The discovery of electricity has dramatically changed our lives, with the wonders of modern science surrounding us everywhere—the light bulb, the electric motor, radio waves. Who would want to be without them? Audio engineers, for one, because all of the above are potential sources of audio pollution that can affect our pristine signal, as it travels between source and destination. Refrigerator motors, light dimmers, radio transmitters, Wi-Fi, etc., can all cause unwanted noise in audio. Of course, the proposition is absurd, since without electricity there wouldn’t be audio engineers and cables, to begin with. However, the point is that what benefits us one way can easily be detrimental in another situation.
There are multiple points at which a signal can degrade in an audio system, but often the first culprit is a microphone cable. Why the microphone cable? What’s the big deal? Attach a microphone to one end and plug the other end into a preamp and you have sound, right? Yes, except that the cheap cable you just bought at the discount store is providing your vocal with an accompaniment featuring a local taxi dispatcher’s instructions, along with some nice 60-cycle hum. Might be what you want if you’re into avant-garde recording, but otherwise those are some serious problems. You run out to the local music store and buy a really expensive cable and suddenly all of those issues are gone—or perhaps not. So, did you spend your money wisely?
Despite cables being ubiquitous, information that helps the typical purchaser make competent decisions about a cable’s performance is hard to come by. Mostly, manufacturers tend to rely on marketing rather than hard facts to promote their cables. Low noise, oxygen-free copper, and gold-plated contacts are all notable features, but they don’t really tell you why a given feature will work, or in what scenario. And sometimes, unfortunately, the manufacturer misstates facts that only confuse. This article aims to dispel the hype, while examining what constitutes a good, balanced microphone cable.
A balanced microphone cable consists of three main components—the conductors (internal wires, or cores), which carry the signal; the shielding, which helps protect the integrity of the information passing through the conductors; and in the case of a microphone cable, three-pronged connectors (XLR) that allow connection of the cable at either end. All three need to work together to ensure proper functioning of the cable, especially with increased lengths, as distance provides additional opportunity to increase electromagnetic and other noise interference.
So, what is it in particular that makes a microphone cable susceptible to noise interference? Microphones generally operate at very low voltages, with the output requiring a relatively hefty amount of amplification applied to it before the signal can successfully travel on its way through an audio system, at line level. When the signal arrives at the preamp, any noise that has infiltrated the cable becomes greatly amplified along with the original signal. Using a good, balanced cable helps to eliminate the problem.
The solution starts with the signal generated by the microphone capsule being sent over two wires, with one side inverted by 180 degrees. A differential input amplifier at the receiving end re-inverts and then combines the two signals. Since any interference entering the cable during transmission will appear equally in both conductors, the induced noise is now inverted and therefore cancelled (phased) out, leaving the original signal intact. Additionally, the original signal is slightly strengthened by the recombination of the two sides, thus helping to compensate for any reduction in signal strength that occurs on a long cable run. However, it should be noted that any noise inherent in the signal, from say, a noisy microphone, will not benefit from this process.
Having defined the basic operation of a balanced cable, let’s look in detail at each component. Although it is not customary to give away the plot, let me state that the shield is the most important factor in a cable. But it makes sense to start the discussion from inside out, so let’s begin with the conductor, the part that carries the audio signal. Copper has been the material of choice for conductors for well over a century, because it has the greatest electrical conductivity of all non-precious metals. Normally, the stronger a metal is, the less pliable. Copper is unique in that it combines strength with high pliability. It is resistant to the effects of corrosion, which normally weaken other metals and impair their conductivity, and copper is easy to solder, allowing for durable connections.
Generic Balanced Cable
The use of oxygen-free copper in cables is contentious. There are “levels” of oxygen-free copper, with 99.99% OFC (Oxygen Free Copper) being the purest form. Certain audiophiles insist their cabling meets this highest standard of purity. However, in reality the difference between the grades is negligible for most audio applications. The purest copper is required only for extreme scientific situations such as the manufacture of semiconductors, or for use in particle accelerators, etc. Use of such copper in audio is expensive and unnecessary. For audio applications OF (Oxygen Free) and the most common ETP (Electrolytic-Tough-Pitch) copper will meet the needs of audio applications.
