Loudspeakers Hum issues in main speakers

Hi all,
I am a senior in High school and have become the resident fix it guy for our theater. From mic packs to dimmer racks I have worked on it lately. I have the been able to solve almost all of our issues, but there is one I cant solve and it is driving me nuts. Our sound system has had an obnoxious hum in it for as long as I have been in the theater. It is only on 1 speaker, (the center speaker). It is not a blown or damaged speaker because any speaker connected to the line hums, and there is a hum whenever the system is on.
I am led to believe that there is some sort of issue with the way the wire is run/ the type of wire run. It does not look as if the wire is shielded (hard to tell for sure because or the location of the speaker itself)
Is there any way that I can determine what the true issue is and any solutions to those issues would be great. We open a show in less than a month and I would LOVE to have it fixed by then.
Thanks
 

bishopthomas

Well-Known Member
Basic troubleshooting to narrow it down to the cause, then replace as necessary... To get you started, begin at the console. Swap the L/R sends to see the hum switches sides. Do the same at the amps, swap mixers or amps, the basics. Let us know more details about the system for a more detailed response. It shouldn't be too difficult of a project to troubleshoot, and hopefully not a difficult fix, just take it one component at a time.

Also, welcome to Control Booth!
 
Basic troubleshooting to narrow it down to the cause, then replace as necessary... To get you started, begin at the console. Swap the L/R sends to see the hum switches sides. Do the same at the amps, swap mixers or amps, the basics. Let us know more details about the system for a more detailed response. It shouldn't be too difficult of a project to troubleshoot, and hopefully not a difficult fix, just take it one component at a time.

Also, welcome to Control Booth!
The way the system is installed is kind of strange and awkward. My L/R sends are actually tied together before reaching the amp. Then that combined signal is split across 4 different amps because all 7 speakers in the house get their own amp channel. I have no specs on the type of wire run or the speaker itself. I can get info on the amp, but that does not seem to be the problem because I have switched other speakers onto the amp channel and did not get the same problem. (not running the speakers off the same cable run as the existing speaker that Im having an issue with).
I have run the noise through a Smaart and found three seperate peaks between 1k and 4k. Dont have the exact frequencies at the moment but can get them tomorrow when Im back.
Any other info that I need to supply?
 

derekleffew

Resident Curmudgeon
Senior Team
Premium Member
...I am led to believe that there is some sort of issue with the way the wire is run/ the type of wire run. It does not look as if the wire is shielded (hard to tell for sure because or the location of the speaker itself)...
Wire running from amplifier to speaker should not be shielded, and may not even need to be twisted. Highly unlikely that this is the source of the hum. Much more likely that the problem is with the amplifier or upstream.
 

DuckJordan

Well-Known Member
I determined it not to be an amplifier issue because when I unplug the wire run to the speaker and plug in a different wire to another speaker the issue does replicate its self.
What else can I do to determine where the issue lies?


That right there tells me it is something in the amplifier.

Here are the facts i see.
Speakers are run by seperate amps, but given all the same feed by the board (doubtful but possible).
When problem speaker is disconnected from the amp and another speaker is put in its place the same problem persists. THIS right here is the key.

Since it doesn't travel with the speaker it means the problem is higher up in the signal chain. either amplifier or towards the board side. Could you explain the signal chain a bit more.

For what i am seeing is board to amp to speaker. while simple most schools generally throw in EQ's and other such equipment because the companies they hire say they need them.
 
I will retest the amp tomorrow to determine if the amp is possibly bad. When I tested it last week, I could not get the hum on any other speaker. The TD however, claims he has. I will double check tomorrow.
Signal flow is
Tascam DM24 digital board- L/R combined- signal split to 2 EQ racks (one digital w/ limiter) 1 analog.- goes to amp- signal to speaker.
The problem speaker is through the analog EQ. dont know if that matters or not.
 

David Ashton

Well-Known Member
One problem it most definitely IS NOT is the cable.a cable cannot generate hum in a speaker,a cable might pick up a few millivolts which can cause hum in a sensitive microphone circuit, but there is no way a cable could produce enough signal to power a hum in a low impedance device like a speaker.The fault is likely to be an earth loop situation or a gain structure problem.If your amps are set to full, then it is almost certainly a gain structure issue, easily the most common problem I have to fix.
 

bishopthomas

Well-Known Member
David Ashton said:
.If your amps are set to full, then it is almost certainly a gain structure issue, easily the most common problem I have to fix.

