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Some technical articles for your enjoyment. These were written by the Pro Co engineering division to help everyone understand a little more about wire and cable construction, and to explain why the good stuff costs a little more. You can download all three articles which are in Adobe Acrobat format(.PDF). The PDF files also contain some graphs and pictures and as long as Pro Co is given credit for them, feel free to use them if you wish.
1) CABLE ANATOMY I: UNDERSTANDING THE MICROPHONE CABLE
What is impedance?
Impedance is the AC (alternating current) version of the DC (direct current) term resistance, which is the opposition to electron current flow in a circuit and is expressed in ohms. Impedance (often abbreviated as "Z") includes capactive reactance and inductive reactance in addition to simple DC resistance. Reactance depends upon the frequency of the signal flowing in the circuit. Capactive reactance increases as frequency decreases: inductive reactance increases as frequency increases. Because of this frequency dependence, impedance is not directly measurable with a multimeter as
DC resistance is.
What are the differences between high- and low-impedance microphones?
To answer this requires a little historical background. High-impedance microphones are capable of producing higher output voltages than low-impedance types. Until recently, "consumer" audio gear (small P.A. systems, home and semi-pro recording equipment, etc.) was always designed for high-Z mics because their relatively high output level required less amplification or gain. The lower output of low-Z mics required the equipment manufacturer to use input transformers in front of the mic preamplifiers to step up the strength of the signal, which substantially increased the cost of the circuitry. Hence, low-Z mics were rar outside of professional recording and broadcast studios. In these "big-budget" facilities, low impedance lines offered several big advantages. A high-Z mic's high source impedance (approximately 10,000 ohms) combines with the capactive shunt reactance of the mic cable to form a low-pass filter which progressively cuts high frequencies. The severity of the loss is determined primarily by the length and construction of the cable. (See "Understanding the Instrument Cable.") The low source impedance (less than 200 ohms) of low-Z microphones proportionally reduces the high-frequency loss. Equally important, the high load impedances demanded by high-Z lines are much more susceptible to various forms of interference than low-Z lines, especially high-frequency noise and radio. Both of these high-Z liabilities made cable runs longer than 15-20 feet a problem.
Isn't the use of balanced lines the biggest advantage of low-impedance microphones? What is a balanced line? Balanced lines are wonderful, but they are sometimes given credit for benefits that they are not actually responsible for. Balanced, unbalanced, low-impedance and high-impedance are all individual properties. Many people erroneously refer to anything with a 3-pin XLR-type connector as "low impedance" and assume it to be "balanced." Others call any line connecting two pieces of equipment with 1/4" phone jacks "high-Z." In reality, a lot of equipment has unbalanced inputs and outputs that are carried on XLR connectors, and there are even more low-Z lines on phone jacks. Medical instrumentation uses a lot of high-impedance balanced lines for sensors, and most line-level unbalanced outputs are very low-impedance. Electrical systems need a reference point for their voltages. Generally referred to as common or ground, although it may not be actually connected with the earth, this reference remains at "zero volts" while the "hot" signal voltage "swings" positive (above) and negative (below) it. This is referred to as an unbalanced configuration. Physically, the common may be a wire, a trace on a printed-circuit board, a metal chassisÑvirtually anything that conducts electricity. Ideally it is a perfect conductorÑthat is, it must have no resistance or impedance. In a cable connecting two pieces of equipment, the shield is used as signal common. As the complexity and size of the system is increased, the imperfect conductivity of the common (ground) conductor inevitably causes problems. Since it is made of a real material, it must have some resistance, which must (Ohm's Law says) cause voltage drop when current flows through it, which means it cannot be at a perfect "zero volts" at both ends. The larger the system and the greater the distances between the source and load, the less effective this unbalanced configuration becomes. The voltages of a balanced line are not referenced to the ground or common. Instead, the signal is carried on a pair of conductors with the signal applied to this pair differentially. The signals are electrical "mirror images" of each otherÑtheir levels are the same, but their polarities are opposite. In other words, as the applied signal "swings," one conductor will be negative with respect to the common, the other will be positive. These polarities alternate with the frequency of the signal, and the total signal level is the difference between the two individual voltages. For example, if one conductor is at +5 volts, the other will be at -5 volts, and the signal level is +5 volts minus -5 volts or 10 volts. If, for same reason, the two conductors were both at +5 volts simultaneously, the level would be +5 volts minus +5 volts, which is zero volts. Very tricky! Because of this differential signal transmission, two very valuable things happen when using balanced lines. First of all, each piece of equipment can have its circuitry referenced to its own common, because the interconnection of the equipment does not require that the commons are connected in order to move the signal around. This eliminates the major cause of a lot of noisy audio gremlins, ground loops. Secondly, because the signal is differentially transmitted and received, any common-mode interference signal superimposed on the signal in the line will be carried by both sides at identical level and polarity. In other words, if the line has +5 volts of external noise induced, both conductors will have +5 volts of noise on them. This equals a total interference level of +5 volts minus +5 volts or zero volts. The interference cancels itself. This is called common-mode rejection. There are several ways to balance lines. (Actually, the term "balanced" is very often used incorrectly to refer to lines that are actually floating. Properly speaking, a balanced line is one which has equal impedance from each side to ground. An unbalanced signal may be derived from it by using one side of the pair as "hot" and ground as common. A floating line has no reference to ground, and must have on side of the line tied to common to "unfloat" it.) The input transformers once required by low-Z mic preamps also provided a floating input as long as neither side of the transformer's primary winding was tied to common. This is where the "low-impedance-is-balanced" misconception began. The use of balanced lines was actually just a by-product of the requirement for a transformer to step up the low signal level. Using modern low-noise integrated-circuit design, a low-Z mic preamp can be clean, quiet, balanced and a lot cheaper to build without a transformer.
What are the basic parts of a high-Z microphone cable and what does each one do? A high impedance mic has many of the traits of an electric guitar, so the cable used for it is generally a coaxial instrument cable. The "hot" center conductor is insulated with a high-quality dielectric; shielded electrostatically to reduce handling noise and triboelectric effects; shielded with a braid, serve, or foil which is also used as the current return path for the signal; and jacketed for protection. This type of cable is discussed in depth in "Understanding the Instrument Cable."