Audio cables generally use a stranded conductor as opposed to a solid cable. While the latter is cheaper to produce, a solid conductor is more rigid and prone to breaking when flexed. Obviously, microphone cables need flexibility, and a stranded conductor provides that. And if damage to a strand does occur, the whole conductor does not cease functioning. Also, in certain circumstances the stranded conductor can include a higher “volume” of copper than a solid one, aiding in the transmission of the signal. The downside is that termination of the stranded conductor is slightly more finicky, and if the strands aren’t wound tightly enough, the conductor will have a higher resistance, which in turn will require greater electrical energy to maintain the same flow.
As noted, a balanced cable needs at least two conductors (cores), with the majority of cables belonging to this category. Generally, the cores vary in thickness between 26- and 20-AWG (American Wire Gauge), with the smaller number being thicker in diameter. Here, the compromise is between a manageable overall thickness of the cable, as applied to its flexibility and level of conductivity. Some cables feature four cores (two pairs) and are often referred to as quad cables. Four-core cables are more expensive, but in extremely “noisy” environments or where cables run unavoidably parallel to power cables, they can provide at least 20 dB or more of common-mode noise reduction when compared to two-core cables. However, care should be exercised as not all these cables are constructed equally, with some cheaper varieties simply having four parallel cores wrapped in the cable jacket, which can introduce more problems than they solve. To be effective, the four cores need to be wound tightly around each other to achieve the additional noise cancellation that such a configuration provides.
Four Core Cable
Now let’s look at cable shielding. This is where a cable will stand up or fall down. You can have all the OF copper you want, with millions of strands, but if the signal traveling down the cable is compromised with noise, then it is next to useless. To this end, there are multiple layers of protection that address the basic forms of electrical interference, and we will discuss those as we travel through the various layers of a complete cable. It should be noted that not all cables have the same kinds and levels of protection, and most of that comes down to price.
The first order of things is to isolate the two (or four) cores from each other, and not just for the obvious reason that wires that touch will short each other out. Any current travelling down a wire will generate a magnetic field, and the resulting EMI (Electromagnetic Interference) introduces noise in an adjacent conductor. In this case a layer of insulation is applied over each core to contain the magnetic field. These days, polyethylene (PE) is used, since it provides better insulation over the PVC found in earlier cables. Although not a 100% insulator, the voltage levels present in an audio cable are small enough that they will not overwhelm the insulator. Some cables then add a layer of conductive (modified) PVC on top to do the opposite, that is drain off any EMI that might be trying to enter the conductors from outside.
The other form of protection that is common to all cables is a metal-based shield, usually copper but sometimes TAC (Tin Plated Copper), and in the case of foil, an aluminum laminate. This is the third leg of a balanced cable, and is connected to ground via the third pin of the XLR connector. A foil shield provides excellent coverage and is very effective when radio frequency (RF) interference is present. It is also lighter and cheaper to produce, but the down side is that it is far easier to damage when flexed, is more difficult to terminate, and should only be used in fixed installations, or as an absolute last resort.
|Copper Braid||TAC Braid|
The other type of shield is a braid, an interwoven mesh of bare or tinned copper wires, which is much easier to terminate. TAC braiding is more expensive than regular copper, but the tin coating aids in preventing oxidation of the copper and improves wear. The type of braiding also comes into play. Because copper possesses higher conductivity than aluminum and the braid has a larger mass, it is more effective as a shield. It doesn’t provide absolute coverage due to the unavoidable presence of small gaps in the braid, but given that for stationary cable 70% coverage will usually suffice, the 98% offered by the braid is more than sufficient. However, it will add bulk and cost to a cable.
