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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB: The Classic Live Audio Board => Topic started by: Bill Jones on March 18, 2021, 07:02:06 PM
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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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{...} 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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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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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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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).
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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.
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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).
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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
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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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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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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.
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The easy button: Radial Stagebug 5, mic line back to the mixer. This ain't rocket surgery.
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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.
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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?
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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
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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).
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{...} 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
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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.
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{...} 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
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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.
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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.
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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
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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
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{...} 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
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Risky business pulling fibre outside of conduit!
-Russ
perhaps no riskier than having a young person crawl in there ?
Ray
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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?
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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.
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IME HDMI cables and network copper cables tend to be a lot more fragile than a fiber cable.
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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
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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