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electrical wiring to avoid ground loops? (need advice)
Old 5th March 2014
  #1
Gear Head
 
🎧 5 years
electrical wiring to avoid ground loops? (need advice)

Hi everyone, I moved wireless mic receivers (4-shure slx) and antenna distro (UA844) to the front of the stage and when I plug in the power adapter for the antenna distro, I get this low frequency hum thru the speakers. I tried to use a two prong cable in the place of the three prong cable that came with the power adapter and the hum disappears completely. When I did little research, all people say to never use two prong if the equipment asks for three prong. So I started searching for more and more information about ground loops and how to fix it, and most of the sites just telll you what the ground loop is, but no instructions how the outlets should be wired to get rid of the loop.

So I made bunch of drawings showing all the options of different wiring methods, please let me know which one is the best to avoid the ground loops.














Last edited by sania44; 5th March 2014 at 08:21 AM.. Reason: spelling
Old 5th March 2014
  #2
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MIKEHARRIS's Avatar
 
🎧 15 years
The problem comes in the electrical service...unless stage & FOH power are on the same leg you will have ground loops.
We request breaker 1/4/9/12 be fed to stage and FOH...a 3 phase panel alternates phases as you go down in order...with orange outlets & homerun grounds.
Old 5th March 2014
  #3
LX3
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🎧 15 years
Try disconnecting pin 1 on each of the XLR lines from the receivers to the mixing desk. Personally I would disconnect it at the mixing console end rather than the receiver end, but YMMV.

Pin 1 is usually connected to mains earth on both pieces of equipment, so there's an earth loop right there.

Pin 1 doesn't carry any audio signal, so not connecting it at both ends shouldn't be a problem in this situation.

This may well fix your hum issue without removing the earth connection from any mains-powered kit which I agree is a pretty bad idea.

To save you desoldering all the XLRs at first, just try it on one line (leave others disconnected at both ends). See if it hums with just one receiver output connected. Then disconnect pin one on the male xlr plug at the mixer end, reconnect, and see if the hum has gone away. If it has, do all the others as well and see where you're at. If it hasn't, then this isn't the solution.

I'd put some money on this working though.
Old 5th March 2014 | Show parent
  #4
Gear Head
 
🎧 5 years
Quote:
Originally Posted by LX3 ➡️
Try disconnecting pin 1 on each of the XLR lines from the receivers to the mixing desk. Personally I would disconnect it at the mixing console end rather than the receiver end, but YMMV.

Pin 1 is usually connected to mains earth on both pieces of equipment, so there's an earth loop right there.

Pin 1 doesn't carry any audio signal, so not connecting it at both ends shouldn't be a problem in this situation.

This may well fix your hum issue without removing the earth connection from any mains-powered kit which I agree is a pretty bad idea.

To save you desoldering all the XLRs at first, just try it on one line (leave others disconnected at both ends). See if it hums with just one receiver output connected. Then disconnect pin one on the male xlr plug at the mixer end, reconnect, and see if the hum has gone away. If it has, do all the others as well and see where you're at. If it hasn't, then this isn't the solution.

I'd put some money on this working though.
Wouldn't that destroy the idea or a purpose of a balanced cable? will there be interference problems if I lift the ground?
I was trying to find a solution from the power feed side, but I will try the ground disconnect with a jumper cable.
Old 5th March 2014
  #5
Gear Head
 
🎧 5 years
Another question that came up when I was trying track the hum were the power adapters. The power adapters that come with the slx receivers have only two prongs, but the adapter that comes with the UA844 distro has three prongs. Why would it make things unsafe if I will brake the ground prong from the distros power adapter?

