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Cable and Interconnect Construction

Cable and Interconnect Construction

Cables and interconnects are composed of three main elements: the signal conductors, the dielectric, and the terminations. The conductors carry the audio signal; the dielectric is an insulating material between and around the conductors; and the terminations provide connection to audio equipment. These elements are formed into a physical structure called the cable’s geometry. Each of these elements—as well as the geometry—can affect the cable’s sonic characteristics.

Conductors

Conductors are usually made of copper or silver wire. In high- end cables, the copper’s purity is important. Copper is sometimes specified as containing some percentage of “pure” copper, with the rest impurities. For example, a certain copper may be 99.997% pure, meaning it has three-thousandths of one percent impurities. These impurities are usually iron, sulfur, antimony, aluminum, and arsenic. Higher-purity copper—99.99997% pure—is called “six nines” copper. Many believe that the purer the copper, the better the sound. Some copper is referred to as OFC, or Oxygen-Free Copper. This is copper from which the oxygen molecules have been removed. It is more proper to call this “oxygen-reduced” copper because it is impossible to remove all the oxygen. In practice, OFC has about 50ppm (parts per million) of oxygen compared to 250ppm of oxygen for normal copper. Reducing the oxygen content retards the formation of copper oxides in the conductor, which can interrupt the copper’s physical structure and degrade sound quality.

Another term associated with copper is LC, or Linear Crystal, which describes the copper’s structure. Drawn copper has a grain structure that can be thought of as tiny discontinuities in the copper. The signal can be adversely affected by traversing these grains; the grain boundary can act as a tiny circuit, with capacitance, inductance, and a diode effect. Standard copper has about 1500 grains per foot; LC copper has about 70 grains per foot. Fig. 1 shows the grain structure in copper having 400 grains per foot (illustration on the right). Note that the copper isn’t isotropic; it looks decidedly different in one direction than the other. All copper made into thin wires exhibits the chevron structure shown in the illustration of Fig.1. This chevron structure may explain why some cables sound different when reversed.

Conductors are made by casting a thick rod, then drawing the copper into a smaller gauge. Another technique—which is rare and expensive—is called “as-cast.” This method casts the copper into the final size without the need for drawing.

The highest-quality technique for drawing copper is called “Ohno Continuous Casting” or OCC. OCC copper has one grain in about 700 feet—far less than even LC copper. The audio signal travels through a continuous conductor instead of traversing grain boundaries. Because OCC is a process that can be performed on any purity of copper, not all OCC copper is equal.

The other primary—but less prevalent—conductor material is silver. Silver cables and interconnects are obviously much more expensive to manufacture than copper ones, but silver has some advantages. Although silver’s conductivity is only slightly higher than that of copper, silver oxides are less of a problem for audio signals than are copper oxides. Silver conductors are made using the same drawing techniques used in making copper conductors.

 

The Dielectric

The dielectric is the material surrounding the conductors, and is what gives cables and interconnects some of their bulk. The dielectric material has a large effect on the cable’s sound; comparisons of identical conductors and geometry, but with different dielectric materials, demonstrate the dielectric’s importance.

Dielectric materials absorb energy, a phenomenon called dielectric absorption. A capacitor works in the same way: a dielectric material between two charged plates stores energy. But in a cable, dielectric absorption can degrade the signal. The energy absorbed by the dielectric is released back into the cable slightly delayed in time—an undesirable condition.

Dielectric materials are chosen to minimize dielectric absorption. Less expensive cables and interconnects use plastic or PVC for the dielectric. Better cables use polyethylene; the best cables are made with polypropylene or even Teflon dielectric. One manufacturer has developed a fibrous material that is mostly air (the best dielectric of all, except for a vacuum) to insulate the conductors within a cable. Other manufacturers inject air in the dielectric to create a foam with high air content. Just as different dielectric materials in capacitors sound different, so too do dielectrics in cables and interconnects.

