Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Mike Sokol]

I've got a really weird one I'm trying to get traction on. One of my installed church customers has been hearing an 8 kHz tone in their monitors for the last month or so. It comes and goes in all 4 monitor channels, doesn't seem to follow any particular instrument, and is still there after muting all inputs. But they claim that muting the aux sends to the monitor amps will kill the tone. I heard it a month ago and traced it to a single digital keyboard. They took that out of the mix, but it was crazy loud last Sunday. So I went there today and they had hooked up the entire system like it was on Sunday, and we couldn't make anything squeal at 8 kHz. In fact, the original keyboard that I traced it to last month was quiet as well. A few months ago I had traced a 60 Hz ground loop hum to an interconnection between their AVIOM system and a Korg digital drum kit, and found a double bonded AC box under the stage. Correcting that fixed the 60 Hz hum, but I'm wondering if there's some kind of switching power supply that injecting 8 kHz "hum" into the system somewhere. Maybe a line wart (transformer) needs low enough AC voltage or warm enough temperature to start dumping 8 kHz tone into the grounding system. Anyone seen this sort of thing? Right now I'm on call for whenever the monitors start buzzing a 8 kHz and I'll run over there for a look see.

[Corey Scogin]

Do they happen to have an input sourced from the headphone jack of a laptop?  That's the first thing that comes to mind. Something like that could be overlooked if it's at FOH not on the stage.

[Jonathan Johnson]

Electronic feedback loop?

• Maybe a recording device that has its playback outputs mixed back in to the mix being recorded?
• A keyboard (or similar amp) that has an output to the board, and that signal is mixed to a feed going back to the amp? (Some amps support this so you can have a direct monitor from the keyboard and a monitor mix in the same box. But you need to use the correct outputs and inputs, so the monitor input isn't mixed into the output feeding the board.)

I'd expect an electronic feedback loop to be a higher frequency, but who can say for sure?

[Mike Sokol]

Do they happen to have an input sourced from the headphone jack of a laptop?  That's the first thing that comes to mind. Something like that could be overlooked if it's at FOH not on the stage.

The worship leader has a Thunderbolt connection from his Mac laptop to an I-O box on his guitar pedal board. We hooked that up today, but no buzz or other noise. 

[Mike Sokol]

Electronic feedback loop?

• Maybe a recording device that has its playback outputs mixed back in to the mix being recorded?
• A keyboard (or similar amp) that has an output to the board, and that signal is mixed to a feed going back to the amp? (Some amps support this so you can have a direct monitor from the keyboard and a monitor mix in the same box. But you need to use the correct outputs and inputs, so the monitor input isn't mixed into the output feeding the board.)

I'd expect an electronic feedback loop to be a higher frequency, but who can say for sure?

This was a rock solid 8 kHz tone (I had SMAART with me), so maybe it's an electronic feedback. I was postulating something at a much higher frequency that was above the Nyquist limit and causing lower freq alias tones. But until I actually hear the new tone I'm just spit balling.

[David Buckley]

I'm wondering if there's some kind of switching power supply that injecting 8 kHz "hum" into the system somewhere.

That would be a very low frequency for a switching power supply; they typically run above 20KHz to prevent audible annoyance.

What it could be is a variable speed drive, perhaps on the HVAC system; VSDs generate a filthy output, full of nasty edges and harmonics, and do work at frequencies we can hear.  The output of a VSD often has to be treated carefully, often using screened cable or conduit, and grounded in very specific ways, in accordance with them VSD manufacturers instructions.  Get it wrong and these things work like transmitters.

The fact that it doesn't happen when there aren't people there bolsters this possibility; folks generally done cane the HVAC when there is no-one about.

[Mike Sokol]

What it could be is a variable speed drive, perhaps on the HVAC system; VSDs generate a filthy output, full of nasty edges and harmonics, and do work at frequencies we can hear.  The output of a VSD often has to be treated carefully, often using screened cable or conduit, and grounded in very specific ways, in accordance with them VSD manufacturers instructions.  Get it wrong and these things work like transmitters.

This is a big complex with lots of different buildings. I also turned on all the new LED stage lights in hopes that some DMX trash was the cause. But no go...   

[John Roberts {JR}]

I've got a really weird one I'm trying to get traction on. One of my installed church customers has been hearing an 8 kHz tone in their monitors for the last month or so. It comes and goes in all 4 monitor channels, doesn't seem to follow any particular instrument, and is still there after muting all inputs. But they claim that muting the aux sends to the monitor amps will kill the tone.

Just to be clear, muting mixer inputs does not stop it, but muting aux sends does....  Sounds like problem is inside mixer.

I heard it a month ago and traced it to a single digital keyboard. They took that out of the mix, but it was crazy loud last Sunday. So I went there today and they had hooked up the entire system like it was on Sunday, and we couldn't make anything squeal at 8 kHz. In fact, the original keyboard that I traced it to last month was quiet as well. A few months ago I had traced a 60 Hz ground loop hum to an interconnection between their AVIOM system and a Korg digital drum kit, and found a double bonded AC box under the stage. Correcting that fixed the 60 Hz hum, but I'm wondering if there's some kind of switching power supply that injecting 8 kHz "hum" into the system somewhere. Maybe a line wart (transformer) needs low enough AC voltage or warm enough temperature to start dumping 8 kHz tone into the grounding system. Anyone seen this sort of thing? Right now I'm on call for whenever the monitors start buzzing a 8 kHz and I'll run over there for a look see.

There could be any number of rouge noise sources,,, compressors oscillating, switching PS in the water cooler, whatever. But the symptom above seems localized to inside the console.

Divide and conquer...

JR

note: probably not a ground loop... 

[Mike Sokol]

Just to be clear, muting mixer inputs does not stop it, but muting aux sends does....  Sounds like problem is inside mixer. There could be any number of rouge noise sources,,, compressors oscillating, switching PS in the water cooler, whatever. But the symptom above seems localized to inside the console.

Divide and conquer...

JR

note: probably not a ground loop... 

They've got a number of direct connections into the digital mixer from various computers and video decks , and we've had them all on today without any 8K noise. Back to their own diagnostics, I'm not sure all inputs were muted since I've talked to them after my first post they're now confused about what's an input and what's an output. So I'm also not sure that the 8K buzz went away when they muted the outputs. This is hearsay evidence at best, and they tend to get frantic while troublshooting during a service.

To have a chance at figuring this out I need to get this system to buzz when I'm in the room. Then I'll begin to divide and conquer.

[Don T. Williams]

This probably is not your problem, but I had a group that on a weekly basis blew their JBL ring radiator tweeters (it was a some time ago).  After a while, JBL stopped warranting them.  I finally went in with a an oscope to look for the problem.  I found 30KHz out of the amps, back to the crossover, back to the mixer.  A keyboard had a wall wart power supply that had a 30KHz square wave riding the DC output.  Even when the keyboard was off, the 30K went into the mixer.  Make certain it is not a wall wart somewhere (they tend to get moved around a lot) producing that 8K.

Also, and I Know this isn't you problem, but on several occasions I would hear 6-8K "feedback" even with the FOH system muted no monitors.  I would finally find the source was a person in the audience wearing a hearing aid that was feeding back.  It happens!

[ProSoundWeb Community]