Shure SLX receiver adapter


Shure Antenna Distribution adapter
Old 5th March 2014 | Show parent
  #6
Gear Head
 
🎧 5 years
Quote:
Originally Posted by MIKEHARRIS ➡️
The problem comes in the electrical service...unless stage & FOH power are on the same leg you will have ground loops.
We request breaker 1/4/9/12 be fed to stage and FOH...a 3 phase panel alternates phases as you go down in order...with orange outlets & homerun grounds.
Good advice, I will open up the circuitry breakers box to see if the breakers from FOH and stage are fed from different legs.
Old 5th March 2014 | Show parent
  #7
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MIKEHARRIS's Avatar
 
🎧 15 years
Quote:
Originally Posted by sania44 ➡️
Good advice, I will open up the circuitry breakers box to see if the breakers from FOH and stage are fed from different legs.
Carefully...this should be an electrician thing
Old 5th March 2014 | Show parent
  #8
LX3
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🎧 15 years
Quote:
Originally Posted by sania44 ➡️
Wouldn't that destroy the idea or a purpose of a balanced cable? will there be interference problems if I lift the ground?
I was trying to find a solution from the power feed side, but I will try the ground disconnect with a jumper cable.
No, it's pins 2 (hot) and 3 (cold) that make it balanced. Pin 1 is just the shield, and as long as it's connected to earth at one end or other, the cable is shielded.

If I'm lifting pin 1 at one end or other I usually lift it at the destination and leave it connected at the source. But it shouldn't really matter either way.

The only time you definitely need pin 1 connected at both ends is if you need to send phantom power down the cable… e.g. ordinary mic cables, connections to whatever desk is sending phantom, etc.

I do think you should try this first before messing about with the electricity supply. All kinds of funny electricity arrangements exist in venues, but as long as everything's connected back to the same point somehow (and it's all on the same phase), it doesn't generally cause earth loops.
Old 5th March 2014
  #9
LX3
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🎧 15 years
That said, picking up power in radically different parts of a venue, and then connecting audio equipment together….? Anything could happen unless you know where the power is originating from.

Usually the safest thing to do is have a main audio distro somewhere near monitors then run a long, well-made, high capacity cable from that distro to provide all the power to FOH. That way you're pretty close to having a star earthing arrangement, which is the best way to minimise earth loop issues. It also helps ensure that you're on the same phase as all the audio kit on stage.

Not to say you can't still end up with an earth loop.

FWIW, if you're in a venue with old-school analog multicore running from stage to FOH, pin one on all those lines is generally lifted (and phantom has to be done from the monitor desk).
Old 5th March 2014 | Show parent
  #10
Lives for gear
 
🎧 5 years
Quote:
Originally Posted by sania44 ➡️
Wouldn't that destroy the idea or a purpose of a balanced cable? will there be interference problems if I lift the ground?
I was trying to find a solution from the power feed side, but I will try the ground disconnect with a jumper cable.
Regarding the power side of ground loops:
  1. Ground loops are an audio problem and fixing ground loops from the electrical side is typically more expensive and/or more dangerous. Several of the sketches you included that modify the power aspect appear very likely to violate building code (they do violate NEC which is likely what your building code references).
  2. The third or ground prong on power cords is the safety ground and it is how the current is routed to ground should the attached device have a problem. Break that ground path such as by removing the ground pin or using a 'cheater' and if something goes wrong with the related equipment then anyone touching that equipment can become the path for all that current, which can be harmful or potentially deadly.
  3. Some electronic devices are "double insulated" meaning they are constructed such that the exterior will not become 'hot' if there is an internal failure. Those devices do not require a dedicated safety ground and thus typically have a two prong plug. However, if a device has a three prong plug then that device is relying on having a dedicated safety ground path.
  4. Three prong to two prong power adapters adapters are actually intended to adapt modern three prong devices to older AC power distribution systems that did not have a dedicated ground wire and instead used metallic boxes and conduit as the safety ground path. What is often overlooked is that the adapters include a tab or wire that is intended to be connected to a metal screw on the face plate that screws into a metal junction box which is in turn connected to metallic conduit that is connected to an earth ground. Those adapters are not intended to be used with modern power wiring or to have the tab or wire broken off or not connected in order to break the safety ground path. Any adapters that do have the tab or wire removed or broken should be thrown out.
  5. Many aspects relating to building power distribution are only to be addressed or modified by properly licensed professional Electricians, Electrical Contractors or Electrical Engineers.

As far as breaking a ground loop by lifting the audio (not power) ground:
  1. Lifting the audio ground is generally not a problem, however it should not be used where the associated cable may be used to carry phantom power for condenser microphones.
  2. If an audio circuit is balanced to start with then it is still balanced with the audio ground lifted at one end.
  3. Lifting the audio ground should not cause interference, however it may be more susceptible to RF interference. If RFI is a problem with a lifted audio gorund then a 100 picoFarad capacitor can be added between the shield and the equipment ground connection at the 'lifted' end.
  4. While it is not critical, it is generally recommended to lift the audio ground at the receive/destination/input end of the run and to leave it connected at the send/source/output end.

