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Title: Strange hum in audio
Post by: Bill Jones 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?
Title: Re: Strange hum in audio
Post by: Russell Ault 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
Title: Re: Strange hum in audio
Post by: Brian Adams 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.
Title: Re: Strange hum in audio
Post by: Dave Garoutte 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.
Title: Re: Strange hum in audio
Post by: Bill Jones 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).
Title: Re: Strange hum in audio
Post by: Mike Caldwell 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.
Title: Re: Strange hum in audio
Post by: Bill Jones 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 (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).
Title: Re: Strange hum in audio
Post by: Mac Kerr 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 (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
Title: Re: Strange hum in audio
Post by: Ike Zimbel 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.
Title: Re: Strange hum in audio
Post by: Don T. Williams 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.
Title: Re: Strange hum in audio
Post by: Mike Caldwell on March 19, 2021, 01:32:25 pm
The Rolls piece you linked to is a simple lower end AV DI box. They don't have the best transformer in them. It would be better than what your using now!

For the same money you can get the Peavey USB interface.
https://www.bhphotovideo.com/c/product/759075-REG/Peavey_USB_P_USB_P_USB_Direct.html (https://www.bhphotovideo.com/c/product/759075-REG/Peavey_USB_P_USB_P_USB_Direct.html)

To use that you would need to go into the laptop audio settings and assign it to be the main audio output.
Title: Re: Strange hum in audio
Post by: Tim McCulloch on March 19, 2021, 01:36:36 pm
The easy button:  Radial Stagebug 5, mic line back to the mixer.  This ain't rocket surgery.
Title: Re: Strange hum in audio
Post by: Bill Jones on March 19, 2021, 01:52:46 pm
https://www.amazon.com/Radial-StageBug-SB-5-1-channel-Passive/dp/B00CRVWFFS/ref=sr_1_1?dchild=1&keywords=stagebug+sb-5&qid=1616175837&sr=8-1 (https://www.amazon.com/Radial-StageBug-SB-5-1-channel-Passive/dp/B00CRVWFFS/ref=sr_1_1?dchild=1&keywords=stagebug+sb-5&qid=1616175837&sr=8-1)
Would this one work for me? It is for a laptop and has the 3.5mm connection. I would have to convert XLR out to RCA which goes to my sound board.
Title: Re: Strange hum in audio
Post by: Tim McCulloch on March 19, 2021, 02:01:09 pm
https://www.amazon.com/Radial-StageBug-SB-5-1-channel-Passive/dp/B00CRVWFFS/ref=sr_1_1?dchild=1&keywords=stagebug+sb-5&qid=1616175837&sr=8-1 (https://www.amazon.com/Radial-StageBug-SB-5-1-channel-Passive/dp/B00CRVWFFS/ref=sr_1_1?dchild=1&keywords=stagebug+sb-5&qid=1616175837&sr=8-1)
Would this one work for me? It is for a laptop and has the 3.5mm connection. I would have to convert XLR out to RCA which goes to my sound board.

Why do you need RCA at the console?  Are you out of XLR inputs?

Also, what's with the "conversion" of HDMI to VGA and back again, and what is accomplished by linking 2 laptops this way?
Title: Re: Strange hum in audio
Post by: Russell Ault on March 19, 2021, 02:28:06 pm
https://www.amazon.com/Radial-StageBug-SB-5-1-channel-Passive/dp/B00CRVWFFS/ref=sr_1_1?dchild=1&keywords=stagebug+sb-5&qid=1616175837&sr=8-1 (https://www.amazon.com/Radial-StageBug-SB-5-1-channel-Passive/dp/B00CRVWFFS/ref=sr_1_1?dchild=1&keywords=stagebug+sb-5&qid=1616175837&sr=8-1)
Would this one work for me? It is for a laptop and has the 3.5mm connection. I would have to convert XLR out to RCA which goes to my sound board.