What are the basic parts of a low-impedance microphone cable and what does each one do? The basic cable construction for low-Z mic or balanced line applications is the shielded twisted pair. It consists of two copper conductors which are insulated, twisted together (often with fillers), shielded with copper, and jacketed.
What gauge and stranding should the two conductors be?
The amount of copper in any electrical cable is usually dictated by the amount of current it has to carry, or by the tensile strength it requires to perform without breaking. If we take the worst-case situation, where the cable is used for a line-level (+24 dBm) 600-ohm circuit, the current is a negligible 13 milliamperes (that's 13 thousandths of an ampere). The power in such a circuit is 100 milliwatts, or one-tenth of a watt. The current produced by a typical 150-ohm microphone connected to a 1,000-ohm preamp input is less than 1 microamperes (that's 10 millionths of an ampere), with power of less than a microwatt. By these figures it is apparent that not much copper is required to actually move signals around, except in applications demanding extremely long cable runs. Many low-impedance mic cables use 24 AWG conductors with excellent performance, and most multipair "snake" cables have 24 AWG (7 strands of 32 AWG) conductors. Other things being equal, more individual strands in each conductor mean better longevity and flex life. Since singers using hand-held microphones can put a cable through several hours of tugging, twisting, straining and other abuse, these situations call for finer stranding and often larger conductors, sometimes as large as 18 or 20 AWG. However, the sonic properties of the cable may be compromised by using large conductors.
Why are the two conductors twisted together?
As previously explained, the interference-canceling common-mode rejection of the balanced line is based on the premise that the unwanted external noise is induced into both signal conductors equally. Minimizing the distance between the two conductors by twisting them together helps to equalize their reception of external interference and improve the common-mode rejection ratio
(CMRR) of the line. The two conductors also form a sort of "loop antenna" for stray magnetic fields. The farther apart the two conductors are the larger the "antenna" becomes, and the more interference it picks up from sources like transformers, fluorescent lighting ballasts, SCR-chopped AC lines to stage lighting, etc.
Minimizing the loop area of the cable helps to reduce the unwanted hum and buzz from this type of interference, which the cable's shield is almost totally ineffective against. The distance between the twists is called the lay of the pair. Shortening the lay (increasing the number of twists) improves its common-mode rejection, and also improves its flexibility. The typical pair lay in microphone cables is about 3/4-inch to 1-1/2 inches. Shortening the pair lay uses more wire and more machine time to produce the same overall finished length, so of course it increases the cost of the cable.
What is "star-quad" cable?
This four-conductor-shielded configuration can best be thought of as two twisted pairs twisted together. Using four small conductors in place of two large ones allows the loop area of the cable to be further reduced and its rejection of electromagnetic interference (EMI) is improved by a factor of ten (20 dB). This makes star-quad cable very popular for microphones and balanced lines used in applications such as television production, where huge amounts of power cable for lighting and camera equipment surround the performers.
Does star-quad actually sound better?
When used for low-impedance microphones, star-quad construction substantially reduces the inductive reactance of the cable. Inductance was previously mentioned in discussing impedance. An inductor can be thought of as a resistor whose resistance increases as frequency increases. Thus, series inductance has a ow-pass filter characteristic, progressively attenuating high frequencies. While parallel capacitance, the enemy of high-frequency response in high-impedance instrument cable, is largely insignificant in low-impedance applications, series inductance (expressed in microHenries, or uH) is not. The inductance of a round conductor is largely independent of its diameter or gauge, and is not directly proportional to its length, either. Parallel inductors behave like parallel resistors: paralleling two inductors of equal value doesn't double the inductance, it halves it. In cable construction, using two 25 AWG conductors connected in parallel to replace each of the conductors of a 22 AWG twisted pair will result in the same DC resistance, but approximately half the series inductance. This will result in improved high-frequency performance: better clarity without the need for equalization to boost the high end. Also of significance is skin effect, a phenomenon that causes current flow in a round conductor to be concentrated more to the surface of the conductor at higher frequencies, almost as if it were a hollow tube. This increases the apparent resistance of the conductor at high frequencies, and also brings significant phase shift.
What is phase shift?
Phase shift is a term describing the displacement of two signals in time. When we described the two sides of a balanced line as being of opposite polarity, we could have said that they are 180 degrees out of phase with each other. Each time an AC waveform completes a cycle from zero to positive peak to zero to negative peak and back to zero, it travels though 360 degrees (just like a circle). A simple 1 kHz (1,000 cycles per second) sine wave travels through this 360-degree rotation in one millisecond. If we consider its starting point to be zero, it will reach its positive peak one-quarter of a millisecond later, cross zero in another one-quarter of a millisecond, reach its negative peak a quarter-millisecond after that, and return to zero after a fourth quarter of a millisecond has elapsed. Thus, each quarter of a millisecond equals 90 degrees of phase difference. When two identical signals are in phase with one another, their zero crossings and peaks are the
same, and summing (combining) the two will double the amplitude of the signal. When they are 180 degrees out of phase, summing them will result in cancellation of both signals. This property is very straightforward when considering simple sine waves. Sine waves consist only of a single fundamental frequency and have no harmonics. Harmonics are multiples of the fundamental, and are the elements of which complex waveforms are composed. An excellent
example of complex waveforms is called music. The reason a middle C note on a piano sounds different from the same note played on a flute is because the two instruments generate different waveforms the harmonics of the piano are present in different amounts and have different attack and decay characteristics than the harmonics of the flute. When complex waveforms are traveling in a cable, it would be ideal if the amplitude and phase relationships they enter the cable with are the same as those they exit the cable with. When the
effects of phase shift alter those relationshipsÑwhen the upper harmonics that define the initial "pluck" of a string, for instance, are delayed with respect to the fundamental that forms the "body" of
the noteÑa sort of subtle "smearing" begins to occur, and the sense of immediacy and realism of the music is diminished.
How can phase shift be minimized?
The phase lag caused by skin effect is one radian (about 57.3 degrees) per skin depth, and the
effective skin depth of a conductor at a particular frequency is the same whether the conductor is
very large or very small in diameter. For instance, the skin depth of a copper wire at 20 kHz is
about .020 inches, while an 18 AWG conductor has a diameter of about .040 inches. This means
that at frequencies from DC to 20 kHz, the full cross-sectional area of the conductor is utilized.
