Like it or not, here are the facts on RCA verses XLR…I have covered this in the tech literature so I keep no secrets;
XLR - Advantages are low noise as they use a simple common mode circuit to remove that external noise…in a perfect world. Each wire in the cable is NOT “identical” to ground. This will be seen in a test called CUB, Capacitance Unbalance. This defines how much noise the cable radiates through unbalanced loop area, and how much noise it can cancel. BOTH wires to ground need to be the exactly same. They aren’t. Close, yes, but not exact. The difference causes what is called COMMON MODE TO DIFFERENTIAL MODE NOISE CONVERSION. Yes, this makes the NOISE a part of the SIGNAL and also radiates the signal in the cable out of it like a transmit antenna. Good cables aren’t afraid to tell you the CUB spec, and that should be less than 3.0% unbalance.
On XLR and shields. If an XLR is “perfect” we don’t need a shield, yes? But they aren’t so we use a shield to REDUCE the ingress ~ 80 dB with a 95% braid shield and thus the CUB has a lower voltage to deal with that gets converted to a differential signal on the two wires, and is now part of the analog signal. We can improve an XLR with a shield if we design it well such that the shield isn’t screwing up the CUB worse than 3%. A geometrically sloppy shield distance to each wire along the cable isn’t going to be nice to CUB values.
Another trick is to use lower DCR star quads. Yes, this is far more complex to make identical as we have FOUR wires…or even sixteen in my case. But, brute force signal level pushes the noise down farther. Simple signal to noise.
The 4x4 lowers the signal wire DCR to be the same between RCA and XLR. The RCA is the signal wire, and the return shield is “zero” to a simple approximation. The XLR is THERE, and then BACK but we use TWO signal wires in parallel with a star quad so they are half the resistance so the “loop” is essentially the same DCR as the RCA. This is why I don’t make a non star quad XLR. I want to MATCH the RCA’s properties as an RCA is a good cable to match! More to it than that that I can’t get into here. Read the tech papers to see how cable REALLY works.
Even if you use a true differential circuit where there is NO GROUND, just the EMF potential between the two wires, each wire is a “hot” but opposite EMF, we will still be offset with the common mode noise conversion…same as an Ethernet cable. Ten volts on BOTH wires is 10-10 =0 volts. But 10 volts on one wire and nine volts on the other leaves 1 volt of noise from CUB.
The pro’s use XLR because the noise is “generally” ~ 65 dB (not infinity) with a good XLR. The CABLE SIGNAL quality is very good but can’t be as good as an RCA cable signal, here is why;
RCA - this tech uses a SINGLE signal wire to a ground. We don’t have to worry about common to differential noise conversion. The SIGNAL on the cable is better EXCEPT if…we have to worry about inductively coupled noise through the shield between components at differing ground potentials coupling noise to our signal. And, there is ALWAYS a ground difference and at best, it is the DCR of the cable shield. No better. There is always SOME ground current moving in the RCA shield. This is why you need to use a very good ground on RCA and NEVER use a XLR wired as an RCA as they are not designed to be single end optimized. E=I*R and we can control shield R to multiply with I and that reduces E.
XLR are the noise rejection champ as common mode to differential mode noise conversion isn’t as bad as shield current noise on long enough lengths. Short reach and RCA can and are excellent signal leads. Get long enough and XLR are better designed for that with lower wire DCR (if designed right) and noise cancellation.
Be aware that the CABLE is not the “system” with an XLR. The I/O has to be more carefully considered before we make a blanket statement on one cable is better than the other, The CABLE, or the SYSTEM? I can report how the cables work.
Leaving out the XLR’s “balanced” input and output devices is not a good idea. Those I/O circuits ALSO have balance issues and influence what you hear. They are part of the deal and have to be of high quality to make the channel an advantage on noise rejection. Unbalance can come from ANYWHERE in the chain.
The answer isn’t what you really want as it isn’t JUST the cable you are evaluating. Single ended systems are simpler, and cheaper and better with low shield DCR and reasonable lengths. My MC phone uses RCA, and at over thousands of times voltage gains (66 dB). The signal is about where the noise is reduced to in an XLR! ) 0.35 mV volts (0.00035 volts) takes at least a thousand fold gain to be useful.
XLR take better care of longer runs. So use each cable properly, and for heavens sake don’t use an XLR as an RCA. The shield isn’t low enough DCR to make an RCA work well. It will “work” but the ground noise is exponential to the ground current’s induced noise as it is a magnetic property that decreases with the squared law. You want a HEAVY and low DCR with RCA as that is their weakness…as is common to differential mode noise conversion impacts the XLR.
These aren’t rumors, it is just how the cables work. Better understanding on the fundamentals, and getting them right, allow all our mysterious tertiary elements to be on a better foundation.
Best,
Galen Gareis
iconoclast design engineer