Those are good units; if you need stereo (which you probably don't) you'll have to adapt the TRS outputs to XLR (or use balanced cable with TRS connectors).

The output of a DI (any DI) isn't suitable for RCA inputs; you'll need to use the mic inputs on your console.

-Russ
Title: Re: Strange hum in audio
Post by: Bill Jones on March 19, 2021, 02:56:57 pm
We are out of inputs on our Yamaha TF1. There are two additional inputs that have RCA inputs (red and white). These are the ones that come from the laptop's 3.5mm jack that goes up through the attic and then down to the Yamaha.
The video conversions is because our sanctuary is a bit old and has VGA cabling that goes from the stage, back to the Yamaha and it all goes under the floor. Most laptops today have HDMI video output, so we have to change (via a converter) to VGA. At the desk where the Yamaha is, and where the computer is, the VGA cable is changed back to HDMI to connect to the computer's capture card. I know it's not the most elegant solution, but it does work for video. It sounds like it would be easy to just run HDMI cable from the stage to the computer and then we could just get the audio from HDMI. It is possible, but would require some minor construction under the floor since the cabling passes through a barrier under the floor. The other option was to leave the HDMI/VGA/HDMI part the way it is and just run the audio up through the attic (which was quite easy).
That takes me to where I'm at now. If I can just get the SB-5 to connect from the 3.5mm input and ultimately output to the RCA jacks in the sound board, I will be a happy camper.
BTW, this is not high quality audio that we need, just audio without a hum. The audio would be from short videos and PowerPoint with embedded audio (played from a laptop on the stage).
Title: Re: Strange hum in audio
Post by: Russell Ault on March 19, 2021, 03:00:22 pm
{...} That takes me to where I'm at now. If I can just get the SB-5 to connect from the 3.5mm input and ultimately output to the RCA jacks in the sound board, I will be a happy camper.

Not without a preamp; passive DI outputs tend to be closer to mic-level than to line-level.

If HDMI-over-fibre is an option that'd work for you and running cable through the attic is easy, why not leave the VGA cable unused and run an HDMI-over-fibre system through the attic?

-Russ
Title: Re: Strange hum in audio
Post by: Tim McCulloch on March 19, 2021, 04:16:16 pm
Not without a preamp; passive DI outputs tend to be closer to mic-level than to line-level.

If HDMI-over-fibre is an option that'd work for you and running cable through the attic is easy, why not leave the VGA cable unused and run an HDMI-over-fibre system through the attic?

-Russ

THIS!  Right up yonder! ^^^

If we can address the video issue, the audio solution can piggyback with VDO.  A device like the one below will loop-through your HDMI signal to your switcher/projector/whatever and allow you to extract L/R analog audio on RCA jacks.  With an HDMI fiber cable (2nd link) you have a $100 solution

https://www.amazon.com/dp/B00XJITK7E
https://www.amazon.com/dp/B07KG71K8K

On the analog audio side, a pair of ART CleanBox Pro, one on each end, would provide all kinds of ins and outs but you're still running more wire or cable, too.

Bill, you can look at it like this - changing the video cable to a powered, fiber HDMI will fix the video problem you didn't know you had (nudge, wink) and enable a $30-$50 fix for the audio problem you know you have.  You'll only run 1 cable and have fewer fiddly bits.

More thoughts on the SB-5:  if you run 2 wires from stage for left & right and use the TRS outputs, you could unbalance the line at the mixer by wiring pin 3 (ring) to RCA ground and pin 2 (tip) to RCA center pin, and jumping pin 1 (sleeve) to RCA ground.  The question is if you'll have enough level after losing 6dB to the unbalance.
Title: Re: Strange hum in audio
Post by: Russell Ault on March 19, 2021, 08:14:16 pm
{...} More thoughts on the SB-5:  if you run 2 wires from stage for left & right and use the TRS outputs, you could unbalance the line at the mixer by wiring pin 3 (ring) to RCA ground and pin 2 (tip) to RCA center pin, and jumping pin 1 (sleeve) to RCA ground.  The question is if you'll have enough level after losing 6dB to the unbalance.