I'm not sure how these two scenarios are related. I (and everyone I know in the industry) keep my amps at full and use the mixer to control the volume. You can quote specs if you think I'm wrong, but with decent quality gear the rise in noise floor between 80% and 100% is completely inaudible. It certainly wouldn't cause the hum issues the OP is having. Output noise floor has to come somewhere. I'd rather keep it at the amps and have more control at the console.
 

museav

CBMod
CB Mods
Departed Member
The way the system is installed is kind of strange and awkward. My L/R sends are actually tied together before reaching the amp. Then that combined signal is split across 4 different amps because all 7 speakers in the house get their own amp channel.
A bit off topic but are the left and right channel conductors literally tied together or do the two channels go through sum type of summing circuit or device? If the system was installed with left and right simply hardwired together and you have seven speakers all getting the same signal then that may indicate some more general things about the system and installation.

I can get info on the amp, but that does not seem to be the problem because I have switched other speakers onto the amp channel and did not get the same problem. (not running the speakers off the same cable run as the existing speaker that Im having an issue with).
So everything the same but swapping a different speaker and speaker cable to the same amp channel exhibited no noise. Is that correct? Did you also try the problematic speaker on a different amp channel?

I have run the noise through a Smaart and found three seperate peaks between 1k and 4k. Dont have the exact frequencies at the moment but can get them tomorrow when Im back.
"Hum" usually refers to low frequency noise, very commonly 60Hz noise. You seem to be identifying much higher frequency noise, more a squeal or hiss. Clarifying this could held significantly as the frequency of the noise could suggest totally different causes.
 
Okay so im in the theaternow. i just retried everything i have tried so far and got the following results.
Switch from dascam to analog board-issue remains
bypass eq-issue remain
Put issue speaker and cable into spare amp output-issue remains
plug different speaker into problemamp output channel-issue not present in speaker
im not positive if L/R out are tied togetherwith or without a summing device. My sends go into a patchbay and it is impossible to follow the wiring. I can pull the batch bay if need. I was told by TD that rightsend may go to one EQ and the othermay feedthe other EQ but not able to prove this. It is doubtful because there are monitor amps that receive L/R signal that does not go through EQ.
Again i can check more things but have limited acess.
as for gain structuredont know how to. Change it on the amp. It is a ashlyx ftx series u amp.if anyone thinks that is an issue please include how to change it. It should be the same settings as installed by the goup that did the orinial install.
what other info can. Iprovide to help solve this issue.i know i mentiod the frequency those werefrom my TD im not positive on thosenumbers i will douple check later
 

nd925a

Member

DuckJordan

Well-Known Member
is there any way to move the speaker from its current location? If so is there anyway you can hook up a seperate 5 foot cable from amp directly to speaker where you can see the entire signal chain (no wires running into walls and such). You may have to take an amp out of the cab or where ever you have it. It is sounding more and more like a blown speaker. Most hum issues come before amplifier and yours indicate its between the amp and the speaker itself.

If there is anyway to get the speaker from its current location just take an amplifier hooked directly to the speaker then use an ipod or something to generate music and see if the problem persists if it does then you likely have a bad speaker if not your wires connecting amp to speakers may go through a junction box where they could be picking up interference.

In the high school I used to go to we had an issue where our left pro monitor output when turned up would push through to the main PA, found out the contractor who put in the system hired a junk electrician who patched into a power patch bay instead of the audio that was spec'ed then used paper clips to jump signals to multiple outputs. lets just say i got back in there and used the proper materials to get the job done.
 
That is tomorrows project. I have to bring in a couple of my tools inorder to take the speaker down from its current position. I will post after I finished that. If it is a blow speaker is there any way to determine which part is blown. I am not used to working with speaker repair, and knowing my school they wont pay to have it fixed.
 

DuckJordan

Well-Known Member
That is tomorrows project. I have to bring in a couple of my tools inorder to take the speaker down from its current position. I will post after I finished that. If it is a blow speaker is there any way to determine which part is blown. I am not used to working with speaker repair, and knowing my school they wont pay to have it fixed.


Speaker Repair is a very difficult task, without being there hearing exactly whats going on i wouldn't be able to tell you, so unless someone in your school decides to pay for it you may be stuck with a broken speaker. Its one of those things an internet forum just can't walk you through.
 

avkid

Not a New User
Fight Leukemia
Actually speaker repair is fairly straight forward for passive cabinets.
I've done many a diagnosis over the telephone.

With pictures and basic instructions it's fairly easy to figure out what's wrong.
 

Users who are viewing this thread