from Robert Harley's book "The Complete Guide to High End Audio"
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-particularly 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.11-5 shows the grain structure in copper having 400 grains per foot. Note that the copper isn't isotropic; it looks decidedly different in one direction than the other. All copper made into thin wires exhibits a chevron structure, shown in the photograph of Fig.11-5. 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. High-quality terminations are essential to a good-sounding cable. 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, gold is plated directly to the brass. Other materials for RCA plugs and plating include silver and 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 poor conductor, the spade lugs are often crimped to the cable first, forming a "cold" weld that forms 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.
Most 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.11-6). 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. Litz strands push 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 of 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 within copper. Individual strands within a conductor bundle can also interact magnetically. Whenever current flows down a conductor, a magnetic field is set up around that conductor. If the current is an alternating-current audio signal, the magnetic field will fluctuate identically. This alternating magnetic field can induce a signal in adjacent conductors (see Appendix B), and thus degrade the sound. Some cable geometries reduce magnetic interaction between strands by arranging them around a center dielectric, which keeps them farther apart. These are just a few of the techniques used by cable designers to make better-sounding cables.
Some cables and interconnects are not merely pieces of wire, but contain electronic components. These cables are identified by "boxes" on one or both cable ends housing the resistors, inductors, and capacitors that form an electrical "network." Pioneered by Music Interface Technologies (MIT), these products are often called terminated cables (Fig.11-7).
According to MIT, some of the audio signal's voltage is stored in the cable's capacitance, and some of the signal's current is stored in the cable's inductance. The amount of energy stored in the cable varies with frequency, which is manifested as tonal and dynamic emphases at those frequencies. Moreover, the energy stored in the cable is released over time, rather than being delivered to the loudspeaker along with the rest of the signal. This frequency-dependent, non-linear energy storage distorts the size and shape of the soundstage, according to MIT.
Terminated cables are designed to prevent such energy storage, and to deliver all the signal to the loudspeaker with the correct phase (timing) relationship. MIT claims that terminated cables are essential to realizing correct bass weight, smooth tonal balance, and a full-sized soundstage with precise image focus. Because terminated cables form a low-pass filter (i.e., they don't start filtering until about 1MHz, well above the audioband), they are useful for connecting very-wide-bandwidth electronics. The Spectral brand of electronics, for example, has bandwidths as high as 3MHz-not because the designer wants to pass those frequencies, but to make the circuits behave better in the audioband. Terminated cables filter those very high frequencies, ensuring that loudspeakers are never fed energy in the megahertz range, and making the system more stable.
There's a lot of hype and just plain misinformation about cables and interconnects. Manufacturers sometimes feel the need to invent technical reasons for why their cables sound better than the competition's. In reality, cable design is largely a black art, with good designs emerging from trial and error (and careful listening). Although certain conductors, dielectrics, and geometries have specific sonic signatures, successful cable designs just can't be described in technical terms. This is why cables should never be chosen on the basis of technical descriptions and specifications.
Nonetheless, some cable and interconnect specifications should be considered in some circumstances. The three relevant specifications are resistance, inductance, and capacitance. (These terms are explained in detail in Appendix B.)
A cable or interconnect's resistance, more properly called DC series resistance, is a measure of how much it opposes the flow of current through it. The unit of resistance is the Ohm. The lower the number of ohms, the lower the cable or interconnect's resistance to current flow. In practice, cable resistance is measured in tenths of ohms. Resistance isn't usually a factor in interconnect performance (except in some of the new non-metallic types), but can affect some loudspeaker cables-particularly thin ones-because of their higher current-carrying requirements.
The sounds of interconnects and loudspeaker cables can be affected by inductance. It is generally thought that the lower the inductance, the better, particularly in loudspeaker cables. Some power amplifiers, however, need to see some inductance to keep them stable; many have an output inductor connected to the loudspeaker binding post (inside the chassis). When considering how much inductance the power amplifier sees, you must add the cable inductance to the loudspeaker's inductance.
Capacitance is an important characteristic of interconnects, particularly when long runs are used, or if the source component has a high output impedance. Interconnect capacitance is specified in picofarads (pF) per foot. What's important isn't the interconnect's intrinsic capacitance, but the total capacitance attached to the source component. For example, 5' of 500pF-per-foot interconnect has the same capacitance as 50' of 50pF-per-foot interconnect. High interconnect capacitance can cause treble rolloff and restricted dynamics. (A full technical discussion of interconnect capacitance is included in Appendix B.)
The interface between a power amplifier and a loudspeaker through a cable is a critical point in a playback system. Unlike interconnects, which carry low-level signals, loudspeaker cables carry much higher voltages and currents. Loudspeaker cables thus react more with the
components they are connected to.
