Cables: RCA Interconnects

There is no magic to good cables; it is adherence to strict design rules that also encompass those “magic” tertiary variables.

In the three-part series, Cables: Time is of the Essence,  [ Part 1 Part 2 Part 3-Ed.]we talked about signal distortion in audio cables. The most demanding variables in cable design involve the time-related distortions the ear is most sensitive to.

Good cable design mitigates time-based issues through the audio band. This article covers the journey through the design process to arrive at a satisfactory design. I must stress all quality cable designers have to work with the exact same variables to solve problems at audio frequencies. Distortions can’t be totally eliminated, so every cable is a compromise of some sort.

RCA Design Brief

RCA cable provides the most pristine electromagnetic properties possible due to a seemingly simplistic design.

1) Conductors

There is a lot of mystery around copper. The grains, or the molecular arrangement of the crystals

themselves, were recently found to not be what we thought.

[https://phys.org/news/2017-07-fundamental-breakthrough-future-materials.html]

The web article on Phys.org “Fundamental breakthrough in the future of designing materials” says, “… granular building blocks in copper can never fit together perfectly, but are rotated causing an unexpected level of misalignment and surface roughness. This behaviour, which was previously undetected, applies to many materials beyond copper and will have important implications for how materials are used and designed in the future…”

The decision to use copper is based on several factors, none of which was price. Copper offers the best material for affordable cables, as well as a significant level of performance in superior electromagnetic designs. Far more expensive materials in lesser designs won’t work, and far more expensive materials in superior designs won’t work… for most of us anyway.

Copper is available in several process treatments and after-process treatments;:

  • Electrolytic tough pitch copper (ETPC)
  • Oxygen-free high thermal conductivity copper (OFHC)
  • Ultra-pure, Ohno continuous casting copper (UP-OCC). What is often called long-grain type
  • Cryo treatments. Used to improve copper’s physical properties only
  • Grain direction options. Music is AC. Which polarity would you like first, and at what frequency?

Belden offers the three fundamental copper grades; ETP, OF, and UP-OCC, as they sound different in the exact same electromagnetic R, L, and C referenced design. (Impendance, inductance, and capacitance.) We don’t know why they sound different, but they do. Belden does not offer options that cost more, but offer no benefits that I can hear as a designer. Sorry, but I’ve yet to hear cryo treatments (which are intended to improve the wire’s physical strength or grain direction) change the sound.

So why copper? It has a very low direct current resistance, a reasonably deep skin depth to manage current coherence, is pretty high in tensile strength for processing, and in most applications resists severe oxidation. The grain structure is clearly visible in form, but that alone is not what makes the different grades sound different. In fact, these traits from drawing the cable do not have as much effect on the sound as you would  be led to believe. Belden’s manufacturing process allows the cable to be used in any direction, as the wires grains all go the same way.

Solid or stranded wire? This, at least, is easy. Solid wire wins every consideration. It is better suited to the way audio cable is used. It is cheaper. It is easier to process. It allows for better terminations. It has fewer of the gremlins that I call tertiary variables (stuff there isn’t a measurement or calculation for).

Conductor Size

So we’ve now chosen solid copper wire. The size of this wire sets the foundation, since the cable’s structure is supposed to allow a conductor to be as near zero R, L, and C measurement cable as we can design.

For the RCA cable, we want as small a wire as we can process as this will force the best current coherence through the wire, i.e. the same current magnitude at all frequencies. The exact skin depth calculation is a tool we use to gain the knowledge to reduce the wire size in audio cables. Skin depth is covered in more detail in Part 2 of the overview. [https://www.psaudio.com/article/cables-time-is-of-the-essence-part-2/]

RCA cables terminate into a theoretically infinite (47K-120K or there about) input resistor. When impedance is so high and current is so low, it is effectively an open circuit. The most logical solution is to use as low a diameter of wire as possible. However, a wire that’s too small can’t be reliably terminated, might permanently stretch too easily, and may break if there are any surface imperfections.

Calculations and testing resulted in the selection of a 0.0176” diameter wire for Belden ICONOCLAST. This is half the diameter necessary for an 18-mil skin depth at audio, so there is greatly improved current coherence. (Remember from the overview that skin depth is dependent on frequency. The smaller the wire, the larger the inner-current magnitude relative to the surface current.)

2) Dielectric material and geometry

How do we retain all that our carefully selected conductor can provide? That’s easy, just stick it in air and find an infinitely low ground potential for our unbalanced / single ended wire! Air is free, and by far the best dielectric to have, but packaging air with a cable isn’t as easy or cheap.