Terminations

The terminations at the ends of cables and interconnects are part of the transmission path. We want a large surface contact between the cable’s plug and the component’s jack, and high contact pressure between them. RCA plugs will sometimes have a slit in the center pin to improve contact with the jack. This slit is effective only if the slit end of the plug is large enough to be compressed by insertion in the jack. Most high-quality RCA plugs are copper with some brass mixed in to add rigidity. This alloy is plated with nickel, then flashed with gold to prevent oxidation. On some plugs, the brass is directly plated with gold or silver or rhodium.

RCA plugs and loudspeaker cable terminations are soldered or welded to the conductors. Most manufacturers use solder with some silver content. Although solder is a poor conductor, the spade lugs are often crimped to the cable first, forming a “cold” weld that makes a gas-tight seal. In the best welding technique, resistance welding, a large current is pulsed through the point where the conductor meets the plug. The resistance causes a small spot to heat, melting the two metals. The melted metals merge into an alloy at the contact point, ensuring good signal transfer. With both welding and soldering, a strain relief inside the plug isolates the electrical contact from physical stress.

 

Geometry

How all of these elements are arranged constitutes the cable’s geometry. Some designers maintain that geometry is the most important factor in cable design—even more important than the conductor material and type.

An example of how a cable’s physical structure can affect its performance: simply twisting a pair of conductors around each other instead of running them side-by-side. Twisting the conductors greatly reduces capacitance and inductance in the cable. Think of the physical structure of two conductors running in parallel, and compare that to the schematic symbol for a capacitor, which is two parallel lines.

This is the grossest example; there are many fine points to cable design. I’ll describe some of them here, with the understanding that I’m presenting certain opinions on cable construction, not endorsing a particular method.

Cable and Interconnect Construction

Many designers agree that skin effect, and interaction between strands, are the greatest sources of sonic degradation in cables. In a cable with high skin effect, more high-frequency signal flows along the conductor’s surface, less through the conductor’s center. This occurs in both solid-core and stranded conductors (Fig.2). Skin effect changes the cable’s characteristics at different depths, causing different frequency ranges of the audio signal to be affected by the cable differently. The musical consequences of skin effect include loss of detail, reduced top-octave air, and truncated soundstage depth.

A technique for battling skin effect is litz construction, which simply means that each strand in a bundle is coated with an insulating material to prevent it from electrically contacting the strands around it. Each small strand within a litz arrangement will have virtually identical electrical properties, pushing skin-effect problems out of the audible range. Because litz strands are so small, many of them bundled together in a random arrangement are required to achieve a sufficient gauge to keep the resistance low.

A problem with stranded cable (if it isn’t litz construction) is a tendency for the signal to jump from strand to strand if the cable is twisted. One strand may be at the outside at a point in the cable, then be on the inside farther down the cable. Because of skin effect, the signal tends to stay toward the outside of the conductor, causing it to traverse strands. Each strand interface acts like a small circuit, with capacitance and a diode effect, much like the grain structure of copper.

Individual strands within a conductor bundle can also interact magnetically. Whenever current flows down a conductor, a magnetic field is set up around it. If the current is alternating, the magnetic field will fluctuate identically. This alternating magnetic field can induce a signal in adjacent conductors. Some cable geometries reduce magnetic interaction between strands by arranging them around a center dielectric, keeping them farther apart—one of many techniques used by cable designers to make better-sounding cables.

 

What to Listen For in Cables

Cables must be evaluated in the playback system in which they will be used. Not only is the sound of a cable partially system-dependent, but the sonic characteristics of a specific cable will work better musically in some systems than in others. Personal auditioning is the only way to evaluate cables and interconnects.

Fortunately, evaluating cables and interconnects is relatively simple; the levels are automatically matched between cables, and you don’t have to be concerned about absolute-polarity reversal. One pitfall, however, is that cables and interconnects need time to break in before they sound their best. Before break-in, a cable often sounds bright, hard, fatiguing, congested, and lacking in soundstage depth. These characteristics often disappear after several hours’ use, with days or weeks of use required for full break-in. You can’t be sure, however, if the cable is inherently bright-and hard-sounding, or if it just needs breaking-in. Note that break-in wears off over time. Even if a cable has had significant use, after a long period of not being used it may not sound its best until you’ve put music through it for a few days.

