A guided tour of audio cables and their applications.No matter what type of audio system you use, cables are necessary to connect the components. Recording studios, live-sound rigs, guitar racks, DJ setups - without cables, none of these systems is much more than a stack of silent boxes. Cables are the highways that audio signals travel on; without them, no sound gets through.
Clearly, cables are essential, critical pieces of "gear" in any audio system. But because they are not as fun and sexy as microphones, preamps, and processors, nor as engaging and multifaceted as samplers, sequencers, and recording devices, cables often get overlooked or ignored. Consequently, cables remain something of a mystery for many people, even those who have invested thousands of dollars in them and rely on them daily.
QUALITY COUNTSIt is common knowledge that good-quality cables are essential for trouble-free system performance and that worn out or damaged cables can result in hums, crackles, and intermittent signals. But what some people might not realize is that an inferior cable - even if it's fully functional - can also have an adverse effect on the sound quality of a system. Poor-quality cables can filter signals, introduce noise, and cause distortion; in addition, they're more prone to crap out just when you need them most.
Worse still, the problem is a cumulative one. A single inferior cable might contribute only the slightest sonic compromise, but the tiny degradations introduced by several such cables can add up to a substantial loss of sonic quality. For this reason, it is vitally important to spend ample time and money selecting appropriate, high-quality cables for your system. (Some manufacturers provide inexpensive molded cables with their audio gear just to help you get up and running; but it's usually advisable to replace these with better-quality cables.) Not only do high-quality cables make a difference sonically, but their greater reliability increases your system's longevity as well.
Of course, sonic quality is a balancing act between all the components in a system. Poorly made cables can compromise your audio, but a poor-quality system might be incapable of registering the enhancements offered by premium cables. Don't be surprised if those $1,000 speaker cables don't improve the sound of your $300 studio monitors!
ANATOMY OF A CABLEThere are three main types of analog audio cables: microphone, line/guitar, and speaker. (To understand the distinction between analog and digital audio cables, see the sidebar "Appearances Can Be Deceiving.") Fig. 1 shows the basic parts of a line cable. The main component is the central conductor, which carries the audio signal. The conductor is covered by a nonconductive insulating material that is typically made of plastic or rubber.
A conductive shield encases the conductor and insulator core, protecting it from outside interference and also providing an electrical ground, which is necessary to transmit a signal. A nonconductive jacket surrounds all of this, providing structure and durability.
Audio cables typically use copper wire as a conductor because copper is inexpensive, pliable, and a good electrical conductor. However, copper oxidizes when exposed to air, forming copper oxide. This material is a poor conductor, which can cause a cheaply made cable or connection to degrade in performance over time.
Cables typically have hardware connectors fastened to the ends. Mic cables normally use XLR connectors, while line cables generally use 11/44-inch or RCA connectors. Speaker cables often terminate in bare wires, but they can also use banana plugs or spade lugs. Cables with high-quality connectors, including those made from superior materials, are always a good investment. Gold is a good choice for cable connectors not only because it is an excellent conductor, but also because it is soft and will deform slightly to fill in all the gaps within a socket and create the best possible connection.
SIZE MATTERSThe diameter of a wire is an important consideration in cable design because it affects the wire's ability to transmit audio effectively. The American Wire Gauge (AWG) protocol defines wire diameter on a scale where larger numbers indicate smaller diameters. For audio wire, typical gauges range from 4 AWG to 30 AWG. (See the table "Wire Gauge Diameters" for a list of wire gauges and their diameters.)
Here are some rules of thumb for estimating wire gauges. Solid-wire diameters change by:
a factor of 2 for every 6 gauges;a factor of 3 for every 10 gauges;a factor of 5 for every 14 gauges.
From this information and the "Wire Gauge Diameters" table, we can deduce that 18-gauge wire is approximately 40.2 thousandths of an inch (40.2 mils) in diameter, 12-gauge wire is 80.4 mils, and 6-gauge is 160.8 mils.
A wire's cross-sectional area can also be estimated with another simple rule of thumb: when you change the gauge by 3 AWG, the cross-sectional area changes by a factor of two. For example, 17-gauge wire has twice the cross-sectional area of 20-gauge wire. This means that a 2-wire strand of a given gauge is the equivalent of a single wire three gauges lower.
This is an important concept in cable geometry, which refers to the physical configuration of the wire or wires used to make the cable. For example, some cables use a single, solid wire as a conductor. However, this is uncommon for audio cable, because a solid wire would make the cable stiff and difficult to work with. Most audio cable uses stranded conductors in which many thin wires are woven together to form a single conductor. This way, the conductor can offer the benefits of a large cross-sectional area but with greater cable flexibility.
