Disconnected ground wire in balanced cable causes ...

[Clint Goss]

Thought I had some idea how balanced cables worked, but then I hit this:

The ground pins on a balanced cable from my mixer to one of my near field monitors was not connected. Got a signal, but also significant HUMM. Should a speaker (Mackie HR824 Mk2) even produce a signal with no ground / shield connection?? Both Tip and Ring were connected ...

Any info / thoughts welcome!

** The Full Story - not really important to the question, but I've been at this for HOURS so felt the need to document it ...
I had a 50 ft balanced cable route through my drop ceiling to my near field monitors, and got significant HUMM. "Must be the halogen lights," I surmised and laid my cables on the floor for about two years. No Hum.
On a whim today I again tried the ceiling cable and this time tried turning off the lights. Still Hummed. Then I ran another 50 ft cable through the ceiling and got ... NO HUM ... Lights or No Lights. Tested the HUMMing cable and found that the ground pins were not connected.
*** END of Full Story

-Clint Goss

 

[fpitas]

Grounding the shield, at least at one end, is a good idea. It also depends on the common mode rejection ratio (CMRR) of the line receiver, and its common mode range. So if for example you get 50V of 60Hz on the line, that may exceed what the line receiver can handle. In extreme cases you can also overload the line transmitter.

 

[sergeauckland]

A balanced cable doesn't need the screen connected, as the balanced input only cares about the difference in voltage between the two signal wires. Common-mode signal, i.e. those on both signal wires like hum should be fully rejected. However, and it's a big however, it depends on how much hum there is, and how well balanced both the output and input are. For long cables, especially those near to lighting circuits, they can pick up sufficient hum for the common-mode rejection (which is never infinite, typically something like 40dB) to leave enough to be audible. That's why for long cables or microphones, a well screened cable is necessary even with balanced circuits.

S.

 

[fpitas]

A balanced cable doesn't need the screen connected, as the balanced input only cares about the difference in voltage between the two signal wires. Common-mode signal, i.e. those on both signal wires like hum should be fully rejected. However, and it's a big however, it depends on how much hum there is, and how well balanced both the output and input are. For long cables, especially those near to lighting circuits, they can pick up sufficient hum for the common-mode rejection (which is never infinite, typically something like 40dB) to leave enough to be audible. That's why for long cables or microphones, a well screened cable is necessary even with balanced circuits.

S.

True, and as I noted, any line receiver has definite upper limits on common mode voltage. Past that voltage the rejection falls rapidly. That is a place where transformers shine.

 

[Clint Goss]

A balanced cable doesn't need the screen connected,

Thank you for your responses!!!

I clearly have a pretty hazy picture of how this all works. I figured that, without the ground wire, there would be an open circuit and there would effectively be no audio signal. Do I get a signal because the audio signal is basically alternating current??

What actually happened: I replaced cable "A" with the disconnected ground with another cable "B" which has the ground pins connected. Same source, same destination, same path past the in-ceiling lighting. Noise level went from untenable to pretty much zero.

 

[sergeauckland]

Thank you for your responses!!!

I clearly have a pretty hazy picture of how this all works. I figured that, without the ground wire, there would be an open circuit and there would effectively be no audio signal. Do I get a signal because the audio signal is basically alternating current??

What actually happened: I replaced cable "A" with the disconnected ground with another cable "B" which has the ground pins connected. Same source, same destination, same path past the in-ceiling lighting. Noise level went from untenable to pretty much zero.

On a balanced circuit, the audio is carried on the two signal wires, and the receiver responds to the difference in voltage. The screen doesn't carry any signal, it's just there to screen the two signal wires from external interference, like hum from mains wiring. The balanced circuit rejects anything that's common to the two wires, but if the interference is large, it can overwhelm the common mode rejection and allow audible hum to get through.

S

 

[Clint Goss]

So ... will an unbalanced circuit "work" (i.e. carry sound) if (assume 1/4" TS connectors) only the Tips are connected and not the Sleeves? Will a stereo unbalanced (e.g. to a headphone) cable work if the Sleeves are not connected?

 

[MRC01]

On a balanced circuit, the audio is carried on the two signal wires, and the receiver responds to the difference in voltage. The screen doesn't carry any signal ...

It's worth adding that the signals carried by the 2 signal wires are mirror images of each other, meaning opposite voltages at every instant in time.
Differential signalling and balanced are technically 2 different things but in the realm of audio they are often conflated.

 

[sergeauckland]

So ... will an unbalanced circuit "work" (i.e. carry sound) if (assume 1/4" TS connectors) only the Tips are connected and not the Sleeves? Will a stereo unbalanced (e.g. to a headphone) cable work if the Sleeves are not connected?

Yes and no! An unbalanced signal is (normally) referenced to ground, so if only the tip is connected, then it won't work. However, if the two items have a common ground, either through the the power connection or other items connected somewhere, then just the tip will carry the signal, although it may have a lot of hum if unscreened as there's no common mode rejection on an unbalanced circuit.

 

[Speedskater]

Normally a XLR balanced interconnect cable only needs the shield connected to the chassis at the send end. There are exceptions.
a] If the shield is NOT connected to the chassis at the receive end, the high frequency shielding is diminished. So a hybrid connector at the receive end is better. That's a radio frequency capacitor is connected from the shield to pin #1.
b] a phantom powered mic always needs the shield connected at both ends.
c] if one of the units is unpowered or battery powered, the shield should be connected at both ends.