With those cautions in mind, you’re ready to evaluate cables and interconnects. Listen to the first interconnect for 15 minutes to half an hour, then replace it with the next candidate. One way of choosing between them is merely to ask yourself which interconnect allows you to enjoy the music more. The other method is to scrutinize what you’re hearing from each interconnect and catalog the strengths and weaknesses. You’ll often hear trade-offs between interconnects: one may have smoother treble and finer resolution than another, but less soundstage focus and transparency. Another common trade-off is between smoothness and resolution of detail: The smooth cable may lose some musical information, but the high-resolution cable can sound analytical and bright. Again, careful auditioning in your own system is the only way to select the right cables and interconnects. Keep in mind, however, that a better cable can sometimes reveal flaws in the rest of your system. You should also know that cables and interconnects sound better after they have “settled in” for a few days.

Cables and interconnects can add some annoying distortions to the music. I’ve listed the most common sonic problems of cables and interconnects.

Grainy and hashy treble: Many cables overlay the treble with a coarse texture. The sound is rough rather than smooth and liquid.

Bright and metallic treble: Cymbals sound like bursts of white noise rather than a brassy shimmer. They also tend to splash across the soundstage rather than sounding like compact images. Sibilants (s and sh sounds on vocals) are emphasized, making the treble sound spitty. It’s a bad sign if you suddenly notice more sibilance. The opposite condition is a dark and closed-in treble. The cable should sound open, airy, and extended in the treble without sounding overly bright, etched, or analytical.

Hard textures and lack of liquidity: Listen for a glassy glare on solo piano in the upper registers. Similarly, massed voices can sound glazed and hard rather than liquid and richly textured.

Listening fatigue: A poor cable will quickly cause listening fatigue. The symptoms of listening fatigue are a feeling of relief when the music is turned down or stopped, or an impulse to do something other than listen to music, or the feeling that your ears are tightening up. This last condition is absolutely the worst thing any audio component can do. If a cable or interconnect causes listening fatigue, avoid it no matter what its other attributes.

Lack of space and depth: Using a recording with lots of natural depth and ambiance, listen for how the cable affects soundstage depth and the sense of instruments hanging in three- dimensional space. Cables also influence the sense of image focus. Poor cables can also make the soundstage less transparent.

Low resolution: Some cables and interconnects sound smooth, but they obscure the music’s fine detail. Listen for low-level information and an instrument’s inner detail. The opposite of smoothness is a cable that’s “ruthlessly revealing” of every detail in the music, but in an unnatural way. Musical detail should be audible, but not hyped or exaggerated. The cable or interconnect should strike a balance between resolution of information and a sense of ease and smoothness.

Mushy bass or poor pitch definition: A poor-quality cable or interconnect can make the bass slow, mushy, and lacking in pitch definition. With such a cable, the bottom end is soggy and fat rather than taut and articulate. Low-frequency pitches are obscured, making the bass sound like a roar instead of being composed of individual notes.

Constricted dynamics: Listen for the cable or interconnect’s ability to portray the music’s dynamic structure, on both small and large scales. For example, a guitar string’s transient attack should be quick, with a dynamic edge. On a larger scale, orchestral climaxes should be powerful and have a sense of physical impact (if the rest of your system can portray this aspect of music).

I must reiterate that putting a highly colored cable or interconnect in your system to correct a problem in another component (a dark-sounding cable on a bright loudspeaker) isn’t the best solution. Instead, use the money you would have spent on new cables toward better loudspeakers—then go cable shopping. Cables and interconnects shouldn’t be Band-Aids; instead, cables should be the finishing touch to let the rest of your components perform at their highest level.

Excerpted and adapted from The Complete Guide to High-End Audio (fourth edition). Copyright © 1994–2013 by Robert Harley. hifibooks.com. To order call (800) 841-4741.

Robert Harley

By Robert Harley

My older brother Stephen introduced me to music when I was about 12 years old. Stephen was a prodigious musical talent (he went on to get a degree in Composition) who generously shared his records and passion for music with his little brother.

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