RAISE YOUR SHIELDSMany types of interference can affect an audio signal traveling along a wire. A common type is radio-frequency interference (RFI). If you've ever picked up a radio station in your system, you've experienced RFI. Typically, the culprit is a poorly shielded audio cable acting like an antenna.
RFI is a kind of electromagnetic interference (EMI) that is caused by radio-frequency transmissions. Other types of EMI include emissions from various sources, such as coils in motors, fluorescent lights, and rheostat lighting dimmers. Electrostatic (ES) noise is another potential source of problems in an audio system. Electrical sparks and static electricity are types of ES noise, either of which can cause artifacts and distortion in an audio signal.
Cable shielding helps protect audio from problems that are caused by RFI, EMI, and ES noise. In general, audio wire employs shielding between the conductor and the jacket to keep interference from reaching the conductor. For some types of EMI, however, cable shielding is not effective, and the only solution is to use balanced cables (see the sidebar "Balancing Act"). Shielding is essential for mic, guitar, and line cables. That's because the signals from these sources are very low in level and must be amplified - which in turn amplifies the interference to the point of being audible.
BRAZEN BRAIDSOne type of cable shielding consists of a braid of wire that completely encircles the conductor (see Fig. 1). Called braided shields, they offer excellent structural integrity, flexibility, and flex life, which is why they are commonly used in mic and guitar cables.
Braided shields typically cover 90 to 97 percent of the conductor. The higher the percentage of braid coverage, the more effective the protection. Cheap cables frequently have loosely wrapped braiding that covers a smaller percentage of the conductor. In this case, interference noise can still penetrate through the gaps in the braid and be picked up by the conductor; as the cable loosens with age, the problem only gets worse.
FOILED AGAINAnother type of cable shielding uses metal foil wrapped completely around the conductor (see Fig. 3). Called foil shields, these consist of aluminum foil laminated to a polyester film. Not only are foil shields lighter and less bulky than braided shields, but they are less expensive to make as well.
Foil shields provide 100 percent coverage of conductors, which improves protection from RFI, EMI, and ES noise. In addition, foil shields use a drain wire to aid in grounding electrostatic charges. This wire runs along the length of the foil and attaches to the connector at the output end, which leads these charges away from the destination of the signal. If you've ever cut open a cable with foil shielding and found one more wire than you expected, the extra one was the drain wire.
Foils shields have a shorter flex life than braids, so they are best suited for permanent cable installations. This makes foil-shielded cable a poor choice for any application that requires the cable to be moved, flexed, or otherwise handled regularly. Foil shielding is therefore not used for guitar and mic cables, because the foil would BREAK with use.
THE OLD ONE, TWO, FOURLine cables typically have either one or two conductors. Single-conductor cables (see Fig. 1) carry unbalanced audio signals from electronic instruments, studio equipment, guitars, consumer audio gear, and so forth, and they generally use tip-sleeve (TS) 11/44-inch or RCA connectors to accommodate the conductor and shield. Braided shields are typical on unbalanced cable, although foil shielding is available for some unbalanced studio connections. Instrument cables (also known as guitar cables) are often designed for greater durability than studio cables and may include reinforced braided shields and extra-thick jackets.
Balanced audio signals are often used in professional audio gear, and they require dual-conductor cables (see Fig. 3 and 4). They also require XLR or tip-ring-sleeve (TRS) 11/44-inch connectors (which look just like 11/44-inch stereo headphone connectors) to accommodate the two conductors plus the shield. These cables come in many design configurations, and the shield can be either a braid or a foil, depending on the intended application. The two conductors are usually twisted together inside the cable to maximize the benefits of balanced connections. Twisting the conductors exposes them equally to any external interference, thereby increasing the efficacy of the noise cancellation that is inherent in balanced transmission.
A variation of dual-conductor cable is quad cable, which uses four conductors arranged in pairs. This configuration uses two conductors, twisted together and attached at each end, to form a single conductor. The two pairs are then twisted together, making a tight winding of all four conductors. Quad cables maximize the effects of noise cancellation in balanced transmissions by exposing all four wires equally to any interference.
Mic cables use either two conductors or a quad configuration and are similar in construction to balanced line cable. The difference is that good-quality mic cable offers increased flexibility and durability as well as reduced handling noise. Because they will be moved around in potentially extreme RFI and EMI fields, mic cables usually employ extra shielding. In addition, mic-cable jackets are typically made of a material that can withstand extra abuse and a wide range of temperatures.
