# DMX lighting/speakers buzzing



## Anthony1966 (Dec 17, 2013)

I've just started a church theatre group and my first production didn't go according to plan after the speakers started buzzing. During the set up in the afternoon there were no problems but then it started and didn't go away. When you press the flash button for all the channels to come on it's really loud. I am using two T bar lighting stands with 6 par 64 on each and 2 qtx dimmer packs and a 24 channel transcension lighting control and dmx cable. For the last scene we had to turn the lights off and put the house lights up. I've used the same set up in different churches so i am not sure what's different this time. These are different dimmer packs though so maybe they are faulty, I haven't used them before.


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## DavidNorth (Dec 17, 2013)

Welcome to CB!

Believe it or not, but the dimmer packs aren't the problem. Dimmers are one of the best tools out there to expose ground loops in audio systems. Most commonly, we find that inputs to the audio mixer, such as DI boxes and amplifiers, are on a different power source and/or ground than the mixer and amplifiers. That's where you need to concentrate your efforts.

The mixer and amplifiers should be plugged into the same outlet. Then, I ask people to remove inputs from the mixer one by one until the noise goes away. Usually they will find one or two inputs that are the noise generators and that where they need to reconnect power and/or use the ground lift switch on the DI boxes. Oh, make sure that all the band members and their instruments are there and in place before troubleshooting.

Others here may have additional suggestions.

David


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## JD (Dec 17, 2013)

What Dave is saying is right on the money. Often lighting equipment gets the blame. Although the nature of phase-chop dimmers (almost all) does produce noise, it needs to get into the sound system through a window of opportunity. This window can often be the ground layout of the building. If a mixer is plugged in at one end of the building, and the power amps at the other, then any ground differential voltage in the building will find an alternate path through the shielding run of the sound system. Even two outlets that are near each other may be wired to different power panels and produce the same problem. To avoid this, the sound system must have a single point ground. In other words, the equipment all needs to be plugged into the same place. Now, add a band on stage and a few direct boxes and pretty soon you have a rubix cube of buzz! Most instruments, like keyboards, enter the sound system using direct boxes. These boxes have a switch labeled "ground lift." It is your best friend! Even with all of this covered, sometimes musicians instruments themselves may be poorly shielded and pick up the noise. Single-inline guitar pickups come to mind!

Oh, and of course, you do want to avoid plugging any shoe-box dimmers into the same outlets that are used for sound and stage


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## Anthony1966 (Dec 17, 2013)

DavidNorth said:


> Welcome to CB!
> 
> Believe it or not, but the dimmer packs aren't the problem. Dimmers are one of the best tools out there to expose ground loops in audio systems. Most commonly, we find that inputs to the audio mixer, such as DI boxes and amplifiers, are on a different power source and/or ground than the mixer and amplifiers. That's where you need to concentrate your efforts.
> 
> ...



Thanks for that. The amps and mixing desk were plugged in at different places. On the night i was only using a laptop for sound effects and a mic on the mixing desk. After reading your post i looked up ground loops and found a lot of info, advice etc, and found it all really interesting, concurring with what you said. Can't wait to go and implement my new found knowledge.


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## Anthony1966 (Dec 17, 2013)

JD said:


> What Dave is saying is right on the money. Often lighting equipment gets the blame. Although the nature of phase-chop dimmers (almost all) does produce noise, it needs to get into the sound system through a window of opportunity. This window can often be the ground layout of the building. If a mixer is plugged in at one end of the building, and the power amps at the other, then any ground differential voltage in the building will find an alternate path through the shielding run of the sound system. Even two outlets that are near each other may be wired to different power panels and produce the same problem. To avoid this, the sound system must have a single point ground. In other words, the equipment all needs to be plugged into the same place. Now, add a band on stage and a few direct boxes and pretty soon you have a rubix cube of buzz! Most instruments, like keyboards, enter the sound system using direct boxes. These boxes have a switch labeled "ground lift." It is your best friend! Even with all of this covered, sometimes musicians instruments themselves may be poorly shielded and pick up the noise. Single-inline guitar pickups come to mind!
> 
> Oh, and of course, you do want to avoid plugging any shoe-box dimmers into the same outlets that are used for sound and stage



Thanks for replying. Yeah the mixer and amps are plugged in at different ends of the building, and the dimmers in between. I'll try a single power point for all the sound next time and see how it goes.


