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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 => AC Power and Grounding => Topic started by: Nicholas Bailey on March 19, 2016, 09:58:23 AM
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We have a ground loop roll on the projectors at our church and LCD in the lobby. It disappears with adding the death ground lift adaptor on the computer. Whats the next step to make it safe?
Three of the projectors are in the worship center. I can see that the 4 square box the outlet that powers two of them are mounted on the red iron. The other projector is in the sound booth. There is also a projector and LCD in another room.
I really do not want to leave the ground adaptor in place.
I look forward to you replys.
Nick
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We have a ground loop roll on the projectors at our church and LCD in the lobby. It disappears with adding the death ground lift adaptor on the computer. Whats the next step to make it safe?
Three of the projectors are in the worship center. I can see that the 4 square box the outlet that powers two of them are mounted on the red iron. The other projector is in the sound booth. There is also a projector and LCD in another room.
I really do not want to leave the ground adaptor in place.
I look forward to you replys.
Nick
Rather than lifting the ground on the sound system, you should consider eliminating the ground loop current coming from the projectors. We typically run CAT-5 or CAT-6 with Baluns (Bal-Unbal transformer) adapters for long projector runs plugged into different outlets. CAT-5 is good for 1080P HD, while CAT-6 is called for to send 4K video.
This follows my general grounding mantra of "ground locally / isolate remotely". Each piece of AV gear should get its safety ground from the local EGC conductor in its local power outlet, and then you should isolate the shields interconnecting the gear. I have one church site where we installed a 1-in/4-out active HDMI DA (Distribution Amp) with four baluns feeding video screens and projectors in three different rooms. Not a hum bar to be seen or hum to be heard once we got that in place.
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We have a ground loop roll on the projectors at our church and LCD in the lobby. It disappears with adding the death ground lift adaptor on the computer. Whats the next step to make it safe?
Three of the projectors are in the worship center. I can see that the 4 square box the outlet that powers two of them are mounted on the red iron. The other projector is in the sound booth. There is also a projector and LCD in another room.
I really do not want to leave the ground adapter in place.
I look forward to you replys.
Nick
Mike answered your question, I just want to jump in and hold up your post as the template all should follow. You identified the problem. You described how you proved that is the problem, and then asked how to fix it in a safe way. Good solid post.
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Rather than lifting the ground on the sound system,
He lifted the ground from the computer, not the sound system.
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We have a ground loop roll on the projectors at our church and LCD in the lobby. It disappears with adding the death ground lift adaptor on the computer. Whats the next step to make it safe?
Three of the projectors are in the worship center. I can see that the 4 square box the outlet that powers two of them are mounted on the red iron. The other projector is in the sound booth. There is also a projector and LCD in another room.
I really do not want to leave the ground adaptor in place.
I look forward to you replys.
Nick
Are you using VGA or HDMI?
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Are you using VGA or HDMI?
I've been at a few churches that still have composite video running on BNC/coax, and that seems to be with worst for ground loop hum bars. They also make video isolation transformers that will lift that kind of ground loop.
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They have VGA cable pulled to all the projectors and the one LCD.
I agree with you Mike if I were to do the install today I would use a cat6 solution.
Any other sugggestions without changing the cabling?
Nick
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They have VGA cable pulled to all the projectors and the one LCD.
I agree with you Mike if I were to do the install today I would use a cat6 solution.
Any other sugggestions without changing the cabling?
A quick Google search for "vga isolation transformer" returned this as the top result, so they are out there:
http://www.allenavionics.com/VHumElim/VI-VGA.htm
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A quick Google search for "vga isolation transformer" returned this as the top result, so they are out there:
http://www.allenavionics.com/VHumElim/VI-VGA.htm
Extron also has one as does Jensen transformers. These would be RGBHV, 5 wire video isolation transformers that are going to have a significant cost. Probably somewhere between about $500.00 and $1000.00 per computer output.
The Jensen is linked here http://www.jensen-transformers.com/product/vbh-5bb/
They are $735.00 ea. at Broadcast Supply. The Jensen is lower cost than the Extron and I would choose Jensen over Extron. The Allen Avionics HEC-5000 referenced by Jonathan is $616.25 from the same supplier.
You will want to look at the cost of pulling new cable and purchasing the appropriate converters to utilize the twisted pair options that are out there vs. simply adding the isolation transformers.
Lee
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While I agree that the perfect solution is a VGA iso, I'm still not clear on what running a laptop (if it is a laptop) with no AC ground is an issue.
I have seen plenty of laptops that don't have 3 pin AC plugs.
My all metal Macbook has no ground but my all plastic Lenovo does have a ground. Makes me wonder.