Because the skin depth (.020") is never less than half the diameter of the conductor (.040"), there is
never more than one radian of phase shift present.
In short, star-quad cables seem to offer lower inductance and lower phase shift, both of which are
parameters that directly affect the clarity and coherence of high-frequency complex waveforms.
Their inherently superior noise-rejection also reduces intermodulation distortion, a type which is
particularly offensive because it produces "side-tones" not harmonically related to the fundamental.
While the improvement may not be as dramatic as changing the microphone, an increasing number
of audio professionals seem to be embracing the sonic benefits of star-quad construction.
What about the insulation used? Does it affect the sound?
Even though the effects of cable capacitance are much less than that encountered in high-impedance
applications, the use of low-loss, high-quality (low dielectric constant) insulation materials such as
polyethylene and polypropylene are still preferred, especially when long cable runs are necessary.
Because of the desire to keep cable diameter to approximately 1/4Ó, the insulation thickness of a
typical two-conductor microphone cable is generally about .020 inches, half that of a coaxial-type
instrument cable. Because of this relatively thin wall, soldering requires good heat control to prevent
melting. For very thin (.010") applications, cross-linked polyethylene insulation is sometimes used.
The cross-linking process (similar to that used in manufacturing heat-shrinkable tubing) greatly
reduces the problems of insulation meltdown and shrinkage during soldering.
Why does some cable have string-like fillers twisted with the conductors?
The primary use for fillers is to make the core of the cable round to eliminate convolution in the
finished cable. A twisted-pair is not round, and without fillers the finished cable will have an
undulating, "wavy" appearance unless a very thick jacket is applied, which will greatly affect its
flexibility and make it very difficult to strip. A good example of convolution is found in the various
thinly-jacketed twisted-pair cables used for pulling in conduit in permanent installations. Such cable
is designed for economy and easy termination and so is not required to be round, only flexible and
Fillers also help to stabilize the cables shape and strengthen it, allowing some of the tugging, twisting
and other stresses encountered to be absorbed by the filers rather than the conductors or shield.
Some special miniature cables used for the "tie-clip" lavalier microphones use conductors that are
literally copper strands wound around cores of synthetic kevlar fiber. This cable is less than 1/8-inch
in diameter, yet is enormously strong. (Unfortunately, it is also very difficult to terminate because of
the necessity of sorting out the unsolderable kevlar from the solderable copper strands.)
Why don't low-impedance cables require electrostatic shielding like high-impedance
The "noise-reducing" semiconductive tape wrap or conductive PVC layers used on coaxial cable
are used to "drain off" static electricity generated by the shield rubbing against the inner conductor
insulation. When the source impedance is very high, these static charges will be heard as "crackling"
noises as the cable is flexed and handled. A low source impedance has a damping effect on this type
of static generation which minimizes its effect. There are cables available which use conductive
textile or plastic shields for 100% coverage, with copper drain wires or very low-coverage copper
braid added for ease of termination and low DC resistance. While this type of construction is very
flexible, its shielding effectiveness suffers greatly as frequency increases, offering very little effect
above 10 kHz because of its low conductivity.
What about handling noise?
The triboelectric effect that causes impact-related "slapping" noise as the cable hits the stage or is
stepped upon during use is related to capacitance, specifically the change in capacitance that takes
place as the insulation or dielectric is deformed. This causes it to behave as a crude piezoelectric
transducer, a relative of an electret condenser microphone. Because such transducers are extremely
high-impedance sources, the drastic impedance mismatch presented by a low-impedance
microphone and its preamp or input transformer makes the extraneous noise generated by
triboelectric effects negligible except in cases involving very low-level signals. In low-impedance
applications, handling noise is best addressed by using soft, impact-absorbing insulation and jacket
materials in a very solid construction with ample fillers to insure that the cable retains its shape. Note
that it is totally invalid to evaluate the handling noise of a low-impedance mic cable without using a
resistive termination to simulate the microphone element. A cable with no termination essentially
presents an infinitely high source impedance, a situation that is beyond worst-case!
What special considerations should be given to shielding low-impedance cables?
Low-impedance microphone cables are shielded using the same basic methods as coaxial-type
instrument cables. Woven copper braid generally offers the best high-frequency shielding
performance and protection from radio-frequency interference (RFI). This is due to the very high
electrical conductivity of the braid, and to its low-inductance, self-shorting configuration. Its
disadvantages are primarily economic; it is the most expensive to manufacture and the hardest to
Spiral-wrapped copper serve shields are very inductive in nature, as they resemble a long coil of
wire when extended. This can compromise high-frequency shielding and is not recommended when
effective shielding above 100 kHz is required. Serve shields are relatively inexpensive and easy to
terminate, making them a popular choice for medium-quality cables.
Foil-shielded cable is very heavily used for permanent installation work and for portable multipair
"snake" cables. The extremely low cost, light weight and slim profile makes foil very advantageous in
applications involving pulling cable into conduit. In these cases the conduit (if metallic and properly
grounded) can greatly enhance the RFI and EMI shielding properties of the thin mylar/aluminum foil
generally used. The 100% coverage of the foil shield, which should be of great benefit at radio
frequencies, is somewhat compromised by the inductive nature of the copper drain wire typically
used for terminating it. At low frequencies, performance is hampered by the relatively low
conductivity of the foil/drain configuration. In applications involving repeated flexing and coiling, the
metallized mylar tape will begin to lose its aluminum particles, opening up gaps in the shielding. This
can be a particular problem with multipair cable used for touring systems, where the shield
breakdown may lead to increased crosstalk between channels and to annoying radio pickup
Does the use of 48-volt phantom power affect the performance of the shield?
The current typically drawn by a phantom-powered condensor microphone is generally limited by
6.81 kohm resistors, resulting in a current of less than 15 mA total. This is not a significant factor
unless the shield begins to break down mechanically due to use: tearing or fraying are possible,
which could create intermittant changes in shield resistance. This has lead a few professionals to
prefer the use of three-conductor microphone cables, with the common carried by a drain wire in
addition to the shield.
2) CABLE ANATOMY I: UNDERSTANDING THE SPEAKER CABLE
What are the main parts of a speaker cable and what does each one do?
