RCA Design Brief

In a previous paper I covered several issues that create signal distortion in audio cables. The most demanding variables involve the TIME related distortions that the ear is most sensitive to. Consideration must be made during cable design to mitigate the TIME based issues through the audio band. The following paper is the journey through the design process to arrive at a satisfactory RCA and XLR cable design. I must stress ALL quality cable designers have to work with the exact same known variables to solve problems at audio. Every cable is a compromise of some sort as distortions can’t be eliminated.  ICONOCLAST has made the outlined design decisions to arrive at, what we think, is an industry leading design based on real measurements. 

SOUND DESIGNS CREATE SOUND PERFORMANCE™

RCA DESIGN BRIEF

1.0 Conductors
1.1 Copper Size
2.0 Dielectric Material(s)
3.0 Dielectric geometry
4.0 Shield material and design considerations
5.0 Jacket design and material considerations

The design process will start with the RCA cable as this provides the most pristine electromagnetic properties possible due to the seemingly simplistic design.  Once all is said and done it is “simple” looking. The more complex XLR will have to, somehow, match the RCA’s electromagnetic properties if it is to be an “equal” on measured attributes. If the RCA isn’t any good, I may as well start over again!

1. Conductors / RCA

There is a lot of mystery around copper. The grains, 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).

“…granular building blocks in copper can never fit together perfectly, but are rotated causing an unexpected level of misalignment and surface roughness. This behavior, 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 battle for material supremacy continues. However, what we tend to discount is that while the overall design of the tire we put on the car is important, the rest of the car has more to do with what that tire does than just the tire. We over spec the tire and vastly under spec the car. I’m intent on building the car, not the tire.

The decision to use copper is based on several factors, none of which were price. Copper offers the best material for affordable cables with a significant level of performance in more ideal 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;

ETPC (as good as what used to be OF grade)

OFE (differing process, but far from vastly lower impurities content)

UP OCC (what is often called long grain type, and again a differing process).

Cryo treatments (used to improve copper’s PHYSICAL properties)

Grain direction (music is AC. Which polarity do you like first and at what frequency?)

I don’t use wire “quality factor” as a design element since every contemporary draw science wire is of vastly better quality than ever. Sure, some processes are more $$$ but there is scant repeatable measurement that I can do other than conductivity, a passive resistive measure that will influence R, L and C. The conductor type is an option for the customer to listen to, only. There are differences. Belden just isn’t in the position to create a pet project to define what isn’t yet scientifically defined. That’s not our thing.

Belden offers the three fundamental copper grades; ETPC, OF and UP OCC, as they DO sound different in the exact same electromagnetic R, L and C referenced design. No changes other than the copper, so we know what the culprit is. What we don’t know, is WHY it is the culprit. Instead of making up a big old story, again, about the material, we don’t. It is what it is in use and we leave it that way.

What we don’t offer is what I can’t hear as a designer. Sorry, but I’ve yet to hear CRYO treatments, intended to improve the wire’s PHYSICAL strength or grain direction, change the sound. As far as grain direction goes, you can flip the leads in any direction you want, as the wire’s grains all go the same way due to the manufacturing process that we use. If you can hear the direction switch, flip them any way you like. We won’t send you a bill for that!

Any material used in a superior design SHOULD sound as good as it can, and cost isn’t a direct line to better sound. I ignored cost when I designed ICONOCLAST™, either high or low. If my system didn’t allow me to hear it, I didn’t use it (materials) or do it (process / design).

This isn’t a paper on conductors, although I may have some things to say about alternatives to copper them later on in another paper based on some measurements and calculations I’ve done. We’re talking copper in this paper as it is the very best economical solution that we have right now.

Copper has a very low DCR, 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. It is a trait of the draw science, but does not have as much effect on the  sound as you would be lead to believe.

Use solid or stranded wire?  This, at least, is easy. Is stranded better for the way the cable is used? Is stranded more, or less, expensive? Is stranded easier or harder to process? Is the termination of the cable better or worse with stranded wire versus solid? Are any gremlins that I call tertiary variables (stuff there isn’t a measurement or calculation for) removed if the truly measureable variables are accounted for between stranded and solid?

ANSWER – Solid wire wins hands down for this application. Every question is in solid wire’s court. End use, costs less, processing cost, ease of termination and lack of tertiary elements (all those diode effect “arguments” between strands and more).

On that, though, a note: the first generation of Iconoclast interconnects use single solid wires for the signal-carrying conductors and that’s what’s discussed in this paper.  Our second generation product (suitable for analog but not for digital due to impedance issues) uses a star-quad arrangement of four separate wires, placed around a separator, in place of each of these conductors for improved inductance; for details see the fourth paper in this series.  Other than this change in the signal conductors, the “Gen 1” and “Gen 2” interconnects are the same.

1.1       Copper Size / RCA

We now have SOLID copper wire. The size selected sets the foundation for the whole thing if we consider that 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. 

