the braided screen in a cable as Ground. Just to keep the idea straight in my own head.

 

Sure removing the Earth in the mains plug from a buzzing guitar amp makes the buzzzz go away, it also turns the whole plot into an accident waiting to happen. HSE guy described a `live` electric guitar as like gripping a switched on electric fire.

 

Even expensive dimmers can induce hum into a poorly GROUNDED sound system, it may not be a poor connection , it may be a ground loop where the ground has 2 routes to go. Sure good advice on tracing and solving these will follow from hum head, er, sound orientated Blue Room members.

 

From lighting side always blame the sound guys ;-)

[indyld]

From lighting side always blame the sound guys ;-)

 

 

If they don't blame you first. AV and sound guys always blame a) the mains b) the lampies c) the room - for any weak points in their set up. ** laughs out loud **

 

I did a little gig with the brilliant Seasick Steve last week and he started to tell me that the lighting was being picked up by his pickups causing the buzz on his amp. He was a jolly nice chap and tried his spare amp instead. That sorted it out even though he reckoned the second amp was usually worse. Must have got our mains wired back to front this time ;-)

 

It's easy to blame the lights when actually, as has been said, there is any number of causes but a bit of good practice can make sure you have done all you can. Like not wrapping a load of Socapex around the sound multi or putting a moving head on top of the amp rack.

 

I too have always tried to keep the phases for lighting and AV/sound separate, but this is more for practical reasons rather than eliminating buzz (same earth and neutral etc). For a start, it is the first thing the noise boys ask you when their systems buzzes like good 'un.

[gazzer1uk]

Paulears,

 

Apologies if I caused offence in respect of the word amateur.

 

I did not mean it in respect of paid or unpaid, the reason for posting on this forum is that there is a wealth of knowledge from a variety of highly skilled people undoubtedly all behaving in a professional manner, whether paid or otherwise. However if we drill down to the various interpretations, I would describe myself as unskilled, interested and learning, which I think in context could be considered amateur.

 

However I will describe myself as in need of help due to a lack of knowledge and understanding and suffering a problem.

 

Many thanks for the information so far, of course I do recognise the quality value cost equation, but think this wont be the sole cause of issues. I look forward to hearing more and trying out some of the thoughts and advice so kindly provided.

 

Obliged to you all,

 

 

Regards,

 

Gazzer

[amillar]

A few things to try

1) Make sure that your DMX signal is only earthed at one end

2) Many reasonable quality PA amps have "Earth Lift" switches on them - try adjusting these.

3) You can get isolating transformers for audio signals going into the PA amps - this may also solve the problem.

 

Good luck.

 

Andrew.

[gazzer1uk]

amillar,

 

Thanks for that I think you may have hit on something........

 

I was going over the installation in my head and I know that I forgot to place a terminator in the last dimmer pack in the DMX chain...... also were using XLR microphone cables instead of DMX, might this be a worthy thing to try and change?? I had assumed the seperate sale of DMX cables was more a marketing exercise than reality, perhaps I am wrong again!!!!!

 

Gazzer

[Ike]

While it is good practice to use DMX cable and terminate the end of every run this is to prevent errors in the lighting control signals and won't have any affect on the buzzing.

[Just Some Bloke]

Have a look here and here for starters, though there's probably more!

[amillar]

While it is good practice to use DMX cable and terminate the end of every run this is to prevent errors in the lighting control signals and won't have any affect on the buzzing.

Ike is correct - lack of terminator should not impact the buzz. In my suggestion though I was not referring to DMX termination but to the grounding of the DMX signal - if the DMX cable between the desk and the dimmer pack has the shield connected at both ends to PIN1 then this can cause problems (I know from experience!).

 

Andrew.

[ben.suffolk]

if the DMX cable between the desk and the dimmer pack has the shield connected at both ends to PIN1 then this can cause problems (I know from experience!).

 

If by Shield you mean the XLR shell, then this should not be connected to PIN 1 at either end. If you mean the shield in the cable, then this should be connected to pin 1 at both ends (but not to the shell)

 

Regards

 

Ben

[KevinE]

Poor quality screened audio cable is a big problem when picking up dimmer noise. They skimp on the price of the cable by poor co-ax outer conductors. Some audio leads currently being peddled will pick up the noise radiated by your own hands if you grab them! By default, it's best to use balanced wiring if your equipment is capable, (some DJ and semi pro stuff isnt, and simply unbalances a balanced connection). Check the screens of audio leads are intact, often one end has been yanked or open circuit, the audio will often work coutesy of a common earth somewhere, but will act as a lovely dipole aerial to any audio frequency noise ie 50Hz hum.

 

I endorse the experimental flipping of your ground lift switches if you have them for the same reason.

[amillar]

If by Shield you mean the XLR shell, then this should not be connected to PIN 1 at either end. If you mean the shield in the cable, then this should be connected to pin 1 at both ends (but not to the shell)

 

Ben - I do mean the shield in the cable. I have had PA buzz problems in some venues when using a cable that has the cable shield connected to Pin1 at both ends - simply removing the connection at one end resolved the problem - consequently my 3 to 5 pin adapter cable that I use at my desk end does not have Pin1 connected. Whilst it may not be the DMX standard (and I haven't checked this out) surely as long as you have Pin1 connected at one end for the signal ground then this is OK?

 

The other thing I should point out (before everyone goes cutting Pin 1 connections) is that this has worked for me at a few venues but this does not always remove the PA buzz :) as this can be caused by a multitude of things as posted earlier...

 

Andrew.