Copper Spiral Braid
A spiral shield utilizes helical, wrapped strands for up to 97% coverage and provides excellent flexibility, but gaps in the weave can open up when the cable is flexed. Some manufacturers implement dual-spiral shielding where each core gets its own shield. This provides better cable flexibility while ensuring the integrity of the shield. For very noisy environments, combination shields consisting of more than one layer of shielding are employed. For broadcast and studio applications, a braided shield with conductive PVC is the best solution, whereas where flexibility is required, as in live-sound situations, dual-spiral with conductive PVC is the preferred answer.
Dual Copper Spiral Braid
Another issue is the presence of microphonic noise. This has nothing to do with the microphone, but is noise generated within the cable when it is flexed. A poorly designed or damaged cable allows friction to be created between the two conductive cores, resulting in static discharges which in turn are amplified by the high gain of a microphone preamp. To prevent such movement, cotton yarn is placed between the insulated conductors and the braid. Finally, the outer jacket has no effect in regard to noise, and while a heavier one will help protect the cable innards in high-traffic areas, heavier doesn’t necessarily mean better. It’s all a question of application.
The issue of frequency-response degradation in cables is sometimes brought up, and while there is a small potential for a cable to do this, the problem is largely immaterial except when dealing with extremely long cable lengths. It takes close to four hundred feet of cable to produce 1 dB of attenuation at 20 kHz (nominal), which in most live situations, is virtually inaudible.
And so we come to the connectors. XLR connectors are favored for two reasons. The first is the design of the female XLR connectors, which allows pin 1 (the earth pin) to connect before the other two pins, (which carry the signal) when the male XLR connector is inserted. With the ground established first, it is possible to insert or remove the connectors, especially in live-equipment situations, without picking up external signals. (TRS phone connectors mate the ground last, which can cause short bursts of hum, or cracks and pops.) The second advantage of the XLR connector is the external locking mechanism on the female connector that provides an additional level of security.
There are several different types of XLR connectors and methods of termination. Due to its modular design, the internal, four-part cable connector as developed by Neutrik allows for field serviceability in the event of a cold solder joint or other damage around the area of the connector. This design is also used on connectors from Rean and Attaché. Manufacturers of other types of XLR plugs include Switchcraft and Amphenol, along with various Far East companies that produce unbranded plugs. Molded connectors, (those that are permanently affixed to the cable ends) while cheaper, will involve greater cost down the road, if the wiring inside the connectors fails and the cable needs to be completely replaced.
Modular XLR Female Connector
Another question often asked is: which are better, gold- or silver-plated contacts? Gold is not as good as a conductor as silver but has a greater level of corrosion resistance, and many people prefer the look of the gold-plated connector. But unless the cables are to be used in harsh environments (close to sea water for example) the added cost is mostly unwarranted. Other considerations for a good XLR connector are what kind of chuck-type strain relief is provided, and the quality of the boot that covers the connector points to alleviate bending stress. The internal thread on the shell should be protected from damage, and some cable manufacturers include shrink-wrapped solder points for added protection and to meet broadcast specifications.
Neutrik Connector (Exploded View)
The “XX” connector from Neutrik is a redesign of the company’s older “X” design, generally considered the best, but more expensive choice for premium cables. It employs a strengthened grounding spring on the female connector for a more robust ground connection, while the female receiving sockets offer more points of contact with the male pins (eight points versus three), and a tighter tolerance of the pins and sockets for improved connectivity.
|Neutrik "X" Connector||Neutrik "XX" Connector|
In summation, the following points need to be considered. Use a cable that is suited to the application. In environments with high EMI and RFI (these days that could be almost anywhere, with the proliferation of wireless communications), a braid/conductive PVC combination should be considered. If the cable will be subjected to repeat flexing, a spiral shield might offer better performance than a braided shield. Again, foil shielding should be avoided since continuous flexing will damage the foil. Make sure the connector provides as effective shielding as the cable itself, and make sure that the equipment being connected is properly matched. A too-high output from a preamp can overdrive and distort a down-stream circuit, while a low-level signal will not make use of the full dynamic range of the complete system. Following this advice will ensure clean and problem-free audio.
