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Three pin XLR balanced cables / Chassis connections

ntom

pfm Member
Can anyone tell me why 3pin XLR plugs with metal bodies offer a fourth connection to plug metalwork (& therefore chassis grounding).

Wondered when you might use this. Would it ever be wise to connect Chassis (& presumably earth grounds together). These days with many audio products double insulated & therefore not earthed could it be potentially dangerous, or might it be beneficial to have the cases earthed (assuming signal ground is separated from the casework on your double insulated device).

My new shiny Neutrik XLRs have the three signal pins for balanced connection plus the 4th pin for Chassis ground. Why have this if it's never used with a cable?

Is there any standardisation when it comes to whether signal screens are independent:
-safety / mains ground
-casework where double insulated (doesn't the casework become an extension of the cable screen in this case).

Confused!
 
What Stefan says, in most cases if you cut the xlr holes yourself its not needed, just check you have good continuity between socket and chassis. Use stainless not anodized screws to mount the sockets and you'll be fine
 
Standard balanced cable is 2 core plus screen. So are you suggesting the screen which would normally be connected to pin 1, should also be connected to the metal plug casing too....?

The the picture attached. There are three, plus a further contact which makes connects with the plug body when assembled & has a solder tag. i.e. there are four connections to the plug.


https://images-na.ssl-images-amazon.com/images/I/41Sr1CF22dL.jpg
 
Thanks for the link. Interesting read. Hows does this relate to actual devices?

Recently bought a Cambridge 851E pre. Partly for the balanced inputs that allows me to connect a Benchmark ADC, a Cyrus Phono and a Slim Devices Transporter.

All of these are earthed wrt to mains, and casework is connected to this, so the 4th XLR socket connections are all connected to this earth. The shield pin on both the Benchmark & Transporter are both also connected to earth.

The Cambridge has a 47ohm resistance between mains ground and XLR pin 1 (& between phono signal ground and main ground). I presume this is to deal with potential ground loop probs?

I presume if making my own cables I short the screen of the plugs to pin 1 it would defeat this. Assuming no hum problems it would be fine. If I found I had hum problems then disconnection of one end should sort it.
 
I presume most XLR / XLR cables that are available, leave the plug ground connection un-terminated for precisely this reason i.e. to avoid affecting earthing arrangements of connected equipment.
 
so long as the signal 0v is connected somewhere else, i.e. not right next to the shield, then all is fine. normally 0V would be connected as close to the mains earth as possible. Older gear tends to use this resistor to chassis thing but most modern gear doesn't. The whole point is that the shield is as continuous with the chassis a possible. Read up on the AES48 standard.
 


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