Typically a speaker cable has two stranded copper conductors, covered with insulation, twisted
together with fillers and sheathed with an overall jacket.
How big should the conductors be?
The required size (or gauge) of the conductors depends on three factors: (1) the load impedance;
(2) the length of cable required; and (3) the amount of power loss that can be tolerated. Each of
these involves relationships between voltage (volts), resistance (ohms), current (amperes) and
power (watts). These relationships are defined with Ohm's Law.
The job of a speaker cable is to move a substantial amount of electrical current from the output of a
power amplifier to a speaker system. Current flow is measure in amperes. Unlike instrument and
microphone cables, which typically carry currents of only a few milliamperes (thousandths of an
ampere), the current required to drive a speaker is much higher; for instance, an 8-ohm speaker
driven with a 100-watt amplifier will pull about 3-1/2 amperes of current. By comparison, a
600-ohm input driven by a line-level output only pulls about 2 milliamps. The amplifier's output
voltage, divided by the load impedance (in ohms), determines the amount of current "pulled" by the
load. Resistance limits current flow, and decreasing it increases current flow. If the amplifier's output
voltage remains constant, it will deliver twice as much current to an 8-ohm load as it will to a
16-ohm load, and four times as much to a 4-ohm load. Halving the load impedance doubles the
load current. For instance, two 8-ohm speakers in parallel will draw twice the current of one
speaker because the parallel connection reduces the load impedance to 4 ohms.
(For simplicity's sake we are using the terms resistance and impedance interchangeably; in practice,
a speaker whose nominal impedance is 8 ohms may have a voice coil DC resistance of about 5
ohms and an AC impedance curve that ranges from 5 ohms to 100 ohms, depending on the
frequency, type of enclosure, and the acoustical loading of its environment.)
How does current draw affect the conductor requirements of the speaker cable?
A simple fact to remember: Current needs copper, voltage needs insulation. To make an analogy, if
electrons were water, voltage would be the "pressure" in the system, while current would be the
amount of water flowing. You have water pressure even with the faucet closed and no water
flowing; similarly, you have voltage regardless of whether you have current flowing. Current flow is
literally electrons moving between two points at differing electrical potentials, so the more electrons
you need to move, the larger the conductors (our "electron pipe") must be. In the AWG (American
Wire Gauge) system, conductor area doubles with each reduction of three in AWG; a 13 AWG
conductor has twice the copper of a 16 AWG conductor, a 10 AWG twice the copper of a 13
AWG, and so on.
But power amp outputs are rated in watts. How are amperes related to watts?
Ohm's Law says that current (amperes) times voltage (volts) equals power (watts), so if the voltage
is unchanged, the power is directly proportional to the current, which is determined by the
impedance of the load. (This is why most power amplifiers will deliver approximately double their
8-ohm rated output when the load impedance is reduced to 4 ohms.) In short, a 4-ohm load should
require conductors with twice the copper of an 8-ohm load, assuming the length of the run to the
speaker is the same, while a 2-ohm load requires four times the copper of an 8-ohm load.
Explaining this point leads to the following oft-asked question:
How long can a speaker cable be before it affects performance?
The ugly truth: Any length of speaker cable degrades performance and efficiency. Like the effects of
shunt capacitance in instrument cables and series inductance in microphone cables, the signal
degradation caused by speaker cabling is always present to some degree, and is worsened by
increasing the length of the cable. The most obvious ill effect of speaker cables is the amount of
amplifier power wasted.
Why do cables waste power?
Copper is a very good conductor of electricity, but it isn't perfect. It has a certain amount of
resistance, determined primarily on its cross-sectional area (but also by its purity and temperature).
This wiring resistance is "seen" by the amplifier output as part of the load; if a cable with a resistance
of one ohm is connected to an 8-ohm speaker, the load seen by the amplifier is 9 ohms. Since
increasing the load impedance decreases current flow, decreasing power delivery, we have lost
some of the amplifier's power capability merely by adding the series resistance of the cable to the
load. Furthermore, since the cable is seen as part of the load, part of the power which is delivered
to the load is dissipated in the cable itself as heat. (This is the way electrical space-heaters work!)
Since Ohm's Law allows us to calculate the current flow created by a given voltage across a given
load impedance, it can also give us the voltage drop across the load, or part of the load, for a given
current. This can be conveniently expressed as a percentage of the total power.
How can the power loss be minimized?
There are three ways to decrease the power lost in speaker cabling:
First, minimize the resistance of the cabling. Use larger conductors, avoid unnecessary connectors,
and make sure that mechanical connections are clean and tight and solder joints are smooth and
Second, minimize the length of the cabling. The resistance of the cable is proportional to its length,
so less cable means less resistance to expend those watts. Place the power amplifier as close as
practical to the speaker. (Chances are excellent that the signal loss in the line-level connection to the
amplifier input will be negligible.) Don't use a 50-foot cable for a 20-foot run.
Third, maximize the load impedance. As the load impedance increases it becomes a larger
percentage of the total load, which proportionately reduces the amount lost by wiring resistance.
Avoid "daisy-chaining" speakers, because the parallel connection reduces the total load impedance,
thus increasing the percentage lost. The ideal situation (for reasons beyond mere power loss is to
run a separate pair of conductors to each speaker form the amplifier.
Is the actual performance of the amplifier degraded by long speaker cables?
There is a definite impact on the amplifier damping factor caused by cabling resistance/impedance.
Damping, the ability of the amplifier to control the movement of the speaker, is especially noticeable
in percussive low-frequency program material like kick drum, bass guitar and tympani. Clean, "tight"
bass is a sign of good damping at work. Boomy, mushy bass is the result of poor damping; the
speaker is being set into motion but the amplifier can't stop it fast enough to accurately track the
waveform. Ultimately, poor damping can result in actual oscillation and speaker destruction.
Damping factor is expressed as the quotient of load impedance divided by the amplifier's actual
source impedance. Ultra-low source impedances on the order of 40 milliohms (that's less than
one-twentieth of an ohm) are common in modern direct-coupled solid-state amplifiers, so damping
factors with an 8-ohm load are generally specified in the range of 100-200. However, those
specifications are taken on a test bench, with a non-inductive dummy load attached directly to the
output terminals. In the real world, the speaker sees the cabling resistance as part of the source
impedance, increasing it. This lowers the damping factor drastically, even when considering only the
DC resistance of the cable. If the reactive components that constitute the AC impedance of the
cable are considered, the loss of damping is even greater.
