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 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.