How do we use air as a dielectric? I take inspiration from semi-solid-core-dielectric radio-frequency cables. These partially suspend a wire in a tube with a spirally wrapped thread. However, tweaking the design to suit audio frequencies is difficult. The audio signal is very sensitive to the dielectric effects of the plastics near it, so the beading thread that maintains the air gap in the dielectric tube has to be carefully chosen. The picture shows the beading around the wire, which is a glass thread coated in pure TEFLON®. [NOTE TO ED: The picture mentioned is on page 4 of RCA and XLR Design Brief REV8.pdf. It’s quite small, so the original file will be needed.] Why the glass? It isn’t possible to make a solid Teflon bead at this size, and maintain dimensional linearity. The strength of the glass also allows the beading to be processed at production speeds.

So why Teflon? First, it has the lowest dielectric constant of any solid plastic. Its high tensile and elongation properties mean it is strong and durable in even thin walls, and allows the bead to stay round even under side-wall pressure. Maintaining the dimensions is critical, as there isn’t a lot of room to play with in this tube, since there is only one optimum asymptotic wire size based on a given tube inner diameter.

The ratio of the tube inner diameter (with an 80% air void) to the inner braid surface will determine the capacitance. Maximizing the air content will improve the efficiency of the dielectric so the smallest loop area for inductance will also yield the smallest measured capacitance.

As the wire gets bigger inside a given tube, it crowds out the air. We could drastically increase tube inner diameter and wire size, but we want to hold inductance and signal coherence in check. Inductance is the loop area between the wire and the inner braid, and that needs to be infinitely close, the opposite of capacitance. For a given tube size, we want the maximum air void, and the smallest possible wire-to-braid distance.

This means the conductor wire size has to be as small as you can process, with the desired capacitance. As the tube gets larger, capacitance will drop but inductance will rise, and the opposite with a smaller tube. The design target is 11.5 pF/foot on the bulk cable, and assembly capacitance would be 12.5 pF/foot.

Inductance isn’t as critical as current in high-impedance cables, in which current is effectively ride time limited by inductive reactance. This is near zero. In my listening test, cable with near zero on both L and C attributes sounded best, and a balance needs to be considered. The cable isn’t big or small; it is what it needs to be to work. The wire size we start with sets this all into motion.

3) Shield material and design considerations

We have a core tube and know the electricals. Now what? The braid.

This is much more important than people think, and for differing reasons than people think. No, it isn’t shielding. True, a double 90%+ braid has 90 dB RF shield properties but, I sure hope your equipment isn’t that sensitive to RF. Foils are much better and more economical for RF than a single 80% braid, and reach the 90 dB mark far more cheaply.

Lets look at how unbalanced circuits work. They share a ground… or do they? They are supposed to share a ground, but they don’t. RCA unbalanced cables use the chassis as a ground to the wall outlet, or in some cases, is floating, but the reference between the grounds is still there.

In all cases, there is that pesky thing called the shield between the ground points on every piece of RCA equipment you use. Though that shield wire should drain away stray current, it has resistance of its own, and that resistance creates a ground potential difference. This causes current to flow between the two end grounds, creating a voltage in the center wire of the cable. The hum you hear as a result is called SIN, Shield Induced Noise. We can reduce this resistance in the shield by using two 98% copper braids. This is expensive, but the right thing to do. The lower the braid direct-current resistance, the better the SIN rejection.

However, these braids will not shield magnetic interference. You need a low-permeability shield to block low frequency magnetic waves (anything below about 1 MHz starts to have a considerable effect on the electric field). If an RCA cable is to shield magnetic fields, the shield would allow a magnet to stick to it. This is an indicator that the material is “influencing” magnetic field flux lines, both into the metal and out in the air. We can manage the SIN noise with a good ground, but true extraneous magnetic noise is still tough to manage with unbalanced cables.

4) Jacket design and material considerations

The Belden ICONOCLAST uses a Fluorinated Ethylene Propylene (FEP) jacket, for good reason. FEP is the most chemically inert material there is, protecting your cables from chemicals, and UV-exposure through those nice picture windows in your house. Lesser plastic material isn’t as stable, or inherently flame-retardant. Nor can many materials be used in thinner walls.

Plasticizer migration out of the cable, especially near heat, is a real issue. A polyester or nylon carpet can take on the color of your cable laying on it! FEP does not have this issue, and will look nice for decades to come. Yes, it costs more , but these cables are an investment into the future, and can follow your system several steps above where you may be now. Based on durability, stability, and inertness to solvents, FEP is the best choice for the long haul.

RCA Summary

Knowing that RCA cables aren’t as “shielded” at audio as we think, what can we do about that? If you don’t have an interference problem, you’re good to go! RCA is a great-sounding cable with good fundamental electromagnetic design. This is why it was created. It does have issues with magnetic noise immunity, though.

This is where XLR cables come in, except that they are far, far harder to make as good as an RCA electrically. We will examine the design of this balanced cable in the next part.