You can’t use a conductor you can’t process. 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 (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. At RF, we use it to tell us how much copper to put over a STEEL support structure to maximize RF attenuation. Audio is not RF, and the ENTIRE wire is used to move the signal and at ALL frequencies concurrently, not the same issue at all in RF cable design.

RCA cables terminate into a theoretically infinite (47K-120K or there about) input resistor. We say impedance, but it is really as resistive as it can be made at the input op-amp level. Yes, purists will point out that input impedance DROPS some at higher frequencies.

If the impedance is so high and the current is so low (it looks like an open circuit) just use as small a wire as you can! Well, yes and no. It has to be reliably terminated and secure in the end product, and it has to process evenly under tension and not fracture from surface issues.

A review of the end of process design backs into the initial design requirement. Calculations and testing selected a 0.0176” diameter wire for ICONOCLAST. The process has to handle less than 4-3/8 pound tension to avoid permanent wire stretching. Wire was tested for the process requirement.

The 0.0176” diameter wire (0.0088” radius) is one half the diameters necessary for one full 18-mil skin depth at audio, so we have significantly improved current coherence through the wire @ 0.0176” diameter wire. Skin depth is FREQUENCY driven for a given material. The smaller the wire the larger the inner current magnitude will be relative to the surface current. We want as good a shot of that as we can get.

The RCA cable’s loop DCR will essentially be the center conductor in an RCA, if it is made right, and ICONOCLAST is. The center wire governs attenuation. The outer conductor is, in theory, infinitely low impedance so it nearly drops out of the loop DCR calculation and leaves the center wire.  The length of the cable relative to the input impedance allows a SMALL wire at audio. At least attenuation works in our favor at audio as it is a LOG relationship and gets really high very quickly as you go up in frequency. For audio, we can relax a bit on attenuation as it is low for the lengths we use and is in the right frequency range to stay low. Attenuation is a passive “distortion” and is VERY hard to hear over TIME based distortions.

• Dielectric Material(s)

We’ve already made a critical choice in our cable. The wire material and size. We’ve used good engineering practice to KNOW what the decision will yield. Now, how to RETAIN all that the material / size wire can provide? That’s easy, just stick it in air and find an infinitely low ground potential for our unbalanced / single ended wire!

OK, this IDEA is easy. The execution isn’t. I don’t care about speed of the process and / or costs as I’ve used REASONABLY affordable material as my conductor. We can always go back and break the bank on conductor materials. AIR is free, but expensive to get. Air is by far the best dielectric to have, and especially nearest the wire were the influences are the worst on group delay. The closer to the wire the dielectric is, the more it influences the overall velocity of the composite structure (wire / beading/ then plastic tube thickness / then braid)

I decided to go the tough route and use air. We can use RF as a HINT at what to do overall. We have used designs called semi-solid core dielectric RF cables. These partially suspend a wire in a tube with a spirally wrapped thread. The problem is that the wire SIZE and the core tube properties aren’t suitable for audio frequencies. Even the choice of materials isn’t as important at RF as we can reach a set impedance vector (real + the reactive inductive and capacitive parts all added together) by tweaking the thread and tube dimensions.

3.0 Dielectric geometry

The audio signal is very sensitive to the dielectric effects of the plastics near it. I chose a specially made beading thread to get the job done.

The above picture beading around the wire is a glass thread coated in pure TEFLON®.  I use a ROUND beading shape versus square, as it touches the wire at the tangent points for the very LEAST effect nearest the wire. The electromagnetic field sees the entire cross section of the plastics and material between the wire and the inner braid, so I use GLASS thread inside the beading as it is a good dielectric, too. Why is the glass there? A solid TEFLON® bead can’t be processes at this size and keep consistent dimensional linearity. The glass is the true STRENGTH member in the beading, not the plastic. The plastic is to set and hold the shape. The glass lets me process the beading at production speeds.

Why TEFLON®, really? OK, I’ll tell you. It has the lowest dielectric constant of any solid plastic. It is TOUGH in thin walls for end product dynamic stability; the bead should STAY round under side-wall pressure. This is a SMALL bead, so I need that toughness. TEFLON® has high T and E’s (tensile and elongation) properties for process toughness. We don’t have much process room, as I’ve calculated backwards how big this bead would need to be in this design and wire size.

How big should the conductor be based on a tube ID? There is ONLY one optimum asymptotic wire size driven MAX AIR volume (%) based on the tube ID. The ratio of the tube ID with the 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.

Here is what happens when we CHANGE the wire size;

Tube ID (IN)	Wire Size (IN)	Air %
0.070	0.014	80
0.098	0.020	80
0.123	0.022	80
0.150	0.030	80

As the wire gets bigger or smaller inside a given tube ID, it crowds out the air. We COULD go drastically big in the ID of the tube and wire size (0.150” tube ID)…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 ID size we want the maximum amount of 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, and with the desired capacitance. As the tube ID gets larger, cap will drop but inductance will rise, and the opposite with a smaller tube ID. The design target is 11.5 pF/foot on the bulk cable to assembly capacitance would be 12.5 pF/foot.