BTW... have the same issue at a local gig and we use an SDI to VGA converter which seems to do the trick.
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While I agree that the perfect solution is a VGA iso, I'm still not clear on what running a laptop (if it is a laptop) with no AC ground is an issue.
I have seen plenty of laptops that don't have 3 pin AC plugs.
My all metal Macbook has no ground but my all plastic Lenovo does have a ground. Makes me wonder.
BTW... have the same issue at a local gig and we use an SDI to VGA converter which seems to do the trick.
I assumed that the laptop was also supplying the audio for the sound system, and thus was "grounded" to the sound system's ground. If the laptop is also supplying the audio, then the next question is does the sound system also have hum? If your layout "grounds" the laptop via a shield to the sound system, then lifting the EGC ground on the laptop should be perfectly safe. Perhaps you're accidentally bonding the isolated grounds in your laptop by "grounding" its power supply EGC to its audio ground plane.
Laptops are their own brand of grounding craziness since they have lots of internal switching power supplies to create all the needed voltages. One thing that would be really interesting to try it to put a clamp-on ammeter around the various VGA cables to look for any ground loop current. I do this all the time for XLR audio cables, but have never tried it for VGA cables. I assume it will be about equivalent with maybe 10 to 100 mA of current causing visible hum bars, and 100 to 1,000 mA ground loop current making the picture unstable. And how does this work for HDMI cables? Gosh, that sounds like an interesting experiment, doesn't it?
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Here's a pretty good discussion on ground-loop video hum bars and how to troubleshoot it. A lot of the answers here are incorrect, but it does show just how complicated and confusing video hum bars can be.
http://www.film-tech.com/ubb/f16/t000377.html
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While I agree that the perfect solution is a VGA iso, I'm still not clear on what running a laptop (if it is a laptop) with no AC ground is an issue.
I have seen plenty of laptops that don't have 3 pin AC plugs.
My all metal Macbook has no ground but my all plastic Lenovo does have a ground. Makes me wonder.
BTW... have the same issue at a local gig and we use an SDI to VGA converter which seems to do the trick.
I don't really understand the distinction, either. I think the laptop "ground" often only goes to the external power supply, and may not pass through to the laptop.
Nick hasn't said whether he is using a laptop or a desktop. Based on the report, I am guessing that it is a desktop, which very likely bonds the VGA ground with the mains ground.
Is the VGA signal (-) tied to ground or is it a balanced signal? If it is balanced, not ground referenced, you should be able to break the shield of the VGA cable (on one end) to safely eliminate the problem. (I don't know the VGA cable spec/pinout.)
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Is the VGA signal (-) tied to ground or is it a balanced signal? If it is balanced, not ground referenced, you should be able to break the shield of the VGA cable (on one end) to safely eliminate the problem. (I don't know the VGA cable spec/pinout.)
From what I can see from the pinout diagram, the VGA cable spec is unbalanced with a common return path from each of the signals, that are typically bonded together. Also, there's no real spec for the cable, so some VGA cables are coax shielded while others aren't. If they are indeed unbalanced, then lifting the shield (ground?) on one end of the cable will only make the hum bars much worse. I'll contact one of my video manufacturers to learn more.
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Thanks for the reply guys.
As usual to fix it correctly is not cheqp.
I will do some more testing when I install a center projector for the church.
I was running sound the morning the lady running the powerpoint ran in and plugged in the ground lift.
Nick
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I was running sound the morning the lady running the powerpoint ran in and plugged in the ground lift.
While plugging in a ground lift plug on the computer isn't probably immediately dangerous, when something else goes wrong you can get into really big trouble, fast. When that perfect storm of failures occurs, death can be the result. So every safety device we disable puts us one step closer to catastrophe. That's why it's so easy to ignore these safety devices. You don't need it until you really need it, and then you need it badly.
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If you must lift a safety ground, a GFCI outlet will still work and protect against hazardous shock currents. One of those GFCI power drops plugged into your 3/2 plug adapter isn't crazy.
JR
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If you must lift a safety ground, a GFCI outlet will still work and protect against hazardous shock currents. One of those GFCI power drops plugged into your 3/2 plug adapter isn't crazy.
Good call. I've posted a link to this before, but for the newbies here, here's something I wrote about how GFCI's perform this current balancing act that doesn't require a safety ground to operate. It's just flat-out ingenious. http://noshockzone.org/rv-electrical-safety-part-viii-gfci/
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While I agree that the perfect solution is a VGA iso, I'm still not clear on what running a laptop (if it is a laptop) with no AC ground is an issue.
I have seen plenty of laptops that don't have 3 pin AC plugs.
