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


I have a somewhat nice RCA analogue interconnect with one conductor, referred to as a coaxial Cable I guess.   But I see higher end RCA cables with two conductors and ground wire. Which is better?

Is better detail provided when connections are made with two conductors? 

jumia

Your thinking is flawed @kijanki. You have an erroneous understanding of how noise induces (or does not) induce current into these circuits and hence have drawn incorrect conclusions about what the result will be. If the noise is flowing in a loop, through the speaker (or cartridge), it is not common mode noise, it is differential.

Let us change the problem to a phono cartridge and a pre-amp. By your understanding, even if one side of the phono amp input is grounded, the system is "differential", and hence will receive the full benefits of twisted pair wiring. That is wrong. I noted that @atmasphere who supported you above uses a true differential input on the phono inputs to his preamplifier (for improved noise rejection).

The flaw in your logic is that with common mode noise, the current does not (have to) flow through the turntable cartridge (or speaker as the case may be). A simple case is electrostatic coupling which capacitively couples a current onto both wires (common mode). No wire "loop" is required to induce current as the parasitic capacitors forms the circuit. However, since you have one side of the phono pre-amp "grounded" the current in that wire flows into the ground (or some portion of it depending on whether hard ground, resistor, or capacitor. The current in the other wire flows into the loading network, offset by whatever current did not flow to ground (from the other side) but also flowed into the loading network. The result is of course a noise voltage. Coax has superior electrostatic noise rejection compared to even shielded twisted pair. Without a proper fully differential system, there is no benefit of twisted pair.

For audio I would assume we can ignore RF antenna coupling onto both wires (common mode), or at least we can ignore the effects which could be similar to electrostatic coupling. If we include them, again, the single ended connection defeats any benefit of twisted pair wiring as above, while the twisted pair has less rejection than coaxial, even shielded.

For magnetic EMI coupling, there would need to be a differential current generated at the point of interference as a complete electrical loop, for practical purposes, would need to exist to have a magnetically induced current. A twisted pair does, obviously, reject magnetic coupling of EMI, but so does a coax (through arguably a different mechanism) and it does it rather well. In most practical cases, because the differential current from magnetic interference happens at the point of interference and not at the load/receiver, the balanced connection does not matter for magnetic interference. (note the experiment link I provided that showed in that case superior rejection of magnetic interference with coax).

So, @jumia, I go back to my original statement, still true, explained in great detail, apparently in agreement with others knowledgeable about the subject, that typically coax will be the superior cable for single ended connections.

 

@jumia , this is not touting my horn, but questioning the competence of many cable companies, but I expect few of them, based on the technical quality of their communication, have the understanding to either refute or agree with what I write. Some of them I am sure do have that understanding.

 

@jumia 

Is it better to connect the ground to each side of the RCA cable?

I think you mean the SHIELD, i.e. if there is one, is it better to connect it on both sides. No, it's best connected (to ground) on one side only.

 

But if you leave the ground on one side unconnected wouldn't that risk creating a noise problem because the unconnected wire within the interconnect rca cable becomes an antenna??? Where noise has nowhere to go except to negatively impact the two connectors?

@jumia

A shielded RCA cable (not coax) is 2 conductors, one for ground, and one for signal, plus a 3rd "conductor" which is the shield. The shield will extend the full length of the cable, but only connect on one end. The shield is effectively a Faraday cage preventing RF energy from getting into cables inside of it. Yes, the shield is potentially an antenna. However, if it is connected on only one end, any RF energy should be shunted harmlessly to ground.

 

If you connect the shield on both ends, and the RF energy generates a large current in the shield, then it could induce a difference in ground voltage which will (or could) show up in the signal on a single ended system. That would also require the stars to align such you had a huge amount of RF and that your audio system converts the RF into something audible. More likely is an electrostatic conduction path which the shield will provide some protection for, or not and/or a magnetic path for which it may provide protection or not. There is more complexity than this and it would take far too much space to put here so I found you a good link. (and RF is obviously both electric and magnetic field).

 

Most fields have a lot of lore, pass down through the ages, repeated, reinforced, and wrong. Just look at this hobby! Electrical Engineering is no different (and nor is physics). What works in one situation may not work in another, hence rules of thumb often cause bruised thumbs.

 

This particular link takes a very hard line on cable shielding (not system level noise). From a practical stand point as an end user, you cannot ignore system level noise in order to perfect shielding, but your takeaway should be that unless everything is designed properly (his examples of mil-spec and FAA), then there is no 1 correct way for every instance, only a correct way for your particular instance. I know most of this paper will likely be over your head, but I think perusing it will give you a feel for what the issues:

https://www.emcstandards.co.uk/cable-shield-grounded-at-one-end-only