Need some help with our School's Sound System

[DisplaceFly]

Hello there, I was looking for some advice on our school system’s PA system. We have a constant buzz coming from the speakers and I can’t pinpoint what is causing this. The buzzing is independent from the volume of the system. I’ve removed all inputs (one by one), checked every cable for a loose connection, the one way I have gotten the buzz to stop is by removing the output from the power amplifier that goes into the matrix processor. I don’t think it is a ground looping issue, but I could be wrong. I’ve included pictures of our system in the post.

We have four Shure microphones, UHF Antenna/Power Distribution, A Power Center, Disc Player (unplugged and no in use), Crown 28M Preamp/mixer, Ashly 24.24 Matrix Processor, 3 pa-9324 power amplifiers feeding to separate rooms), and lastly Crown CDi 2000 power amplifier which is sent to 6 Electro Voice PA speakers.

Let me know if a video of the buzzing/system wiring/rack components would help! Thank you very much for your time!

 

[TLS Guy]

Hello there, I was looking for some advice on our school system’s PA system. We have a constant buzz coming from the speakers and I can’t pinpoint what is causing this. The buzzing is independent from the volume of the system. I’ve removed all inputs (one by one), checked every cable for a loose connection, the one way I have gotten the buzz to stop is by removing the output from the power amplifier that goes into the matrix processor. I don’t think it is a ground looping issue, but I could be wrong. I’ve included pictures of our system in the post.

We have four Shure microphones, UHF Antenna/Power Distribution, A Power Center, Disc Player (unplugged and no in use), Crown 28M Preamp/mixer, Ashly 24.24 Matrix Processor, 3 pa-9324 power amplifiers feeding to separate rooms), and lastly Crown CDi 2000 power amplifier which is sent to 6 Electro Voice PA speakers.

Let me know if a video of the buzzing/system wiring/rack components would help! Thank you very much for your time!
View attachment 61267View attachment 61268

Common things commonly happen, so it is probably a ground loop. Ground loops are caused by resistances between grounds so there is a voltage potential between grounds. So the solution is bonding all grounds back to one single point.

Is there any Ethernet cable involved here or other cable from an outside provider? Ethernet cables are a frequent source of ground loops.

So you will need to disconnect all grounds and reconnect each one separately until you find the offending grounds, and then deal with them one by one. Don't miss one ground, as is easy to miss one or two.

This is a relatively complex system. Planning and designing the ground plane of the system is part of the basics. Unfortunately this frequently gets overlooked.

I think the equipment is in a metal rack. Bonding the units chassis to a metal rack, and then boding that rack back to a secure ground (the electric panel if possible) is a good plan to follow.

 

[DisplaceFly]

Thank you for the advice and appreciate your time! yes there is a ethernet cable but once I unplug it there is no change in the buzzing. I will go through the grounds in order to find the offending ground.

 

[TLS Guy]

Thank you for the advice and appreciate your time! yes there is a ethernet cable but once I unplug it there is no change in the buzzing. I will go through the grounds in order to find the offending ground.

The best thing to do, is to remove the grounds and replace them one at a time. Then you will find the offending grounds.

 

[Mike L. Long]

Hello there, I was looking for some advice on our school system’s PA system. We have a constant buzz coming from the speakers and I can’t pinpoint what is causing this. The buzzing is independent from the volume of the system. I’ve removed all inputs (one by one), checked every cable for a loose connection, the one way I have gotten the buzz to stop is by removing the output from the power amplifier that goes into the matrix processor. I don’t think it is a ground looping issue, but I could be wrong. I’ve included pictures of our system in the post.

We have four Shure microphones, UHF Antenna/Power Distribution, A Power Center, Disc Player (unplugged and no in use), Crown 28M Preamp/mixer, Ashly 24.24 Matrix Processor, 3 pa-9324 power amplifiers feeding to separate rooms), and lastly Crown CDi 2000 power amplifier which is sent to 6 Electro Voice PA speakers.

Let me know if a video of the buzzing/system wiring/rack components would help! Thank you very much for your time!
View attachment 61267View attachment 61268

I can also recommend checking the grounding of the equipment, this may be the cause of unwanted noise. See if there are any cross magnetic fields from electrical appliances. I remember how at our university there was a fire due to wires throughout the school audio system. Then everyone studied offline and I often pay for homework, I found https://edubirdie.com/pay-for-homework for this. I also recommend paying attention to possible locations of grounding faults. I hope no one has such problems.