My all metal Macbook has no ground but my all plastic Lenovo does have a ground. Makes me wonder.
BTW... have the same issue at a local gig and we use an SDI to VGA converter which seems to do the trick.
Mac float all over the place. The definitely need to be isolated as much as possible. Fortunately I've not been involved with using one to feed both video and audio. Just audio, which I fixed with a Radial transformer DI.
I just got back from China and plugging a Mac into the 240VAC mains over there results in leakage on the case you can feel. This odd buzzing tingle that makes the case feel textured. One of these days I'm going to get hold of a meter and see what the potential between the case and a real ground is.
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Good call. I've posted a link to this before, but for the newbies here, here's something I wrote about how GFCI's perform this current balancing act that doesn't require a safety ground to operate. It's just flat-out ingenious. http://noshockzone.org/rv-electrical-safety-part-viii-gfci/
In the unlikely event that a person contacts both the hot and neutral, and is otherwise insulated from the surrounding environment, a GFCI will not protect against that shock. :o
One scenario where it could happen is if a "grounded" device is plugged into a bootleg ground receptacle, and another "grounded" device is plugged into an RPBG receptacle, and the person contacts the chassis of both devices. A GFCI upstream may not protect from that. Of course, it's likely that the GFCI would trip well before that happens (because of the bootleg grounds), but hey, it's a conceivable problem.
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In the unlikely event that a person contacts both the hot and neutral, and is otherwise insulated from the surrounding environment, a GFCI will not protect against that shock. :o
One scenario where it could happen is if a "grounded" device is plugged into a bootleg ground receptacle, and another "grounded" device is plugged into an RPBG receptacle, and the person contacts the chassis of both devices. A GFCI upstream may not protect from that. Of course, it's likely that the GFCI would trip well before that happens (because of the bootleg grounds), but hey, it's a conceivable problem.
Perhaps a few too many "and thens".
I ASSume you mean bootleg ground to neutral (not RPBG). If the GFCI outlet and normal(?) bootleg outlets are used in parallel (not plugged into each other), the bootleg safety ground connection is effectively on the input side neutral of GFCI protected outlet. In that scenario any line to safety ground (to neutral via input side bootleg) fault current is outside the GFCI loop and will trip the GFCI outlet at the expected >5mA.
As I've warned before, if a GFCI bootleg ground is connected to neutral at the output side of the GFCI, that will defeat the protection for faults to that bootleg ground. If it is bootlegged to the input side neutral the GFCI will work, but still DON'T DO THAT it is always unsafe practice to bootleg.
Better IMO to just float the safety ground and let the GFCI do it's magic.
JR
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Mac float all over the place. The definitely need to be isolated as much as possible. Fortunately I've not been involved with using one to feed both video and audio. Just audio, which I fixed with a Radial transformer DI.
I just got back from China and plugging a Mac into the 240VAC mains over there results in leakage on the case you can feel. This odd buzzing tingle that makes the case feel textured. One of these days I'm going to get hold of a meter and see what the potential between the case and a real ground is.
We use Apple laptops (yes the aluminum Unibody ones) all the time for video and audio and have not had issues or need "isolation" for VGA, DVI, HDMI, and even the headphone jack on occasion (I use both Radial and Peavey USB DIs.) The AC adaptor/power supply on Macs can be used with the the 2 prong ungrounded plug (plugs directly into outlet) or the 3 prong grounded power cord. Which did you use?
John R.
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I've run across plenty of laptops, and owned one, that caused this issue and some audio noise as well. Almost every time all noise went away when the laptop ran on batteries. Maybe this is the solution rather than disconnecting any safety grounds if you have enough battery life to last through your average event.
Another common fix, although less universal than running on batteries, that usually helps with audio and video noise is a ground lifted DI box connection to the console.
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We use Apple laptops (yes the aluminum Unibody ones) all the time for video and audio and have not had issues or need "isolation" for VGA, DVI, HDMI, and even the headphone jack on occasion (I use both Radial and Peavey USB DIs.) The AC adaptor/power supply on Macs can be used with the the 2 prong ungrounded plug (plugs directly into outlet) or the 3 prong grounded power cord. Which did you use?
John R.
Typically when traveling we just carry the two prong adapter that goes right on the PS. I'll bring the 3 wire cord and a travel adapter next time and see if I can still feel the leakage.
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Typically when traveling we just carry the two prong adapter that goes right on the PS. I'll bring the 3 wire cord and a travel adapter next time and see if I can still feel the leakage.
If you can feel it you can measure it with a VOM... I measured close to 120VAC and 2 mA leakage from my power strip with the broken surge protection circuitry to a real ground return.
JR