How should I connect the audio ground and shield wires in a Cat5 cable for analog audio?

Question

I would like to carry 3x balanced line level (not mic level) signals to another part of my living room and I found that the cable space in the skirting boards would not allow 3x balanced cables (XLR connectors at the ends).

I think I can safely carry the (analog) signals using an unmodified Cat5 cable using custom converters at the ends, each twisted pair of the cable carrying a balanced signal.

How should I handle the remaining twisted pair? I think I don't have high frequency interferences in the room, but I want to ensure the best protection against ground loops and 50 Hz interferences.

Should I use both wires for signal ground and ignore the shielding of the XLR connectors? this would allow me to use cheaper plastic XLR connectors. Should I rather use one wire for the shield (connected to the metallic XLR connectors, at one side only of the cable) and one wire for signal ground? Or should I use a STP Cat5 and use both wires for signal ground, and connect the cable shield to the metallic XLR connectors at one side only?

I guess that stranded vs solid copper wires make no difference.

Related but not same: Analog Audio Signal over dedicated Ethernet/Twisted Pair Cable

Edit: it is worth mentioning that the source device is the same one for all 3x signals, therefore the signal ground of the sources is actually the same.

Answer 1

You can treat the 4- twisted-pairs inside a standard CAT-5 cable as simple unshielded twisted pairs for audio applications. If both the sending end and receiving end are fully balanced, you don't need to do anything else. You can try using the 4th pair as a ground conductor but even that is not normally necessary.

On the other hand: if either the sending end or receiving end are un-balanced, you will need to play around to see what works for you. I would normally insert audio balancing transformers at the receiving end - this works well even in the presence of significant common-mode noise.

I use CAT-5 cable to connect RTS Adam Intercom Panels from remote locations back to the Intercom Frame. A standard Adam analog port uses 3- pairs: balanced audio to the frame, balanced audio from the frame, RS-485 data for key-panel data. Even with cable runs that exceed 2000 feet, I find that I don't need a ground conductor between the Intercom frame and the key-panel. My concern, of course, is keeping the RS-485 data transceivers operating within their common-mode range. Just hasn't been a problem - ever.

In your situation (short cable run within a single room: I wouldn't expect any problems whatsoever. This assumes that you are talking about balanced audio signals. I'm making that assumption because you are talking about using standard mic cables with XLR connectors.