 

[highfigh]

Hello there, I was looking for some advice on our school system’s PA system. We have a constant buzz coming from the speakers and I can’t pinpoint what is causing this. The buzzing is independent from the volume of the system. I’ve removed all inputs (one by one), checked every cable for a loose connection, the one way I have gotten the buzz to stop is by removing the output from the power amplifier that goes into the matrix processor. I don’t think it is a ground looping issue, but I could be wrong. I’ve included pictures of our system in the post.

We have four Shure microphones, UHF Antenna/Power Distribution, A Power Center, Disc Player (unplugged and no in use), Crown 28M Preamp/mixer, Ashly 24.24 Matrix Processor, 3 pa-9324 power amplifiers feeding to separate rooms), and lastly Crown CDi 2000 power amplifier which is sent to 6 Electro Voice PA speakers.

Let me know if a video of the buzzing/system wiring/rack components would help! Thank you very much for your time!
View attachment 61267View attachment 61268

Have any recent changes been done to the system or the electrical service in the area?

When did this buzzing begin to be noticeable? Was it after a thunderstorm?

Checking the grounding will require some equipment that may not be available to you or the school maintenance staff- I would recommend calling the contractor who installed it and if they're no longer in business, find someone who installs this kind of equipment. I don't know where you're located, but most cities have equipment distributors who can be called, for info about the contractors who buy from them. You can also contact the equipment manufacturers to get dealer info in your area- if more than one mentions the same companies, call those.

Can you post photos with higher resolution? These are too small to see any detail.

The fact that the buzz stops after disconnecting the Crown power amp's output means that the buzz is entering into the system in or before that piece- someone needs to use a noise 'sniffer' to find out if the noise is being radiated by other equipment nearby.

Have you tried turning off the microphone system? Do that. If it helps, connect it to another system and see if it makes the same noise.

Are the 3 pa-9324 power amplifiers the same as what is seen in the link?

file:///C:/Users/USER/Downloads/hfe_inter-m_pa-9312_9324_9336_9348_en.pdf

Who provides the internet? Do they also provide a cable TV feed? Make sure that feed is grounded as close as possible to where it enters the building- if it's not, call their service department and have this rectified because it's an Electrical Code violation.

 

[Audiophile Heretic]

Is there any Ethernet cable involved here or other cable from an outside provider? Ethernet cables are a frequent source of ground loops.

I am neither agreeing with you nor disagreeing with you. I am asking a real question in an attempt to understand, not a rhetorical question to make a point. I am tired of the trolls on Audioholics forums who beat me up for asking questions.

Isn't Ethernet cable balanced? How does balanced cable create ground loops?

 

[TLS Guy]

I am neither agreeing with you nor disagreeing with you. I am asking a real question in an attempt to understand, not a rhetorical question to make a point. I am tired of the trolls on Audioholics forums who beat me up for asking questions.

Isn't Ethernet cable balanced? How does balanced cable create ground loops?

First of all a balanced cable does nothing to prevent ground loops. Ground loops are caused by a resistance between grounds.

Ethernet and cable systems are both a potent source of ground loops as there is often a resistance between the system ground and ground at the service entry point.
Installers of Internet service and cable system installers are notorious for poor quality work and often infringing code. More often than not I have had to either stand over them and direct them, or redo their work after they leave.

 

[Audiophile Heretic]

First of all a balanced cable does nothing to prevent ground loops. Ground loops are caused by a resistance between grounds.

Ethernet and cable systems are both a potent source of ground loops as there is often a resistance between the system ground and ground at the service entry point.
Installers of Internet service and cable system installers are notorious for poor quality work and often infringing code. More often than not I have had to either stand over them and direct them, or redo their work after they leave.

That is the source of my confusion. Ground loops are caused by a resistance and voltage difference between ground points, but with balanced cables, the grounds on devices are not connected as with unbalanced or single ended connections. The balanced cable does not connect ground between devices. If there is a ground potential difference, it is the same polarity on both conductors. The receiver differential input only responds to the voltage difference between balanced conductors, not the difference between one unbalanced conductor and ground.

There is no problem of ground loops between computers and Ethernet switches with different ground points on a large computer network, and there is no problem with ground loops between analog audio devices with different ground points using balanced cables as long as the shield of the balanced signal cable is only connected at one end.

 

[TLS Guy]

That is the source of my confusion. Ground loops are caused by a resistance and voltage difference between ground points, but with balanced cables, the grounds on the two devices are not connected as in unbalanced or single ended connections.

