Mysterious hum, I give up.

[MrMagLit]

Hey CB,

So, here's a challenge for all of you people. I have a nasty hum coming from my right main speaker (QSC K12) and the KSub connected to it. Standard troubleshooting failed, and after an entire few days messing with it, I give up. Here's the setup:

• A&H MixWizard, LR out
• Multipin plugged into wall receptacle
• Multipin cable runs in underground conduit to XLR panels around the stage
• Right speaker (Stage Left) to an XLR outlet for KSub
• Solder pins on XLR outlet split off
• Split off signal goes to K12 line input, 0dB gain

The speakers are all on the same circuit, and the mixer is on a different one. That being said, they are all on the same panel.

What I know:

• Putting mixer and speakers on the same circuit does not fix the problem.
• Plugging the mixer directly into the speaker (long XLR cable across auditorium) DOES remove hum.
• Tested entire multipin run for continuity, all pins work fine.
• Powering board "off" amplifies hum by around 20-30dB.
• Removing either the KSub or the K12 has no effect.
• Adding additional inline equipment such as equalizers or compressors will amplify the hum by a small amount.
• Board output does not matter, it is the same speaker whether plugged in to L, R, or M.

My guess is it has something to do with the run from the wall receptacle to the speaker, since bypassing it with a long XLR cable solved the problem.

Any wisdom would be appreciated...

 

[AudJ]

Is this a new install, or a new problem on an existing system that was working properly?

 

[TDN]

Probably picking up some interference on the long run through the wall. If you've got access to it and can do it in a way that won't anger anyone else around, you could try flipping breakers one at a time and see if the hum goes away. Might be something on one of the circuits or the power run itself that is causing the issue.

Are you using all of the circuits on the multipin? Is there anyway you can try rerouting the signal down another line? Swapping L and R, for example, and seeing if the problem follows along.

 

[Evans Poulos]

Can you grab another pair on the multipin and bypass what sounds like a bad pair?

 

[MrMagLit]

I was told that the install happened some 3 or 4 years ago. And apparently, the hum has been there for the majority of those years... I doubt it's been there since day one, only because the install company is very reputable in the area.

There are many free circuits, I might try one of them. Only bother is that accessing them would be an absolute nighmare as they were all cut fairly far away from the rest of the used pairs.

I'll try the breaker idea as soon as I can get some time alone in the building. The facility is a local high school, and the non-dim auditorium circuits are shared with other classrooms...

 

[FMEng]

I seriously doubt you have a noise source getting into just one pair of a multipair cable. It would be polluting mic channels far more than a line level send.

The fact that a separate cable is clean tells the story. The shield of that line is grounded to something when it should be isolated. If you check continuity from pin 1 to the enclosures at each end, with both ends disconnected, I bet you will find it isn't open. In that case, inspect the XLR connectors to ensure that the shield only goes to pin 1 and is insulated so that it cannot come in contact with anything else. The same is true for the cables plugged between mixer and snake, and the speaker and snake. A few misguided souls connect the shield to both pin 1 and the shell grounding lug. That's a no-no.

The other thing you can do is lift the shield from pin 1 inside the connector at the speaker. For balanced, line level, there is no need to have the shield grounded at more than one end. Grounding it at both ends just encourages ground loops on long runs. If you drop the shield in a cable, make sure the cable is marked so that it doesn't get re-purposed where it would cause trouble.

 

[MNicolai]

Worth taking a multimeter and doing a continuity test between pins and conduit ground. Possible someone connected shield to connector ground as mentioned, but also possible wire got skinned being pulled into conduit and is grounded to the conduit. Continuity test should provide definitive answer.

 

[JD]

There has always been a mix of opinions regarding the shield on XLR cables. The norm is to connect it to pin one, but not the case. This is best practice regarding elimination of ground loops and hum.
BUT---- And this is the real problem- There is a solder lug on XLR connectors for the case, and sometimes the techs doing an install, or modifications to a current install, feel obligated to solder the shield to the case lug, or more commonly to the case lug and pin one. This problem can exist for years before some other thing changes in the system and a phantom hum appears. As stated above, check the pin one to case continuity (with both ends disconnected) and if you find any, check them all!
This second point is important! The usual diagnostic technique breaks down because it is based on the idea that there is a single problem/issue. What can happen is you think you have found it, but after correcting it, the problem still exists because there are several pathways at other points that continue the ground loop/hum problem.