A HIGHER LEVELFor most audio applications, cable shielding is necessary to protect audio signals from EMI and ES noise. Speaker cables, however, do not need such shielding, because they carry signals at a much higher current level, which means that any EMI must be very strong in order to be apparent. Nevertheless, speaker cables should be kept as short as possible to minimize cable impedance (more in a moment).
Speaker cable consists of two conductors inside a jacket or molded covering (see Fig. 5). To handle the high-current signals, the conductors are usually larger in diameter than mic or line cables.
Speaker cables can terminate in bare wire or in a number of different connector types, including 11/44-inch and XLR connectors, banana plugs and spade lugs. Obviously, two of these types - 11/44-inch and XLR - are also used on line and mic cables, so care must be taken not to substitute mic or line cables for speaker cables with identical connectors. The cables may look the same on the outside, but they are quite different on the inside. For example, line cables use the shield as a second conductor. However, a speaker connection requires two conductors of equal electrical performance to work properly. Because the shield in a line cable is not designed for this application, using one in place of a proper speaker cable would result in poor performance from both the amplifier and the speaker. In extreme cases, this can result in equipment damage.
TRANSMISSION PROBLEMSA cable's job is to transmit an audio signal with little or no signal loss. As it turns out, though, the cable itself is a major contributor to problems in signal transmission. The conductors, shield, and insulators that make up a cable all contribute resistance, capacitance, and inductance, which affect the signal. This is a complicated subject, beyond the scope of this article, but here are some basic issues to think about.
All cables exhibit a certain amount of impedance, which is measured in ohms (represented by z, the Greek letter omega). Impedance is a measure of how much the cable impedes the flow of electrical current, and it's actually a combination of three electrical properties: resistance, capacitive reactance, and inductive reactance. Let's take a closer look at all three, starting with resistance, which is the tendency to resist the flow of a direct current. (Of course, audio signals manifest as an alternating current, but resistance is part of a cable's total impedance, so it must be considered.)
Resistance. In simple terms, the larger the diameter of the cable or conductor, the lower the resistance, which is often expressed in ohms per unit length. (See the table "Copper Wire Resistance" for the relationship between gauge and resistance in copper wire.)
A handy rule of thumb is that 500 feet of 16-gauge wire has a resistance of about 4 ohms. Every change of three gauges doubles or halves the resistance, because such a change also doubles or halves the cross-sectional area of the cable. This means that 500 feet of 13-gauge cable has a resistance of about 2 ohms and 19-gauge of about 8 ohms.
In line-level and mic-level connections, cable resistance isn't much of a concern; with speaker cables, however, it becomes more of an issue. Because speakers exhibit input impedances in the range of 2 to 8 ohms, the resistance of the cable can add significantly to the overall load. For example, if a 4-ohm speaker is connected to an amplifier with a cable that exhibits a 4-ohm resistance, the cable will dissipate half of the amp's power before it even gets to the speaker!
Most good-quality speaker cable is of sufficient gauge (12 to 18 AWG) to work at moderate lengths (less than 100 feet); beyond that, however, the cable resistance becomes an important consideration. At 100 feet, for example, 18-gauge cable has an impedance of 1.3 ohms, which could mean a significant power loss. Consider this issue carefully when selecting cable for long cable runs.
Reactance. In line and mic cables, capacitance is more of an issue than resistance. A capacitor is an electrical component consisting of two conductors separated by a space that is often filled with an insulating material. This is precisely how shielded cables are constructed: an inner conductor surrounded by an insulator and a shield. The entire cable is one giant capacitor that resists the flow of an AC current by virtue of its capacitive reactance, which is greatly influenced by the insulating material.
By itself, capacitive reactance diminishes as the frequency of the signal increases. However, when combined with the cable's resistance, it forms a lowpass filter. As the cable gets longer, the cutoff frequency of this lowpass filter drops, resulting in more attenuation. This is one reason it is advisable to avoid long cable runs.
Another property of all conductors is inductance. As an audio signal travels along a wire, it creates a magnetic field that changes along with the signal voltage. This process is called self-inductance, which impedes the signal by virtue of its inductive reactance. Inductive reactance diminishes as the frequency decreases, but it interacts with resistance and capacitive reactance in complex ways.