Although tube amplifiers generally fall far short of sold-state types in damping performance, their
sound can still be improved by the use of larger speaker cables. Damping even comes into play in
the performance of mixing consoles with remote DC power supplies; reducing the length of the
cable linking the power supply to the console can noticeably improve the low-frequency
performance of the electronics.
What other cable problems affect performance?
The twin gremlins covered in "Understanding the Microphone Cable," namely series inductance and
skin effect, are also factors in speaker cables. Series inductance and the resulting inductive
reactance adds to the DC resistance, increasing the AC impedance of the cable. An inductor can be
thought of as a resistor whose resistance increases as frequency increases. Thus, series inductance
has a low-pass filter characteristic, progressively attenuating high frequencies. The inductance of a
round conductor is largely independent of its diameter or gauge, and is not directly proportional to
its length, either.
Skin effect is a phenomenon that causes current flow in a round conductor to be concentrated more
to the surface of the conductor at higher frequencies, almost as if it were a hollow tube. This
increases the apparent resistance of the conductor at high frequencies, and also brings significant
Taken together, these ugly realities introduce various dynamic and time-related forms of signal
distortion which are very difficult to quantify with simple sine-wave measurements. When complex
waveforms have their harmonic structures altered, the sense of immediacy and realism is reduced.
The ear/brain combination is incredibly sensitive to the effects of this type of phase distortion, but
generally needs direct, A/B comparisons in real time to recognize them.
How can these problems be addressed?
The number of strange designs for speaker cable is amazing. Among them are coaxial, with two
insulated spiral "shields" serving as conductors; quad, using two conductors for "positive" and two
for "negative"; zip-cord with ultra-fine "rope lay" conductors and transparent jacket;
multi-conductor, allegedly using large conductors for lows, medium conductors for mids, and tiny
conductors for highs; 4 AWG welding cable; braided flat cable constructed of many individually
insulated conductors; and many others. Most of these address the inductance question by using
multiple conductors and the skin effect problem by keeping them relatively small. Many of these
"esoteric" cables are extraordinarily expensive; all of them probably offer some improvement in
performance over ordinary twisted-pair type cables, especially in critical monitoring applications and
high-quality music systems. In most cases, the cost of such cable and its termination, combined with
the extremely fragile construction common to them, severely limits their practical use, especially in
portable situations. In short, they cost too much, they're too hard to work with, and they just aren't
made for rough treatment. But, sonically, they all bear listening to with an open mind; the differences
can be surprisingly apparent.
Is capacitance a problem in speaker cables?
The extremely low impedance nature of speaker circuits makes cable capacitance a very minor
factor in overall performance. In the early days of solid state amplifiers, highly capacitive loads (such
as large electrostatic speaker systems) caused blown output transistors and other problems, but so
did heat, short circuits, highly inductive loads and underdesigned power supplies.
Because of this, the dielectric properties of the insulation used are nowhere near as critical as that
used for high-impedance instrument cables. The most important consideration for insulation for
speaker cables is probably heat resistance, especially because the physical size constraints imposed
by popular connectors like the ubiquitous 1/4" phone plug severely limit the diameter of the cable.
This requires insulation and jacketing to be thin, but tough, while withstanding the heat required to
bring a relatively large amount of copper up to soldering temperature. Polyethylene tends to melt too
easily, while thermoset materials like rubber and neoprene are expensive and unpredictable with
regard to wall thickness PVC is cheap and can be mixed in a variety of ways to enhance its
shrink-resistance and flexibility, making it a good choice for most applications. Some varieties of
TPR (thermoplastic rubber) are also finding use.
Why don't speaker cables require shielding?
Actually, there are a few circumstances that may require the shielding of speaker cables. In areas
with extreme strong radio frequency interference (RFI) problems, the speaker cables can act as
antennae for unwanted signal reception which can enter the system through the output transistors.
When circumstances require that speaker-level and microphone-level signals be in close proximity
for long distances, such as cue feeds to recording studios, it is a good idea to use shielded speaker
cabling (generally foil-shielded, twisted-pair or twisted-triple cable) as "insurance" against possible
crosstalk form the cue system entering the microphone lines. In large installations, pulling the
speaker cabling in metallic conduit provides excellent shielding from both RFI and EMI
(electromagnetic interference). But, for the most part, the extremely low impedance and high level of
speaker signals minimizes the significance of local interference.
Why can't I use a shielded instrument cable for hooking an amplifier to a speaker,
assuming it has the right plugs?
You can, in desperation, use an instrument cable for hooking up an amplifier to a speaker.
However, the small gauge (generally 20 AWG at most) center conductor offers substantial
resistance to current flow, and in extreme circumstances could heat up until it melts its insulation and
short-circuits to the shield, or melts and goes open-circuit, which can destroy some tube amplifiers.
Long runs of coaxial-type cable will have large amounts of capacitance, possibly enough to upset
the protection circuitry of some amplifiers, causing untimely shut-downs. And of course there is
enormous power loss and damping degradation because of the high impedance of the cable.
3) CABLE ANATOMY I: UNDERSTANDING THE INSTRUMENT CABLE
Are instrument cables used for high-impedance or low-impedance lines?
Generally, the source impedance is the determining factor in cable selection. Instrument cables are
used for a wide range of sources. Many keyboard instruments, mixers, and signal processors have
very low (50 to 600 ohm) source impedances. On the other hand, typical electric guitar or bass
pickups are very inductive, very high impedance (20,000 ohms and above) sources. Typical load
impedances are greater than 10,000 ohms, which limits the electrical current flow to a very small
amount on the order of a few thousandths of an ampere (milliamps).
How much power does an instrument cable have to carry?
The voltages encountered range from a few millivolts, in the case of the electric guitar, to levels over
ten volts delivered by line-level sources such as mixers. By Ohm's Law this represents power levels
of less than a thousandth of a watt.
What kind of frequency response does an instrument cable need? What are the lowest and
highest frequencies produced by the source?
