cable anatomy i: understanding the microphone cable

cable anatomy i: understanding the microphone cable
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 rare 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 chassis—virtually anything
that conducts electricity. Ideally it is a perfect conductor—that 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
Positive peak: Signal= +5 Volts
10
Zero crossing: Signal= 0 Volts
Negative peak: Signal= -5 Volts
1 CYCLE
VOLTS
5
(1/1000th Second)
0
Amplitude of signal
is referred to 0 volts
- 5
-10
UNBALANCED 1.0 kHZ SIGNAL TRANSMISSION
images” of each other—their 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 some 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 commonmode 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-impedanceis-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.”
a
Positive peak: Total = Signal (+5) + Interference (+5) = +10 Volts
10
Zero crossing: Total = Signal (0) + Interference (+5) = +5 Volts
Negative peak: Total = Signal (-5) + Interference (+5) = 0 Volts
VOLTS
5
Total amplitude of signal
is referred to 0 volts
Interference "spikes"
ADD to signal
0
- 5
+5 Volt interference "spikes" riding on signal
-10
UNBALANCED 1.0 kHZ SIGNAL TRANSMISSION with INTERFERENCE
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.
Outer Jacket
Shield
a
Filler
Insulation
Inner Conductors
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 10 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.
Positive peak: Total signal= Signal (+5) - Signal' (-5) = +10 Volts
10
Zero crossing: Total signal= Signal (0) - Signal' (0) = 0 Volts
Negative peak: Total signal= Signal (-5) - Signal' (+5) = -10 Volts
1 CYCLE
(1/1000th Second)
VOLTS
5
One conductor
of balanced line
0
- 5
-10
Amplitude of one signal is referred to
amplitude of o t h e r signal.
Total amplitude equals d i f f e r e n c e
between the two.
10
1 CYCLE
(1/1000th Second)
VOLTS
5
Other conductor
of balanced line
0
- 5
-10
BALANCED 1.0 kHZ SIGNAL TRANSMISSION
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 starquad 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.
Positive peak: Total Signal = Signal (+5) - Signal' (-5) = +10 Volts
Total Interference = Interference (+5) - Interference (+5) = 0 Volts
10
Zero crossing: Total Signal = Signal (0) - Signal' (0) = 0 Volts
Total Interference = Interference (+5) - Interference (+5) = 0 Volts
Negative peak: Total Signal = Signal (-5) - Signal' (+5) = -10 Volts
Total Interference = Interference (+5) - Interference (+5) = 0 Volts
VOLTS
5
One conductor
of balanced line
0
+5 Volt interference "spikes" riding on signal
- 5
-10
Amplitude of one signal is referred to
amplitude of o t h e r signal.
Total amplitude equals d i f f e r e n c e
between the two.
Interference “spikes” have same polarity on
both conductors and cancel each other.
10
VOLTS
5
Other conductor
of balanced line
0
+5 Volt interference "spikes" riding on signal
- 5
-10
BALANCED 1.0kHZ SIGNAL TRANSMISSION with INTERFERENCE
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 low-pass
filter characteristic, progressively attenuating high frequencies. While parallel capacitance, the enemy of highfrequency 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.
CONDUCTOR SIZE
INDUCTANCE OF COPPER CONDUCTORS
28 AWG
26 AWG
24 AWG
0.029
0.028
0.026
22 AWG
20 AWG
18 AWG
16 AWG
14 AWG
12 AWG
10 AWG
0.025
0.024
0.023
0.022
0.021
0.019
0.018
0.00
0.49
0.48
0.47
0.45
0.44
0.42
0.41
0.40
0.38
0.37
0.10
0.20
0.30
0.40
ONE INCH
ONE FOOT
0.50
INDUCTANCE IN MICROHENRIES
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 relationships—when 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 note—a 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.
12 AWG Conductor
Diameter = 0.080"
12 AWG Conductor
Diameter = 0.080"
Insulation
Insulation
Copper
Copper
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At DC: No skin effect.
Current flows in entire
conductor.
At 20 kHz:
Skin depth
= 0.020".
Center of
conductor
has no
current flow.
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 1/4”, 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 cheap.
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 cables?
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 lowimpedance 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 terminate.
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 problems.
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.
BIBLIOGRAPHY
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
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 1984.
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., Milwaukee, 1970.
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.
CHOOSING THE RIGHT MICROPHONE CABLES
■
■
■
■
■
In this chapter, you will learn
about microphone cable
construction and selection
with recommended products
for various types of use. We
will cover:
XLR connectors
Balanced and unbalanced
connections
Wiring of the different types of
microphones
The right wire for microphone
cables, including shielding
A short section for vocalists only
Benchmark
It is really difficult to buy a really
flexible, really reliable, really quiet,
really good-sound, really goodlooking 25' microphone cable for
under 30 bucks.
whatever you want to call them).
In pro audio, microphones are low
impedance (Lo-Z) and are terminated
in 3-pin XLR connectors.
Pin 1
(X)ternal
Shield
Pin 2
(L)ive
Hot (+)
Pin 3
(R)eturn
Cold ( - )
Another typical configuration is
emerging which includes an XLR
female (the output of all professional
microphones is a 3-pin XLR male)
It is really difficult to buy a
really flexible, really reliable,
really quiet, really goodsound, really good-looking
25' microphone cable for
under 30 bucks.
XLR Female
1
3
Lo-Z Unbalanced Microphone Cable
XLR Female
1
The mic cable situation
Microphone cables connect microphones to mixers (desk, consoles,
“One of my problems is making
sure the minister does not trip
on a microphone cable.”