Like resistance, this is not a serious concern in line-level connections, because the current is low, resulting in a weak self-induced magnetic field. However, with high-current signals (such as the ones going to your speakers), the magnetic field can be much larger, making self-inductance a cause for concern. Fortunately, high-quality speaker cables can reduce self-inductance, using specific cable geometries to cancel the magnetic field, which in turn can improve the sound quality of your system.
A cable's magnetic field can also cause inductive reactance in nearby cables in addition to introducing EMI. Again, this is typically only a problem with speaker and power cables, which carry high-current signals. Therefore, you should avoid placing these cables near line and mic cables in your audio system, because doing so can result in an audible hum.
For similar reasons, avoid coiling up excess speaker cable in your studio. Coiling or wrapping the cable forms an inductance coil, in which each coil wrap produces a magnetic field that affects the signal on the other wraps. A better solution for managing excess cable length is to shape the cable into a loose "S" (or similar pattern) on the floor.
ROUGH HANDLINGAs you flex and move a cable, you may hear what is commonly called microphonic, or handling, noise. Poorly manufactured cables in which the components are loose in the jacket often exhibit this problem. In high-quality cables, tight construction and a good-quality insulator will reduce handling noise. Most good-quality audio cables also include some kind of filler - typically made of cotton, jute, or polyester - to prevent the components from shifting.
Handling noise is most problematic with guitar cables, mic cables, and any other cable that normally flexes or moves while in use. Therefore, it's very important to look for tight packing when selecting these kinds of cables. (Many retailers have samples with the ends cut open. If not, you can buy a foot or two of the cable and cut it up right there in the store.) Handling noise also becomes a problem when you use cables meant for other applications as guitar cables. For example, speaker cables and studio interconnect cables are not manufactured with the intention of being moved around while in use and therefore contain little or no filler.
CURRENT EVENTSAnother issue to take into account is the current-handling capability, or ampacity, of the cable. Again, this is mostly an issue for speaker cables and power cables, both of which need to carry high currents. These cables have a maximum current rating, measured in amperes (amps), that indicates what the cable can safely handle. The rating is based on the amperage a conductor can carry before melting either the conductor or the insulation. Therefore, when in doubt about ampacity, check with the manufacturer.
In general, the larger the conductor size, the greater the current-carrying capacity - that is, as long as the jacket is able to handle the heat. For example, 28-gauge wire can handle between 3 and 5 amps, while 4-gauge wire can carry between 125 and 180 amps. Of course, 4-gauge wire is overkill for most speaker applications; a typical speaker cable is 10 to 18 AWG. (Depending on how it's constructed, 18-gauge wire can handle around 10 to 20 amps.)
RELIEF IS IN SIGHTAs a cable hangs off a piece of gear, the connector joint bears the weight of the part of the cable that is not resting on the ground. Eventually, this can cause the internal conductor and braiding to fray and even BREAK. That's why strain relief is another important factor in cable construction.
When selecting cables, be sure there is good strain relief at the ends, usually in the form of a rubber sheath that extends into the connector. Molded cables offer only fair strain relief and cannot be inspected or fixed if a problem arises. Soldered connectors are generally more robust, because they include better strain relief and you can open them up for repair.
Another important consideration - especially for mic and guitar cables - is how well the cable can handle the strain of being pulled on. Inside good-quality cable, the same filler that reduces handling noise also helps relieve pulling strain, taking the load off the connector. With inexpensive cable, however, the strain is borne solely by the solder joint between the conductor and the connector, which can cause the cable to fail after only one or two unfortunate tugs (or overly enthusiastic stage dives). In either case, the trick is to pull on the connector, not on the cable itself.
UPPING THE ANTESo far, I have covered the basics of cable design and described some ways in which cables can affect performance. Fortunately, most reasonably priced, well-made cables address these problems to a high degree. But for those seeking maximum audio performance, many types of so-called "exotic" cables are also available - typically at exorbitant prices.
Is exotic cable really worth the added expense? The answer depends on several variables, including whom you ask, how good your ears are, the listening environment, and the quality of the other components in the system. As suggested earlier, it makes little sense to buy the most expensive cables available if the rest of your system is not of similar quality.
Exotic cable designs can include unusual geometries, such as square or flat conductors, to reduce cable capacitance and/or inductance. For speaker cables, some manufacturers minimize self-inductance by using multiple insulated wires to form each conductor. And capacitance is often reduced in line cables with expensive insulators and jackets. Also common is the use of silver, gold, and other expensive materials in conductors and connectors to improve performance.