The bandwidth spans the entire audible range of frequencies, from the 41 Hz (and below) of bass
guitar and synthesizer to the 20 kHz harmonics of keyboards and cymbals. Recording applications
demand wide bandwidth to preserve the "sizzle" of a hot performance. Even an electric guitar has a
bandwidth of about 82 Hz to above 5 kHz.
How big does an instrument cable need to be? Will a bigger cable sound better? Will a
bigger cable last longer?
In order to be compatible with standard 1/4-inch phone plugs the diameter of the cable is effectively
limited to a maximum diameter of about .265". Larger cable diameters demand larger plug barrels,
which sometimes won't fit jacks that are located close together or in tight places. In terms of both
sound and durability, "it's not how big you make it, but how you make it big."
What are the basic parts of an instrument cable and what does each one do?
The coaxial configuration is generally used for unbalanced instrument cables. At its simplest it
consists of a center conductor, which carries current form the source, separated by insulation from a
surrounding shield, which is also the current return conductor necessary to complete the circuit.
These three components are augmented by an electrostatic shield to reduce handling noise and an
outer jacket for protection and appearance.
What is a stranded center conductor? Why is it important?
A stranded conductor is composed of a number of strands of copper wire bunched together to form
a larger wire. Solid conductors having only one strand are the cheapest and easiest to work with
when assembling cables, because they do not require the twisting and tinning that stranded types
need to prepare them for soldering. The problem with a solid conductor is that it quickly fatigues
and breaks when it is bent or flexed. This makes stranded conductors a must for cables that are
frequently moved around, especially when they are attached to human beings playing music. Finely
stranded conductors increase the cost of the cable because of the increased production time and the
expensive and sophisticated machinery required to assemble very small and fragile strands into a
single conductor. The stranding of the center conductor is only one of a number of factors that
influence the overall flexibility of a given cable, but it is generally true that finer stranding increases
the flexibility and the flex life of the cable.
What is wire gauge? What gauge wire is used in instrument cables?
The diameter of copper wire is typically given in AWG (American Wire Gauge), with the larger
numbers signifying smaller size. For instance, a 20 AWG (or "20 gauge") wire is smaller than an 18
AWG wire. Generally, instrument cable center conductors are in the range of 18 to 24 AWG, with
strands of 32 to 36 AWG. Many American wire mills simply cannot work with wire smaller than 36
AWG because their equipment is too antiquated. The Japanese manufacturers Canare and Mogami
have been leaders in the use of very fine (40 AWG) copper stranding.
What gauge should the center conductor of an instrument cable be?
Since the current involved in instrument applications is negligible, the amount of copper in the center
conductor has only a very slight effect on the strength of the signal reaching the amplifier. In practice,
the center conductor's size is determined primarily by (1) the necessity of obtaining a maximum
diameter of .265" or less while (2) providing sufficient tensile strength to withstand the rigors of
performance without breaking. The 20 AWG center conductor has become quite standard,
normally in the form of 26 strands of 34 conductor has become quite standard, normally in the form
of 26 strands of 34 AWG(26/34) or 41 strands of 36 AWG (41/36). A 20 AWG conductor has a
breaking point of approximately 31 lbs. Reducing conductor size to 22 AWG reduces breaking
point to about 19 lbs. (a reduction of 39%); increasing it to 18 AWG increases the strength to over
49 lbs. (an increase of 58%). The most common cause of failure for instrument cables is broken
What are the differences between tinned copper and bare copper stranded conductors?
Sometimes the individual strands of the center conductor are run through a bath of molten tin before
assembling them into a wire. Tinned copper wire is often easier to solder, especially if a lengthy
(months to years) shelf life is required, because the tin coat prevents copper oxides from forming. If
the cable is to be used immediately upon manufacture pre-tinned strands are not required and add
unnecessary expense. Furthermore, an electrical phenomenon known as skin effect makes the use
of tinned conductors a potential threat to the high-frequency signal-carrying properties of the cable.
However, the aging effects of the formation of copper oxides on untinned conductors may also
cause a gradual deterioration of performance.
What is skin effect and how does it affect tinned copper?
Briefly, skin effect is caused by the magnetic field generated by the current flow in the cable causing
electron flow to be concentrated more and more on the outer surface of the conductor as frequency
increases. If this outer surface is coated with tin, which has higher resistance than copper, the cable
will have a falling high-frequency response and act as an attenuator.
What is oxygen-free and linear-crystal copper? How do they affect sound in cables?
There is a continuing debate concerning the use of oxygen-free and linear-crystal copper wire.
These types of wire contain lower levels of oxide impurities and fewer crystal boundaries than
standard copper. Since these impurities form tiny semiconductors within the cable, the theory is that
the cable itself introduces signal distortion, especially of low-level "detail" information. These claims
have been very difficult to document with scientific test equipment, but numerous listening tests
suggest there is something to them.
What materials are used for insulation of the center conductor?
The insulation that surrounds the center conductor can be made from thermoset (rubber, E.P.D.M.,
neoprene, Hypalon) or thermoplastic (polyethylene, polypropylene, PVC, FPE) materials. The
thermoset materials are extruded over the conductor and then heat-cured to vulcanize them. This
process yields a very high melting pint which makes soldering very easy, but the vulcanizing stage
adds to the cost and introduces unpredictable shrinkage which can make it very difficult to maintain
the desired wall thickness. Thermoplastic insulations are cheaper to process but will return to a
liquid state when overheated, requiring great care during soldering when used to insulate large
conductors. In the past decade the insulation of choice for instrument cable has largely shifted from
rubber or E.P.D.M. to high-density polyethylene, with cost being a major factor.
How does the insulation affect flexibility?
The insulation material and its thickness can be very dominant in determining the flexibility of the
cable. A finely-stranded conductor insulated with a stiff compound will behave much like a solid
conductor, as will a conductor insulated with a very thick layer of a more flexible compound. The
thinner the insulation is, the more flexibility it allows in the overall cable.
How thick does the insulation need to be?
The basic electrical requirement for insulation thickness is called dielectric strength and is determined
by the cable's working voltage. The voltages involved in instrument cable applications are very low
and very little dielectric strength is necessary to prevent the insulation from breaking down.
However, a very important consideration when the cable is to be used for instruments like electric
guitars is the amount of capacitance between the center conductor and shield.