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## Lextech (Dec 17, 2013)

The other thing is making sure that the audio power you are plugging into is in fact grounded. Just because the plug has a ground it does not mean it is attached to anything, especially in older buildings. If you do not know how to use a multi-meter to find out, buy one of these or something like it http://www.amazon.com/Electrical-Receptacle-Outlet-Ground-Tester/dp/B0012DHVQ0 
Also make sure you are using a USB audio output device or a DI on the laptop. Headphone jacks tend to be noisy.


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## Anthony1966 (Dec 17, 2013)

Lextech said:


> The other thing is making sure that the audio power you are plugging into is in fact grounded. Just because the plug has a ground it does not mean it is attached to anything, especially in older buildings. If you do not know how to use a multi-meter to find out, buy one of these or something like it http://www.amazon.com/Electrical-Receptacle-Outlet-Ground-Tester/dp/B0012DHVQ0
> Also make sure you are using a USB audio output device or a DI on the laptop. Headphone jacks tend to be noisy.



Thanks for that. I bought a ground tester that plugs in earlier today similar to the one in the link. I'll check the ground next time in the building.


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## SteveB (Dec 17, 2013)

Anthony1966 said:


> Thanks for that. I bought a ground tester that plugs in earlier today similar to the one in the link. I'll check the ground next time in the building.



Generally, it only takes one mis-wired audio cable in the wrong place to pickup dimmer noise and inject it into the entire system. When we use our house system, which has been well maintained, we don't get buzz. Ditto pro systems from good audio companies. They've learned to use one grounding source and to maintain their cables. It's actually no less an issue then 10 or 20 years ago. I suspect the audio guys have gotten better at learning how to isolate a problem.


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## techieman33 (Dec 17, 2013)

SteveB said:


> Generally, it only takes one mis-wired audio cable in the wrong place to pickup dimmer noise and inject it into the entire system. When we use our house system, which has been well maintained, we don't get buzz. Ditto pro systems from good audio companies. They've learned to use one grounding source and to maintain their cables. It's actually no less an issue then 10 or 20 years ago. I suspect the audio guys have gotten better at learning how to isolate a problem.



It's a lot easier in a professional setting since the audio power is usually isolated coming into the building, or on it's own generator outdoors. I'm sure it's harder in a church that was never designed with any consideration for the things people are doing now.


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## SteveB (Dec 18, 2013)

We don't have a separate power feed to either our house sound system nor our sound company switch, but in truth, it doesn't really matter. One third the battle is using good gear that is well maintained. Another third is making sure the amps/gear on stage is using the same ground as the FOH gear, with no ground loop. The third is paying attention to eliminating noise induction with non balanced gear. 


Sent from my iPad using Tapatalk


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## Les (Dec 19, 2013)

Reminds me of a post I came across on another forum:

"I quieted down my rack system (for jobs) by removing the ground\earth on every piece of equipment except the amplifier. My system is now extremely quiet."

Can't decide if this guy's system is now extremely dangerous or just marginally shady...


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## JD (Dec 19, 2013)

Les said:


> Reminds me of a post I came across on another forum:
> 
> "I quieted down my rack system (for jobs) by removing the ground\earth on every piece of equipment except the amplifier. My system is now extremely quiet."
> 
> Can't decide if this guy's system is now extremely dangerous or just marginally shady...



Lots of people do that kind of thing. Unfortunately, it is not safe and it also side-steps the real problem. Basically, they are now using the shielding as a ground fault path. If there was a short in any piece of equipment, line voltage could get applied across the shield conductor (usually about #22) with predictable results! In addition to the fire/shock hazard, a lot of equipment would get damaged. 

Best to get to the bottom of how the buzz is getting in there to begin with.


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