There is no problem of ground loops between computers and Ethernet switches with different ground points, and there is no problem with ground loops between analog audio devices with different ground points using balanced cables as long as the shield of the balanced signal cable is only connected at one end.

Normally you do connect them at both ends, as you want both ends of the xlr connected. Basically you want all units strongly bonded together. I have always connected balanced cables at both ends. If you buy a balanced cable then both xlrs are grounded.

In system design, you want all grounds strongly bonded to a central point.

 

[Audiophile Heretic]

the one way I have gotten the buzz to stop is by removing the output from the power amplifier that goes into the matrix processor.

I don't understand. The OUTPUT of the power amplifier goes into a matrix processor? Doesn't the OUTPUT of the power amplifiers drive the speakers? Does the output of a matrix processor, whatever that is, drive the INPUT of the power amplifiers?

 

[Audiophile Heretic]

Hello there, I was looking for some advice on our school system’s PA system. We have a constant buzz coming from the speakers and I can’t pinpoint what is causing this. The buzzing is independent from the volume of the system. I’ve removed all inputs (one by one), checked every cable for a loose connection, the one way I have gotten the buzz to stop is by removing the output from the power amplifier that goes into the matrix processor. I don’t think it is a ground looping issue, but I could be wrong. I’ve included pictures of our system in the post.

We have four Shure microphones, UHF Antenna/Power Distribution, A Power Center, Disc Player (unplugged and no in use), Crown 28M Preamp/mixer, Ashly 24.24 Matrix Processor, 3 pa-9324 power amplifiers feeding to separate rooms), and lastly Crown CDi 2000 power amplifier which is sent to 6 Electro Voice PA speakers.

Let me know if a video of the buzzing/system wiring/rack components would help! Thank you very much for your time!
View attachment 61267View attachment 61268

If I were going to guess, I would guess that what you have are wireless microphones that output to a mixer, the output of the mixer goes to a matrix switch, the outputs of the switch go to the amplifiers, and the power amplifiers drive speakers. What happens if you bypass the matrix switch and temporarily connect the output of the mixer to each power amplifier? If the noise goes away, the problem is the switch.

 

[Audiophile Heretic]

In system design, you want all grounds strongly bonded to a central point.

You only need the shield connected between equipment if the connections between equipment are unbalanced aka single ended because the shield is also one of the conductors in the circuit. If the shield conducts current between devices that have different ground potentials, that is a problem.

What if your equipment is so far apart that you can't use a central ground point? What if your mixing console is in the back of the performance venue and your amplifiers are behind the stage close the speakers to keep the speaker cables short?

Since we don't know how the equipment is connected, maybe the problem is some unbalanced cable has a broken shield so chassis ground is acting as a conductor.

Maybe a device has failed. The proper way to troubleshoot is to disconnect or bypass devices and run the systems with as few devices as possible. When you add the device that causes the noise, you found the source of the problem. Disconnect the mics from the mixer and reconnect one at a time. If you still have the noise, disconnect each amplifier from the mixer save for one. Test each amplifier one at a time. First, bypass the matrix switch and connect the mixer directly to each amplifier, one at a time. Do the microphone receivers have output level controls so they can each be connected directly to an amplifier, bypassing the mixer and the switch?

 

[Audiophile Heretic]

Normally you do connect them at both ends, as you want both ends of the xlr connected.

In balanced cable, two conductors connected to XLR pins 2 and 3 or TRS tip and ring carry the audio signal normal and inverted. The outer shield connected to XLR pin1 or TRS sleeve does not carry any signal as it does in unbalanced cable. Some professionals connect the shield at only one end, the source end, but both ends of the signal conductors must be connected. Sometimes XLR pin 1 is signal ground while the shield is connected to the shell to connect chassis grounds between devices. Like you, I usually connect the shield at both ends.

Ethernet is four balanced twisted pairs and does not have a shield or ground connection between ends. Power Over Ethernet POE puts DC voltage on both conductors of separate pairs to power devices like Wireless Access Points or VOIP phones, but the data signals are the difference between conductors in a pair. i work with Cat 6 with POE every day. A run of Cat 6 can be 100 m or 328 ft. There are no ground loops because there is no ground connection between devices at different ground potentials. There may not be a central ground point all devices are connected to, not like an audio system in one rack.

Some sceptics laughed at Monster cable because some unbalanced audio cables were marked with direction. It wasn't because Monster thought the audio signal was directional. They built an unbalanced cable like a balanced cable. They had two equal conductors, signal and ground, between both RCA ends. The shield around the two conductors was connected at only the source end to short the EMI noise to ground at the source end.