 

[RonHebbard]

Hey CB,

So, here's a challenge for all of you people. I have a nasty hum coming from my right main speaker (QSC K12) and the KSub connected to it. Standard troubleshooting failed, and after an entire few days messing with it, I give up. Here's the setup:

• A&H MixWizard, LR out
• Multipin plugged into wall receptacle
• Multipin cable runs in underground conduit to XLR panels around the stage
• Right speaker (Stage Left) to an XLR outlet for KSub
• Solder pins on XLR outlet split off
• Split off signal goes to K12 line input, 0dB gain

The speakers are all on the same circuit, and the mixer is on a different one. That being said, they are all on the same panel.

What I know:

• Putting mixer and speakers on the same circuit does not fix the problem.
• Plugging the mixer directly into the speaker (long XLR cable across auditorium) DOES remove hum.
• Tested entire multipin run for continuity, all pins work fine.
• Powering board "off" amplifies hum by around 20-30dB.
• Removing either the KSub or the K12 has no effect.
• Adding additional inline equipment such as equalizers or compressors will amplify the hum by a small amount.
• Board output does not matter, it is the same speaker whether plugged in to L, R, or M.

My guess is it has something to do with the run from the wall receptacle to the speaker, since bypassing it with a long XLR cable solved the problem.

Any wisdom would be appreciated...

Hello!
I'd like to add the following to comments already posted. Individually shielded multi-pair cables, designed for installation rather than portable use (Belden 8776 for example), often employ metalized mylar foil for the shields wrapped around the individual pairs. Often inexperienced installers don't realize that all of these individual shields are electrically insulated from each other even though they appear to be simply bunched together hard up against each other within the cable's overall exterior jacket. When the individual foils are wrapped around the pairs, they are wrapped with the conductive metalized surfaces inside, against the pairs, and the insulated, non conductive, surfaces on the outside. Where the conductive edges of the foils overlap they're normally interlocked with a Z style fold ensuring the edges of the conductive sides are within the fold leaving the outsides totally electrically isolated from each other. Upon removing a length of the outer jacket to fan the pairs for termination, it's normal to carefully install durable, flexible, insulated sleeving over each individual pair and an overall short length, or two, of heat shrink insulation over the juncture of the sleeves and the cable's original outer jacket. If the installer is not paying close attention, it's easily possible to disrupt the manufacturer's meticulously interlocked Z folds at this point leading to inadvertent, and possibly intermittent, electrical connections between the various shields. If / when this happens at both ends of the run it's easy to end up with ground loops within the cable's shields themselves. If, as someone has already posted, the cable's overall jacket becomes damaged and any of the individual shields come into electrical contact with building ground, within grounded metal conduit for example, the whole problem can be greatly exacerbated. This is another case of where it's better to work a little slower and extremely carefully than to rush only to have to invest days unearthing [Sorry for the inadvertent terrible pun] later.
Toodleoo! Ron Hebbard

 

[RonHebbard]

MrMagLit; Have you been able to resolve your problem?
Toodleoo!
Ron

 

[MrMagLit]

The problem is so far unresolved. @JD had it with multiple problems existing. We ran the mains circuit (that the FOH K12s are on) down to 2 outlets for the KSubs... That reduced the hum slightly. I tried lifting the case-lug-to-pin-one connection at the speaker with no results. There doesn't appear to be any continuity between the shield and the conduit either. I also turned off every circuit other than the audio ones on the panel, and they made no difference. I'm currently building a schematic to try and visually see how many ground loops could be occurring...

 

[FMEng]

You still need to try lifting the shield to pin one connection in the connector that plugs into the speaker. If you don't want to modify the cable, buy one of these:

http://www.zzounds.com/item--HOSGLT255?siid=134206

A ground lift adapter is a good item to have in every audio toolbox.

If all else fails, use a transformer. It'll provide isolation and common-mode noise rejection that is unobtainable from an active, balanced input.

http://www.bhphotovideo.com/bnh/con...oCFQ6SaQod5ckITw&is=REG&ap=y&m=Y&Q=&A=details

 

[howlingwolf487]

An ISO transformer may be the way to go here, but I would troubleshoot by running everything off of one outlet to start with. Then, build and utilize a "hummer" per Bill Whitlock's instructions in his Grounding presentation here or here.

Find out which piece(s) of gear has/have a Pin-1 problem and do what you can to correct it - repair, modify, or replace.

Also, read through the papers listed on this page. You won't be disappointed, and you'll really impress some people.

 

[FMEng]

The hummer tests for a problem that would require INTERNAL modifications of the equipment to resolve. Very few people are qualified and capable of doing those modifications properly, and harm to the equipment or the person doing the work is likely. The hummer is a technic for use in the lab by the people designing equipment. Most hum problems can be resolved through proper wiring methods outside of the equipment.