|Amplifier||Electronic component or circuit for amplifying power, current, or voltage. In this context, a pre-amplifier takes microphone level signals and boosts them to line level.|
|Balanced||Equal distribution of weight, amount, etc. A balanced cable refers to the signal being sent across two conductors, with the phase of one side reversed.|
|Boot||Sheath-like protective covering. Some XLR connectors include a boot that slides over the internal connectors of the plug to provide additional protection.|
|Braided Shield||Multiple strands of material interwoven to form a shield to protect against radio interference.|
|Cable||Insulated electrical conductor, often in strands, or a combination of electrical conductors insulated from one another that transmits electrical information.|
|Capacitance||Ability of a body to store an electrical charge and eventually discharge it.|
|Capsule||Part of the microphone that responds to incoming sound-pressure waves and in turn outputs a corresponding voltage that is sent along the cable to a preamplifier.|
|Chuck||Device for centering and clamping an object. In this case, the cable where it enters the connector.|
|Common-Mode Rejection Ratio (CMRR)||A number that describes how well an input or output will reject noise, or how well "balanced" a balanced line is. A high CMRR is important in applications where the signal is represented by a small voltage fluctuation, as in microphone signal transmission over balanced lines.|
|Conductor||Substance, body, or device that readily conducts heat, electricity, sound. Refers to the cores contained in a cable.|
|Connector||Device for connecting one object to another. In this application, types include XLR female or male.|
|Copper||Malleable metallic element used in large quantities as an electrical conductor.|
|Core||Innermost part of a cable, also referred to as conductor.|
|Corrosion||Process in which a metal is changed by a chemical reaction, causing it to lose conductivity. Copper is relatively good at resisting corrosion, but even when it corrodes, the overall conductivity of the metal is not greatly reduced.|
|Differential Input Amplifier||Type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.|
|Electrolytic-Tough-Pitch (ETP)||Standard type of commercial wrought copper used in the production of electrical wire.|
|Electromagnetic Interference (EMI)||High or radio-frequency disturbance that affects an electrical circuit, due to either electromagnetic induction or electromagnetic radiation emitted from an external source, such as light dimmers or refrigerator motors.|
|Impedance||Description of a circuit’s resistance to a signal, as measured in ohms, thousands of ohms (Kilohms), or millions of ohms (megohms).|
|Line Level||Refers to the voltages used by audio devices such as mixers, signal processors, tape recorders, and DAWs.|
|Magnetic Field||Mathematical description of the magnetic influence of electric currents and magnetic materials. Produced by electric currents in wires.|
|Microphone Level||Very low level signal output from microphones, typically around 2 millivolts (2 thousandths of a volt).|
|Microphonic Noise||Phenomenon in which certain components in electronic devices transform mechanical vibrations into an undesirable electrical signal (noise).|
|Microphone Preamp||An electronic device used to amplify low-level signals. Commonly used to bring microphone outputs up to levels that subsequent equipment can utilize, such as mixing consoles, tape recorders, or Digital Audio Workstations (DAW).|
|Ohm||Unit of electrical resistance. Named after its founder Georg Ohm, Ohm's Law states that current varies in direct ratio to the wires' resistance.|
|Oxygen-Free Copper||Copper that has varying degrees of oxygen removed to increase its purity.|
|Polyethylene (PE)||Synthetic plastc material used in multiple applications, including electrical cable insulation.|
|PVC||Synthetic plastc material also used for electrical cable insulation.|
Generic term for balanced cables that use four conductors
|Radio Frequency (RF)||Interference from signals that are in the radio frequency range.|
|Resistance||The opposition that a substance offers to the flow of electric current. All electrical systems display a certain amount of resistance. The standard unit of resistance is the ohm.|
|Shielding||Various methods for preventing inteference entering a system.|
|Tin-Plated Copper (TAC)||Widely used in the electronics industry because of its ability to protect the copper from oxidation and preserve its solderability.|
|XLR||Standard three-pin connector on which pin 1 is typically connected to the shield of the cabling to provide a ground, whereas pins 2 and 3 carry the actual audio signal, normally in a balanced (out-of-phase) configuration.|