Damping factor is an amplifier's ability to control the woofer's motion after the drive signal has ceased. For example, if you drive a loudspeaker with a bass-drum whack, the woofer's inertia and resonance in the enclosure will cause it to continue moving after the signal has died away. This is a form of distortion that alters the music signal's dynamic envelope. Fortunately, the power amplifier can control the motion; the degree of this control, or damping factor, is expressed as a simple number.
Damping factor is related to the amplifier's output impedance. The lower the output impedance, the higher the damping factor. When you connect a power amplifier and loudspeaker with cable, the cable's resistance decreases the amplifier's effective damping factor. For example, an amplifier's damping factor of 100 may be reduced to 40 by 20' of moderately resistive loudspeaker cable. The result is reduced tightness and control in the bass. Loudspeaker cables should therefore have low resistance and be as short as possible.
One of the most important issues is cable dressing-the way cables are positioned in your system. In an attempt to make their systems look tidy, some audiophiles bundle all the cables together with tie-wraps. But grouping AC cords, interconnects, and digital cables right next to each other can degrade your system's musical performance.
To understand why, consider how a moving-magnet phono cartridge works. The tiny magnets, suspended between coils of wire, are moved back and forth by the modulations in the record groove. The relative motion between the magnetic field and the coils induces (creates) electrical current flow through the coils. When this electrical current flow is amplified and converted into sound by the loudspeakers, we hear it as music.
The same sort of magnetic field around a cable in your rack can induce an unwanted signal in an adjacent cable. Audio signals flowing through a conductor create a magnetic field around that conductor. This magnetic field expands and collapses around the conductor at the same frequency as the audio signal. The magnetic field's expansion and collapse provide the relative motion between the magnetic field and a nearby conductor, inducing an electrical signal in any conductor within that field. This means that AC power cords, which carry 60Hz from the wall to your components, will induce a 60Hz noise in any cable that happens to be nearby. Similarly, a digital interconnect from a CD transport to a digital processor will radiate high-frequency noise (in the megahertz range) into analog interconnects, and even into AC cords. Although you don't hear this contamination as audible noise, it overlays the music with a grainy patina.
That's why the first rule of system setup is to keep AC cords away from interconnects and loudspeaker cables. If they must meet, AC cords and signal conductors (interconnects and speaker cables) should cross at right angles, not run parallel to each other. Crossing cables at right angles minimizes the amount of induced noise.
A simple trick for further reducing interaction between cables is to place 1"-square blocks of Styrofoam between AC cords, interconnects, or speaker cables where they cross. Use the blocks at every cable junction to separate interconnects, speaker cables, and AC cords from each other.
Putting Styrofoam blocks between interconnects at the back of your rack is more of a challenge than laying out cables on the floor. A little glue on the interconnect or power cord will hold the lightweight Styrofoam, provided you're not constantly moving interconnects. Rubber bands can also hold the blocks in place.
Even a small physical separation has a large effect. The "inverse square law" states that the strength of a magnetic field diminishes in proportion to the square of the distance. Consequently, you don't need large distances between cables to isolate them from one another-that's why the 1" Styrofoam blocks are so effective.
Next, avoid stacking two components on one rack shelf. The components will be better isolated from each other electrically, and get better ventilation, when each is given its own shelf. Also pay attention to the positioning of components within the rack. Don't put phono preamplifiers near digital sources (CD players, transports, digital processors), or close to a power amplifier. You can also avoid running a digital interconnect next to analog interconnects by thoughtful component positioning. Take special care with the interconnects that run from your turntable to your preamplifier's phono input; they carry extremely tiny signals that are easily contaminated by radiated noise.
Here are several other tricks to help you keep interconnects and cables from degrading your system's sound:
Because all wire degrades the signal passing through it, the less wire you have in your system, the better. Keep interconnects and loudspeaker cables short.
Keep left and right loudspeaker cables, and left and right interconnects, the same length.
If you have excess cable or interconnect, don't wind it into a neat loop behind the loudspeaker or equipment rack. This will make the cable more inductive and change its characteristics. Instead, drape the cable so it crosses other loops at right angles.
Periodically disconnect all interconnects and loudspeaker cables for cleaning. Oxide builds up on jacks and plugs, interfering with the electrical transfer. Use a contact cleaner (available at most high-end stores). It works. In fact, switching interconnects sometimes cleans the jacks, making the system sound better even though the interconnect may not be intrinsically better.
When connecting and disconnecting RCA plugs, always grip the plug, never the cable. Remember to push the tab when disconnecting XLR plugs.
Ensure tight connection of all RCA plugs, and particularly spade lugs on power amplifiers and loudspeakers. Get lots of contact surface area between the spade lug and post, then tighten down the binding posts.
Avoid sharp bends in cables and interconnects. |
