What's better, one conductor or two conductors for an RCA interconnect?

@tcotruvo, coaxial cable is also the cable of choice in almost any scientific application. My understanding is it is better for electrostatic noise rejection but there may be circumstances where shielded twisted pair is better for magnetic. I am going off memory here. It depends on the shape of the noise field. Primary reason for not using co-axial is it is difficult to work with and maintain its properties.

I would expect you would have to have pretty long runs before cable capacitance became important. I am sure there are some bad tube products that could have issues.

A French audio journalist named Jean Hiraga wrote the paper that proposed using 2 identical twisted wires with a shield that was only connected at one end as a way to assure the waveform being conducted was treated with the same impedance for both positive and negative polarities. This idea was picked up by Noel Lee and his team at Monster Cable and marketed as “Balanced Bandwidth”. They added the prescription that the cables were directional based on which end the shield was attached, and put arrows on the cable. This approach has become entrenched and is accepted as received wisdom by many cable manufacturers today…but not all. Prior to this, virtually all RCA interconnects were coaxial…meaning a center conductor was wrapped with a shield of braided copper separated by a foam dielectric. 
I’m not claiming to pontificate on the validity of this article of faith!

Interesting @crustycoot .  That journalist no doubt discovered what was already being used industrially and in the scientific community no doubt for decades prior.

Greetings 

Jumia consider moving this post to the Cable Forum.

Lots of knowledgeable people on this site to provide you with more input.

Joe Nies

My understanding is it is better for electrostatic noise rejection but there may be circumstances where shielded twisted pair is better for magnetic.

Twisted pair exposes both wires evenly to electric and magnetic fields causing identical induced noise currents that cancel. It is extremely efficient, as long as the pitch of the twist is much smaller than wavelength of the offending electrical noise. Shield works great at higher frequencies where, in spite of being non-magnetic, shields by means of skin effect (noise currents flow on the outside - shield). Shielded twisted pair is the best combination.

As for grounding the shield - I would always ground it at the source and never at the receiver end only. The reason for this is that induced common mode electrical noise (identical in both wires) normally would cancel, but since both wires have impedance and slightly different distributed capacitance to shield, grounding it at the receiver creates two lowpass filters at two different frequencies converting common mode to normal mode signal. We’re talking of few picofarad difference, but at very high frequencies it can makes a difference. Also, intuitively I wouldn’t drain electrical noise from the shield into more sensitive side (input) device.