Using too large a wire hurts frequency coherence so we pushed the wire size DOWN until inductance was moving off spec relative to capacitance. A balance was sought between wire size (coherence) and reactive variables (L and C).

I can do a quick check to see how I’m doing by applying a test ground over a ten foot sample. Using RF frequencies as a “constant” since the velocity has stabilized to an asymptotic maximum, we measure really high VP values, ~ 87%. This is good as it allows me to reference to end capacitance, too. I just treat the cable like an RF cable and work the capacitance backwards from the open – short Impedance; Z = 101670 / Cap * VP. This is about 104.6-ohms so capacitance calculates to 11.2 pF/foot versus a measured value of 11.19 pF/foot.

We know from the previous paper that Capacitance and Inductance are FLAT with frequency, and are actually measured at 1 KHz. Our 11.19 pF/foot bulk cable value is true at 20Hz-20KHz. Inductance is a low 0.15 uH/foot through the audio band as well.

Capacitance @ 1 MHz per ELP 423, Agilent E4980A Precision LCR Meter, Belden’s Cap/Ind Test Fixture

     Spec for Cap @ 1 MHz: 12.5 +/- 1 pF/ft

     PDB1610 B24 Cap @ 1 MHz: 11.1947 pF/ft

Characteristic Impedance per MIL-DTL-17H (ELP 142) using the included equation:

Char. Imp per ELP 142:  Imp =    101670/(C +VP)

       Spec for Impedance: 100 +/- 5 Ohms

       PDB1610 B24 Impedance: 104.631 Ohms

       SEMI-SOLID PDB1610 finished RCA “assembly”

CAP                12.25 pf/foot
IND                0.1450 μH/foot

Inductance isn’t as critical in high impedance leads as current, which is ride time limited by inductive reactance, which is near ZERO, but 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.

The FEP tube is critical to get right. Special processes are used to keep it on-sized and ROUND over the beaded center wire.

• Shield material and design considerations.

We have a core tube and know the electricals, so now what? The braid is much more important than people think, and for a different reason than people think. No, it isn’t shielding, either. True, a double 90%+ braid have 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 the shield reaches the 90 dB mark far more cheaply.

RF cables are “shielded” to RF noise and IMMUNE to low frequency nose (outside their pass band) because the shields have a low resistance to RF, measured as transfer impedance. This is sort of like low DCR at audio frequencies, but relates to how high frequencies work. Audio cables are not RF cables!

We need to look at how unbalanced circuits work. They SHARE a ground…or do they? They are SUPPOSED to SHARE a ground. They don’t. RCA unbalanced cables use the CHASSIS as a ground to the wall outlet or it is floating in some cases but the REFERENCE between the grounds is still there. In ALL cases, there is that pesky WIRE thing called the SHIELD between the ground points on every piece of RCA equipment you use. That wire has RESISTANCE and that resistance creates a ground potential difference so current starts to flow between the two end grounds. E=I*R, remember that? A VOLTAGE is impressed against the center wire and the magnitude of that voltage is the current times the resistance. We can CONTROL the “R” by using TWO 98% copper braids. This is $$$ to do, but it is the RIGHT thing to do.

No, those braids won’t shield MAGNETIC interference. The HUM you hear is more than likely ground loop current through the braids resistance called SIN; Shield Induced Noise. The lower the braid DCR is the better the SIN rejection. You need low permeability shield to block low frequency magnetic waves (anything below about 1 MHz starts to have a considerable B-field bent over E-field). Good audio RCA cables ARE NOT going to shield B-fields. They will shield E-fields and reduce SIN noise.

To shield magnetic B-fields a MAGNET needs to be able to STICK to the shield. This is an indicator that the material is “influencing” the magnetic field flux lines INTO the metal and OUT OF the air. We can manage the SIN noise with a good ground, but true extraneous magnetic noise is still tough with unbalanced cables. Now you know why. It’s the ground system it uses. 

• Jacket design and material considerations

ICONOCLAST uses an FEP jacket for some good reasons. 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 in contact with polyester or nylon carpet that would love to be the same color as your cable laying on it! My previous cables were.  FEP does not have this issue and will look nice for decades to come. Yes, it costs some 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 the problem, you’re good to go! RCA is a great sounding cable by fundamental electromagnetic design. This is why it was created. It does have magnetic noise immunity issues, though. There is no magic to good cables; it is adherence to strict design rules that also encompass those “magic” tertiary variables called wire science.  The same design adjusted for a new material’s skin depth properties can be made to the same “ratio” and match the electricals with differing wire.  The layers of the onion and their thickness can be altered (L and C values) depending on what is most audible. Tests won’t tell you that, this comes from design experience.  This does NOT mean that either L or C can be thrown to the wind.  Both L and C cause TIME based distortions and neither is welcome in good cable.

Then there is the next cable I’m going to talk about that does exactly that, except it is far, far harder to make as good as an RCA electrically. It is called the XLR cable.