Plus the (if my math is right) ~11 dB of signal loss in the DI's transformers (which is actually less than I would have guessed).

-Russ
Title: Re: Strange hum in audio
Post by: Dave Garoutte on March 19, 2021, 09:15:45 pm
Not without a preamp; passive DI outputs tend to be closer to mic-level than to line-level.

If HDMI-over-fibre is an option that'd work for you and running cable through the attic is easy, why not leave the VGA cable unused and run an HDMI-over-fibre system through the attic?

-Russ
D'oh!  I was thinking of under the floor, but the attic is perfect.  An extractor would still work. Add a 1/8 TRS to twin RCA cable and you're done.  The fiber HDMI cable will isolate, so no DI required.
Title: Re: Strange hum in audio
Post by: Bill Jones on March 20, 2021, 08:58:18 pm
Thanks for all the input. You guys are the best.
I think the fiber HDMI cable in the attic sounds like the best option for me. The extractor at the sound board also sounds like to way to go.
Thanks again.
Title: Re: Strange hum in audio
Post by: Bill Jones on March 21, 2021, 07:37:41 am
The hard part in installing the fiber HDMI will be telling the young man that crawled into the attic that he has to remove the wire he just installed a week ago, and then put in another wire.  ;D
Title: Re: Strange hum in audio
Post by: raymondsoly on March 21, 2021, 11:25:43 am
The hard part in installing the fiber HDMI will be telling the young man that crawled into the attic that he has to remove the wire he just installed a week ago, and then put in another wire.  ;D

And then tell him you were kidding  ;D, and that he could just use the existing cable as a pull line for the new cable instead..... 8)

Ray
Title: Re: Strange hum in audio
Post by: Russell Ault on March 21, 2021, 03:22:28 pm
{...} and that he could just use the existing cable as a pull line for the new cable instead..... {...}

Risky business pulling fibre outside of conduit!

-Russ
Title: Re: Strange hum in audio
Post by: raymondsoly on March 21, 2021, 06:15:45 pm
Risky business pulling fibre outside of conduit!

-Russ

perhaps no riskier than having a young person crawl in there ?

Ray
Title: Re: Strange hum in audio
Post by: Bill Jones on March 22, 2021, 06:33:10 am
Risky business pulling fibre outside of conduit!
-Russ

Is the fibre HDMI fragile, more so than any other wire? Will there be a problem bending it around corners?
Title: Re: Strange hum in audio
Post by: Dave Garoutte on March 22, 2021, 01:58:06 pm
Is the fibre HDMI fragile, more so than any other wire? Will there be a problem bending it around corners?
The manufacturer should spec a minimum radius.  The cables are more delicate than an xlr, but not a lot.
Title: Re: Strange hum in audio
Post by: Helge A Bentsen on March 22, 2021, 02:35:45 pm
IME HDMI cables and network copper cables tend to be a lot more fragile than a fiber cable.
Title: Re: Strange hum in audio
Post by: Russell Ault on March 22, 2021, 03:38:30 pm
Is the fibre HDMI fragile, more so than any other wire? Will there be a problem bending it around corners?

The usual warning with fibre is that bending it below its spec'ed minimum radius even once can cause permanent, it-doesn't-work-any-more-style damage (that very few people can fix), which is why pulling it can be risky. Other than that, fibre tends to be pretty durable, and (depending on the fibre) it can have significantly better tensile strength than copper-based cabling (which is why pulling it through properly-shaped conduit is no big deal).

-Russ
Title: Re: Strange hum in audio
Post by: raymondsoly on March 22, 2021, 05:09:51 pm
Is the fibre HDMI fragile, more so than any other wire? Will there be a problem bending it around corners?

The weakest links here might actually be the attic type and /or the 2 x 50'  hdmi, which won't offer much pulling power at the coupler...

Ray