These kinds of tweaks can cost some serious money, resulting in remarkably high-priced cables. But in the right sonic environment with the right gear, they really can provide audible improvements, albeit subtle ones. Whether those improvements are worth the price depends on many factors, starting with your budget and ending with simple common sense. If you are trying to decide whether to spend your inheritance on a pair of speaker cables or a new Jeep, you're probably overdoing it!
THE BIG RUNAROUNDSo, what is the best way to go about upgrading your audio system with better cables? Well, short of simply replacing every cable with the best that money can buy - a costly proposition - one approach is to replace a few key cables and determine for yourself if there's an improvement. For example, you might try purchasing a premium-quality mic cable and a similar-quality line cable for connecting a mic and preamp directly to a recorder. Then, record a source using both sets of cables - the originals and the new ones - and compare the results. If you hear an improvement, you must decide if it's worth the money to replace the rest of your cables.
In the meantime, you can do several things to maximize the performance of the cables presently in your system. For starters, be neat with cable runs, not only to keep a tidy house, but also so that your cables don't cause problems with one another. Specifically, be sure that analog audio cables, digital audio cables, speaker cables, and power cables are separated. It is especially important to keep speaker and power cables separate from audio cables, because they can cause hum and signal degradation. If you have to cross audio and speaker cables (or power cables), do so at a right angle; however, do this only if you can't find a better solution.
It's also important to clean your connectors on a regular basis. Connectors left sitting in jacks for extended periods will build up dirt and oxidation, which degrade the connection. Regular plugging and unplugging helps connectors to self-clean to some extent. In addition, you should periodically use Tweak, Cramolin, or a similar product to directly clean contacts and jacks.
Make every effort to avoid excessive cable runs. Because longer cables have more resistance and capacitance and are more prone to interference, keep your cables as short as possible. Use the most appropriate length for a given cable run - even if means you have to cut and solder the cable. Also, be sure that all permanent cables have good strain relief, and wherever possible, affix cables to a support to take the weight off the connectors. Finally, label all the cables in your studio. Months after you have wired everything up and can't remember what goes where, you'll be glad you did!
Because many digital audio cables use the same connectors as analog cables do, it is tempting to use analog cables to make digital audio connections. But by all means, resist this temptation! Analog and digital cables have very different impedance requirements. In an analog cable, it is not unusual for the impedance to vary from 30 to 90 ohms at different points along the length of the cable. Such impedance fluctuations do not negatively affect the quality of an analog audio signal. However, they could have dire consequences for digital signals.
A digital audio signal is a pulse wave that travels at a very high frequency (around 3 MHz). In order for this signal to be transmitted accurately, the impedance of a digital cable must be matched to the sending and receiving devices, and the impedance must also be consistent along the entire length of the cable. For example, an AES/EBU cable must exhibit a consistent 110 ohms from end to end. This is why an AES/EBU cable is considerably more expensive than a seemingly identical mic cable.
So what happens if you use an analog audio cable in place of a digital audio cable? For one thing, the mismatched impedance can create standing waves and unwanted reflections of the signal in the cable, which in turn can "smear" the shape of the pulse wave. This smearing can also be caused by the cable's capacitance, which can reduce the high-frequency response of the cable and thus directly affect the rise time of the pulse wave.
The transitions between high and low voltages in the pulse wave define the 0s and 1s of a digital signal. If these transitions are smeared because of incorrect impedance and/or capacitance, the receiving device might interpret them as arriving either too early or too late. This timing error, known as jitter, can result in reduced audio quality and data errors. The bottom line is that you should use only digital audio cables to make digital audio connections.
Balanced connections allow noise-free cable runs of 100 feet or more, whereas unbalanced cables are prone to induced noise if they run more than 20 feet or so. An unbalanced connection uses a single-conductor cable with a shield for protection from interference (see Fig. 1)
Balanced connections use signal cancellation to remove interference picked up along the cable. This requires a cable with two conductors, plus a shield (see Fig. 3 and 4). The source device sends the signal on both central conductors, but on one of them, the signal is reversed in polarity with respect to the other. As the signal travels along the cable, both conductors pick up an equal amount of interference. At the receiving end, the destination device returns the inverted signal to its original polarity and combines it with the noninverted signal. The interference noise from each conductor is also combined, but the polarity of one of the noise signals is now reversed. This cancels out the noise. A balanced transmission effectively cancels most stray interference, allowing for cable runs of 100 feet and more.
One point to remember is that a balanced connection requires both ends to be balanced. That is, if you connect a balanced output to an unbalanced input, or vice versa, the connection is unbalanced, and no noise cancellation takes place.