What is capacitance and what does it do?
Capacitance is the ability to store an electrical charge. In cables, capacitance between the center
conductor and shield is expressed in picofarads per foot (pF/ft.), with lower values indicating less
capacitance. Combined with the source impedance, cable capacitance forms a low-pass filer
between the instrument and amplifier; that is, it cuts high frequencies, much as the instrument's tone
Why is low-capacitance cable an advantage? How can cable capacitance be eliminated?
How long of a cable can I run before I lose high frequencies?
Lower cable capacitance allows more of the natural "brightness," "presence," or "bite" of an
instrument to reach the amp, which in turn allows the treble controls to be run lower, reducing "hiss"
and other unwanted noise. High-frequency loss from the cable becomes audible and objectionable
depending on the source, the amplification and other circumstances. Raising the source impedance
or increasing the length of the cable increases the loss; there is no point at which high-frequency loss
suddenly appears or disappears. Guitars typically have much higher source impedances at higher
frequencies because of the inductive nature of their pickups, which aggravates the effect of cable
capacitance. A guitar will often sound noticeably "muddier" when run through a 40-foot cable,
whereas keyboard instruments, samplers, mixers and other line-level devices with low source
impedances can usually drive cable runs of hundreds of feet without problems.
How is low-capacitance cable made?
Given that the overall outside diameter of the cable is limited by the plugs that must be used, cable
capacitance is largely the result of trade-offs between conductor size (and hence strength), insulation
material (cost) and insulation thickness (size and flexibility). The term dielectric constant is used to
rank the insulation quality of a material. Some materials are great insulators but impractical for use as
wire insulationÑglass, for instance! As far as practical materials are concerned, the thermoplastics
are generally far superior to the thermoset family. For instance, polyethylene has a dielectric
constant of 2.3, while that of rubber is 6.5. This allows a cable with polyethylene insulation to have
perhaps one-third of the capacitance of a cable insulated with the same thickness of rubber. This
can make an audible increase in the clarity of the sound.
What is the best all-around insulation material for instrument cables?
Polyethylene is very economical and dielectrically hard to improve upon (teflon is slightly better, but
its cost is far greater, and its flexibility is far from ideal). Its only drawback is a low melting point
which requires a skilled touch with the soldering iron to avoid problems in production.
What does the electrostatic do?
As the cable is flexed and bent, the copper shield rubs against the insulation, generating static
electricity. The electrostatic shield acts as a semi-conducting barrier between the copper shield and
the center insulation which discharges these static electrical charges. Without it any movement of the
cable would result in obnoxious "crackling" noises being generated.
What are electrostatic shields made of?
Electrostatic shields first appeared in cable as a layer of rayon braid. Nowadays
carbon-impregnated dacron "noise-reducing tape" is a common element in any good
high-impedance cable. Increasing in popularity are conductive-plastic (carbon-loaded PVC)
electrostatic shields. Conductive PVC is extrudable just like an insulation, which guarantees 100%
coverage of the insulation with a very consistent thickness and a very low coefficient of friction. The
superior conductivity of C-PVC makes it much more effective than the semiconductive tape in
bleeding off the small electrical charges that cause "the crackles." Extruded C-PVC is also thinner
and more flexible than dacron tape, which is applied longitudinally and restricts the "bendability" of
the cable. Although conductive plastic (with a copper drain wire) has been used to completely
replace copper braid or serve shields, its effectiveness falls off above 10 kHz.
Why are some cables microphonic?
As was noted previously, the center conductor, insulation and shield of a coaxial cable form a
capacitor; and, as many a microphone manufacturer will tell you, when the plates of a capacitor are
deflected, a voltage is generated. (This is the basis of the condenser microphone!) Similarly, when
the plates (conductor and shield) of our "cable-capacitor" are deflected (for instance, by stepping on
it or allowing it to strike a hard floor), a voltage is also generated. Unfortunately, this voltage
generally pops out of the amplifier as a distinct "whap," and can be very hard on ears and
loudspeakers alike. Effects of this type are called triboelectric noise.
How can cable noise be reduced?
The electrostatic shield's charge-draining properties help greatly to diminish triboelectric effects.
Triboelectric impact noise is also reduced by decreasing the capacitance of the cable with thicker
and softer insulation because the deflection of the conductor is proportionally reduced. This is the
main reason that the single-conductor coaxial configuration remains superior to the "twisted pair" for
high-impedance usesÑit allows thicker insulation for a given overall diameter. Triboelectric effects
are accentuated by high source impedances, and are at their worst when the source is an open
circuitÑfor instance, a cable plugged into an amplifier with no instrument at the sending end. Testing
for this type of noise requires termination of the cable with a shielded resistance to simulate the
source impedance of a real instrument.
What does the shield do?
The copper shield of a coaxial cable acts as the return conductor for the signal current and as a
barrier to prevent interference from reaching the "hot" center conductor. Unwanted types of
interference encountered and blocked with varying degrees of success by cable shielding include
radio frequency (RFI) (CB and AM radio), electromagnetic (EMI) (power transformers) and
electrostatic (ESI) (SCR dimmers, relays, fluorescent lights).
What makes one shield better than another?
To be most effective the cable shield is tied to a groundÑusually a metal amplifier or mixer chassis
that is in turn grounded to the AC power line. Cable shielding effectiveness against high-frequency
interference fields is accomplished by minimizing the transfer impedance of the shield. At frequencies
below 100 kHz, the transfer impedance is equal to the DC resistanceÑhence, more copper equals
better shielding. Above 100 kHz the skin effect previously referred to comes into play and increases
the transfer impedance, reducing the shielding effectiveness. Another important parameter to
consider is the optical coverage of the shield, which is simply a percentage expressing how complete
the coverage of the center conductor by the shield is.
What are the characteristics of the three basic types of cable shields? Which is best?
