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Author Topic: Strange hum in audio  (Read 1427 times)

Bill Jones

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Strange hum in audio
« on: March 18, 2021, 07:02:06 pm »

I installed a 3.5mm audio cable that goes from the stage, up into the attic, then down through the ceiling to the Yamaha TF1 sound board. The audio cable is made up of two 50 foot pieces. Our stage area has VGA cable that goes from the stage under the floor to the Yamaha. The object is to have video and audio at the stage. The video typically goes from a laptop (on the stage) to an HDMI to VGA converter, then goes through the VGA cable (under the floor) to the Yamaha, into another converter (VGA to HDMI) and into the desktop computer. The video is not a problem, maybe.
I tested the new 3.5mm cable in the auditorium before I strung it through the attic. No problem. After stringing it in the attic, there is a very obvious hum. If I leave the audio cable plugged into the stage computer and pull the HDMI cable out, the hum stops. I tried two different HDMI to VGA converters, with the same hum.
The only explanation I have is that there is something in the attic interfering (since I tested this before pulling into the attic with absolutely no hum). The other explanation for the hum is that where the audio cable comes down to the Yamaha, it comes down a pole that also has some electrical wires on the same pole. Any ideas of how to troubleshoot this?
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Russell Ault

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Re: Strange hum in audio
« Reply #1 on: March 18, 2021, 08:19:24 pm »

{...} Any ideas of how to troubleshoot this?

There are several things that could be causing the noise; the solution to basically all of them is using balanced cable and a stereo DI with a ground lift.

-Russ
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Brian Adams

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Re: Strange hum in audio
« Reply #2 on: March 18, 2021, 08:34:41 pm »

Your unbalanced cable is closer to lights, ballasts, motors, or something else electrical now that it's up in the ceiling. It doesn't take much to mess with an unbalanced cable, especially one that long.

Russell's suggestion of a stereo DI is the best way to fix this.
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Dave Garoutte

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Re: Strange hum in audio
« Reply #3 on: March 18, 2021, 09:02:37 pm »


Can you replace the VGA cable under the floor?
There are now affordable fiber HDMI cables the can run 300'. 
HDMI carries embedded audio and you shouldn't need another cable.
You can split out the audio at the mixer end with an extractor, which are not expensive.
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Bill Jones

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Re: Strange hum in audio
« Reply #4 on: March 19, 2021, 07:10:52 am »

If I use a DI box, I would need to convert the 3.5mm cable to XLR to go into the DI box, then convert from XLR to 3.5mm coming out of the box. Is this correct?
The other strange thing is that with only the 3.5mm audio cable plugged into the laptop, there is no hum. The hum starts when the HDMI cable is plugged in (and the audio cable is plugged in).
« Last Edit: March 19, 2021, 07:18:20 am by Bill Jones »
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Mike Caldwell

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Re: Strange hum in audio
« Reply #5 on: March 19, 2021, 09:15:35 am »

If I use a DI box, I would need to convert the 3.5mm cable to XLR to go into the DI box, then convert from XLR to 3.5mm coming out of the box. Is this correct?
The other strange thing is that with only the 3.5mm audio cable plugged into the laptop, there is no hum. The hum starts when the HDMI cable is plugged in (and the audio cable is plugged in).

You would use just one DI box as close to the video source/computer as possible.
From the DI box you would run XLR balanced line audio cables directly to the XLR inputs on the Yamaha mixer.
Radial and Whirlwind both make AV style DI boxes with 3.5mm input jacks.

If possible you could use a USB audio interface on the laptop, Peavey makes a simple one that works
great.


Most likely when you plug in the HDMI your creating a grounding issue between the video equipment and the audio system.

Bill Jones

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Re: Strange hum in audio
« Reply #6 on: March 19, 2021, 11:18:02 am »

https://www.amazon.com/rolls-DB14B-Rolls-DB14-Presenter/dp/B0006NAMFE/ref=sr_1_2?dchild=1&keywords=rolls+db14&qid=1616166808&sr=8-2
How about something like this? From a connection point of view it is perfect. That is, there is a 3.5mm male end that would connect to the box and the Yamaha TF1 has a stereo RCA connection (red and white).
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Mac Kerr

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Re: Strange hum in audio
« Reply #7 on: March 19, 2021, 11:33:37 am »

https://www.amazon.com/rolls-DB14B-Rolls-DB14-Presenter/dp/B0006NAMFE/ref=sr_1_2?dchild=1&keywords=rolls+db14&qid=1616166808&sr=8-2
How about something like this? From a connection point of view it is perfect. That is, there is a 3.5mm male end that would connect to the box and the Yamaha TF1 has a stereo RCA connection (red and white).
That box does nothing to solve your problem. Mike Caldwell gives two good solutions right before your post.

Mac
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Ike Zimbel

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Re: Strange hum in audio
« Reply #8 on: March 19, 2021, 01:04:51 pm »

I installed a 3.5mm audio cable that goes from the stage, up into the attic, then down through the ceiling to the Yamaha TF1 sound board. The audio cable is made up of two 50 foot pieces. Our stage area has VGA cable that goes from the stage under the floor to the Yamaha. The object is to have video and audio at the stage. The video typically goes from a laptop (on the stage) to an HDMI to VGA converter, then goes through the VGA cable (under the floor) to the Yamaha, into another converter (VGA to HDMI) and into the desktop computer. The video is not a problem, maybe.
I tested the new 3.5mm cable in the auditorium before I strung it through the attic. No problem. After stringing it in the attic, there is a very obvious hum. If I leave the audio cable plugged into the stage computer and pull the HDMI cable out, the hum stops. I tried two different HDMI to VGA converters, with the same hum.
The only explanation I have is that there is something in the attic interfering (since I tested this before pulling into the attic with absolutely no hum). The other explanation for the hum is that where the audio cable comes down to the Yamaha, it comes down a pole that also has some electrical wires on the same pole. Any ideas of how to troubleshoot this?
Ground LOOPS are called that for a reason. By running one cable through the attic, and the other through the basement, you are creating a very large loop area between the shields on the two cables. Running both cables, near each other, under the floor might solve or minimize the issue.
Russell's suggestion of using a stereo DI (preferably one with good transformers) is a good one, but even then, running the XLR's through the attic will still give you a large loop area, which at the very least would probably require the use of the ground lift switch(es) on the DI(s). There are DI boxes that have 3.5mm inputs, but if you go that route, be very mindful of the fact that there are a LOT of really crappy pre-made 3.5mm cables out there. I've been burned many times with that so now usually make my own.
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Don T. Williams

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Re: Strange hum in audio
« Reply #9 on: March 19, 2021, 01:28:28 pm »



As everyone has said: you have a classic ground loop.  The easiest fix is using an AV type transformer issolating DI box with a ground lift switch.  Be on the lookout (or listening for) power supply buzz if you are using laptop computers.  Not all of them have the problem but you may hear a small buzz when the power supply is plugged into the wall but not when they run off battery.  One common problem I have run into fairly often is distorted audio and background noise out of the headphone jacks.  Taking the audio out of a USB A to D/D to A adapther can greatly improve the audio quality.  Even the cheap UCA 202 from Bheringer makes a world of difference.
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ProSoundWeb Community

Re: Strange hum in audio
« Reply #9 on: March 19, 2021, 01:28:28 pm »


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