One alternative to lifting the audio ground is the use of input transformers. While a more effective solution it is also a more expensive option, especially when addressing multiple connections.
Old 6th March 2014
  #11
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Electrolytic's Avatar
 
🎧 5 years
keep those wall wart transformers as far away from the ariel gear as possible.
Old 11th March 2014 | Show parent
  #12
Gear Addict
 
🎧 5 years
Quote:
Originally Posted by MIKEHARRIS ➡️
The problem comes in the electrical service...unless stage & FOH power are on the same leg you will have ground loops.
We request breaker 1/4/9/12 be fed to stage and FOH...a 3 phase panel alternates phases as you go down in order...with orange outlets & homerun grounds.
Sorry, I have to disagree here. I've used plenty of systems, both temporary live setups, and permanent studios, where technical equipment was spread over all three phases, and there was no significant hum.

Of the drawings shown, Option 1 or 2 would have the best chance of success, but as others have stated, simply lifting the AUDIO ground (pin 1) on each mic line to the console would be even better, and has no safety implications like messing around in electrical panels does.

You haven't told us what you're using at FOH, but it sounds like your console suffers from the "pin 1" problem, which was quite common in older consoles, and not just at the low end of the market. The problem is that pin 1 needs to take the shortest path to chassis (and electrical) ground, not go to audio signal ground where any noise can contaminate the audio.

It is almost inevitable that there will be slight AC leakage (through capacitance or very high resistance) in all AC powered equipment. Therefore, there will be very slight ground current in all the safety ground wires at each outlet that has equipment connected. Since there is current, there will be a very slight voltage drop on that wire too, and since it's very unlikely that the voltage drop will be the same at each device (And impossible if they are on different phases. Maybe that's where that rumour got started), current will flow in the grounds between equipment, and create noise if it contaminates the audio ground. Using an isolated ground scheme (orange outlets) will help keep other equipment (like motors and coke machines) from making the ground noise even worse, but there will always be something there.

Geoff
Old 11th March 2014
  #13
Lives for gear
 
🎧 5 years
I suggest reading "Power Distribution and Grounding of Audio, Video and Telecommunications Equipment White Paper" and the associated Addendum available at White Papers. A great resource with input from multiple experts.

The "Thermal Management White Paper" on that same page is also a good resource.
Old 15th March 2014 | Show parent
  #14
Gear Head
 
🎧 5 years
Quote:
Originally Posted by cbc6403 ➡️

You haven't told us what you're using at FOH, but it sounds like your console suffers from the "pin 1" problem, which was quite common in older consoles, and not just at the low end of the market. The problem is that pin 1 needs to take the shortest path to chassis (and electrical) ground, not go to audio signal ground where any noise can contaminate the audio.


Geoff
at FOH there is a Mackie SR32-4 VLZ PRO -> DBX 260 Driverack -> snake -> powered speakers.
Old 15th March 2014 | Show parent
  #15
Gear Head
 
🎧 5 years
Quote:
Originally Posted by MIKEHARRIS ➡️
Carefully...this should be an electrician thing
sure thing, I am familiar with electrical wiring quite a bit and I do agree that some of my examples in pictures were not safe or up to code, also I should have added to some of my wiring examples that I was going to add GFI receptacles to the wiring examples that don't have ground running back to the main panel, but again as I did more research, GFIs in live sound/pro audio is a no-no, cause of a chance that the gfi can trip in the middle of the show/recording. I decided not to do anything with electrical wiring so far and lift pin1 on XLRs.
Old 15th March 2014
  #16
Gear Head
 
🎧 5 years
another question is: all of our FOH speakers and monitors are powered/active, does lifting the ground (pin1) at monitors and FOH speakers cause any problems with interference if the run to the main FOH cabinets is around 200' ?
Old 15th March 2014
  #17
Lives for gear
 
🎧 15 years
Don´t forget DI boxes with transformers. These are the general all purpose solution to ground loops. Costs a bit though, so we tend to cheep out with other solutions instead. // Gunnar
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