A braided shield is applied by braiding bunches of copper strands called picks around the insulated,
electrostatically shielded center conductor. The braided shield offers a number of advantages. Its
coverage can be varied from less than 50% to nearly 97% by changing the angle, the number of
picks and the rate at which they are applied. It is very consistent in its coverage, and remains so as
the cable is flexed and bent. This can be crucial in shielding the signal from interference caused by
radio-frequency sources, which have very short wavelengths that can enter very small "holes" in the
shield. This RF-shielding superiority is further enhanced by very low inductance, causing the braid to
present a very low transfer impedance to high frequencies. This is very important when the shield is
supposed to be conducting interference harmlessly to ground. Drawbacks of the braid shield include
restricted flexibility, high manufacturing costs because of the relatively slow speed at which the
shield-braiding machinery works, and the laborious "picking and pigtailing" operations required
A serve shield, also known as a spiral-wrapped shield, is applied by wrapping a flat layer of copper
strands around the center in a single direction (either clockwise or counter-clockwise). The serve
shield is very flexible, providing very little restriction to the "bendability" of the cable. Although its
tensile strength is much less than that of braid, the serve's superior flexibility often makes it more
reliable in "real-world" instrument applications. Tightly braided shields can be literally shredded by
being kinked and pulled, as often happens in performance situations, while a spiral-wrapped serve
shield will simply stretch without breaking down. Of course, such treatment opens up gaps in the
shield which can allow interference to enter. The inductance of the serve shield is also a liability
when RFI is a problem; because it literally is a coil of wire, it has a transfer impendance that rises
with frequency and is not as effective in shunting interference to ground as a braid. The serve shield
is most effective at frequencies below 100 kHz. From a cost viewpoint, the serve requires less
copper, is much faster and hence cheaper to manufacture, and is quicker and easier to terminate
than a braided shield. It also allows a smaller overall cable diameter, as it is only composed of a
single layer of very small (typically 36 AWG) strands. these characteristics make copper serve a
very common choice for audio cables.
The foil shield is composed of a thin layer of mylar-backed aluminum foil in contact with a copper
drain wire used to terminate it. The foil shield/drain wire combination is very cheap, but it severely
limits flexibility and indeed breaks down under repeated flexing. The advantage of the 100%
coverage offered by foil is largely compromised by its high transfer impedance (aluminum being a
poorer conductor of electricity than copper), especially at low frequencies.
What type of shield works best against 60-cycle hum from power transformers and AC
The sad truth is that the most offensive "hum-producing" frequencies (60 and 120 Hz) generally
emitted by transformers and heavy power cables are too low in frequency to be stopped by
anything but a solid tube of ferrous (magnetic) metalÑiron, steel, nickel, etc.Ñnone of which
contribute to the flexibility of a cable! For magnetically-coupled interference, the only solution is to
present as small a loop area as possible. This is one of the reasons that the twisted-pair
configuration generally used in balanced-line applications became popular. Fortunately the high input
impedances generally found in unbalanced circuits minimize the effects of such interference. Don't
run instrument cables parallel to extension cords. Don't coil up the excess length of a "too-long"
cable and stuff it through the carrying handle of a ampÑthis makes a great inductive pickup loop for
60 Hz hum!
What does the outer jacket do? What is it made of?
The jacket is both armor and advertisement; it protects the cable from damage and enhances the
marketability of the assembly. As armor, the jacket must resist abrasion, impact, moisture and
sometimes hostile chemicals (Bud Light, for instance). As advertisement, it may be distinctively
colored or printed with the name of the manufacturer or dealer for product identification. The
materials used for jacketing are the same type as those used for the inner insulation (thermoset or
thermoplastic), but the choice is dictated less by electrical criteria and more by physical durability
and cosmetic acceptability.
What is the best cable jacketing material?
For years rubber or neoprene were preferred for their superior abrasion resistance and flexibility,
but modern thermoplastic technology has produced a number of PVC compounds that are soft and
flexible but also very tough. As previously noted, thermoplastic processing is cheaper, faster and
more predictable than that for thermoset materials. Only very specialized situations requiring oil or
ozone resistance or extremes of temperature and climate demand neoprene or Hypalon jacketing.
The use of PVC has two other major advantages. PVC is not as elastic as rubber or neoprene, and
this lack of "stretch" lends additional tensile strength to the resulting assembly by taking some of the
strain that would otherwise be borne solely by the center conductor. This has made a dramatic
improvement in the reliability of currently manufactured instrument cables.
The other important property of PVC is its almost limitless colorability. Once found only in gray or
"chrome vinyl," PVC-jacketed cable now ranges from basic black through brilliant primary colors to
outrageous "neon" shades of pink and green.
¥ Ballou, Greg, ed., Handbook for Sound Engineers: The New Audio Cyclopedia, Howard W.
Sams and Co., Indianapolis, 1987.
¥ Cable Shield Performance and Selection Guide, Belden Electronic Wire and Cable, 1983.
¥ Colloms, Martin, "Crystals: Linear and Large," Hi-Fi News and Record Review, November
¥ Cooke, Nelson M. and Herbert F. R. Adams, Basic Mathematics for Electronics, McGraw-Hill,
Inc., New York, 1970.
¥ Davis, Gary and Ralph Jones, Sound Reinforcement Handbook, Hal Leonard Publishing Corp.,
¥ Electronic Wire and Cable Catalog E-100, American Insulated Wire Corp., 1984.
¥ Fause, Ken, "Shielding, Grounding and Safety," Recording Engineer/Producer, circa 1980.
¥ Ford, Hugh, "Audio Cables," Studio Sound, Novemer 1980.
¥ Guide to Wire and Cable Construction, American Insulated Wire Corp., 1981.
¥ Grundy, Albert, "Grounding and Shielding Revisited," dB, October 1980.
¥ Jung, Walt and Dick Marsh, "Pooge-2: A Mod Symphony for Your Hafler DH200 or Other
Power Amplifiers," The Audio Amateur, 4/1981.
¥ Maynard, Harry, "Speaker Cables," Radio-Electronics, December 1978,
¥ Miller, Paul, "Audio Cable: The Neglected Component," dB, December 1978.
¥ Morgen, Bruce, "Shield The Cable!," Electronic Procucts, August 15, 1983.
¥ Morrison, Ralph, Grounding and Shielding Techniques in Instrumentation, John Wiley and Sons,
New York, 1977.
¥ Ott, Henry W., Noise Reduciton in Electronic Systems, John Wiley and Sons, New York, 1976.
¥ Ruck, Bill, "Current Thoughts on Wire," The Audio Amateur, 4/82.