2
3
1
2
1/4" Phone Plug
2
3
Hi-Z Microphone Cable
Lo-Z Microphone cables can also
be wired “unbalanced” and “Hi-Z”
(high impedance) microphones are
available, for high impedance sound
systems (didn’t take long to get
confusing, did it?).
Hi-Z cables allow the user to plug
a Hi-Z microphone directly into the
input of, say, a guitar amp or the
input of a Hi-Z mixer.
1
1
1/4" Phone Plug
2
2
3
3
Lo-Z to Hi-Z Transformer
XLR Male
XLR Female
Lo-Z Microphone Cable
connector to a 3-pin mini male
(1/8" or 3.5mm) connector for
inputs to laptop computers
and other devices where
space is at a premium.
Most professional mixer’s
microphone inputs are
designed with “balanced”
circuits to help decrease or
eliminate noise and unwanted radio frequency
interference (RFI). The
understanding of
balanced circuits and
low-impedance is
complicated and is
addressed in Pro Co’s
white papers on
microphone cables,
which can be found
at our website:
www.procosound.com.
Unbalancing a balanced microphone by using an unbalanced cable
allows it to sometimes be used in the
input of a Hi-Z mixer. This does not
always work, depending on the input
impedance of the mixer. When this
does not work, a Lo-Z to Hi-Z transformer must be placed in line at the
end of a standard Lo-Z mic cable.
These commercially available transformers make the proper change
from XLR to 1/4" for you.
The real world problems
with microphone cables
The quality and type of cables
needed depend on the application:
“Cannot fail” situations:
■ Cables used for live concerts,
amateur and professional, TV/
Radio recording and broadcast,
ENG (electronic news gathering),
recording studios and churches
and all other situations where
perfection is demanded and
failure and noise are not options.
Brutal environments:
■ Workhorse cables for touring
bands and hard use situations
such as A/V (audio visual) rental.
Normal duty use:
■ Light-duty church and
auditorium use, weekend bands
and rehearsal halls.
Light duty/little use
■ Beginner use and “thrown-inwith-the-deal” while buying the
microphone, that work enough
to get you started.
Mission Impossible:
■ Lead signers in rock bands who
tend to try to destroy mic cables, a
real life test of durability.
Note: With the addition of Kevlar
to our Ameriquad and Merlin brands
of microphone cable assemblies (1998),
these cables can be used in situations
where everything else breaks.
The Solutions
About Microphone Wire
Microphone wire consists of a
twisted pair of copper conductors
(typically 22 - 24 AWG — American
Wire Gauge). These conductors are
covered with one of three types of
shielding: braided, spiral (also called
“serve” shield), and foil shielding
which includes a drain wire. Foil
shields work great in snakes, but
prove to be unreliable in cables
designed for portable use.
Braided shield is best for mic cables
and spiral is a little more flexible and
less expensive than braid.
Microphone Connectors
XLR audio connectors come in a
variety of contact materials, gold,
silver and tin. The trade generally
likes silver for sound, gold for tarnishfree contacts and tin for price.
With the addition of Kevlar
to our Ameriquad and Merlin brands of microphone
cable assemblies (1998),
these cables can be used in
situations where everything
else breaks.
There are about four good suppliers
of XLR connectors on the planet and
20 or so copy houses, which wreck
havoc on the trade, since they look
like industry standard connectors,
but are not properly dimensioned.
Microphone cables, unlike guitar
cables, use a female connector on one
end and a male connector on the
other. This enables microphone cables
to be daisy-chained, hooked end to
end, to increase length when necessary.
This requires a very narrow tolerance for size and pin locations in the
connectors, to ensure that the female
XLR (the one with the locking
mechanism) will lock and unlock
when mated to its male counterpart.
Complicating these problems is
one manufacturer in America who
uses English measurements, a lot of
oriental copiers who have approximated the English measurement
with metric measurements, and the
manufacturers who are not copiers
and make great, “to spec” connectors
using metric measurements.
Yup! You guessed it. There are
compatibility problems. Furthermore,
there are some budget minded
equipment manufacturers who will
use oriental knockoffs in their back
panels, exacerbating the problem.
This gets you a cheaper price on
the original unit and lots of headaches hooking it up, night after night.
Aside from these occasional
compatibility problems and types of
connector contact finishes, most XLR
connectors will work just fine for
most situations. We suggest
buyng cables which use Neutrik or
Amphenol (the original ITT-Canon)
connectors for best results.
Let’s talk technical
about mic cables
Try at all costs to avoid Hi-Z and
Lo-Z unbalanced mics. You are
not doing your performances any
favors by using these products,
regardless of price.
4-conductor (quad) mic cable
is so dramatic in its noise reduction that the only reason not to
use quad mic cables everywhere
in your sound system is price.
As sound system operators
find out how much quad mic
wire reduces noise compared to
well-designed and built two
conductor assemblies, they are
turning to the wire as a logical
step up in their system performance.
A friend of Pro Co’s, who
operates several county fair P.A.
systems, working in the absolutely worst conditions imaginable, has found that with the use
of Pro Co’s Ameriquad wire to
help eliminate “hiss” from his
systems, that the artists, often
times, are unable to detect that
the sound system is “on”. They are
so used to listening to hiss as an
indicator that the equipment is
working that this unsettles them
greatly. From an engineering
standpoint all we can say is, “We
get it right and they still complain”. Good grief!
Page 9
CHOOSING THE RIGHT MICROPHONE CABLES
Solutions
Most professional Lo-Z microphone
outputs can easily be run up to 500
feet. However, Hi-Z microphones
have the same roll off problems that
guitar cables have and their lengths
should be limited to 20' or less to avoid
high frequency attenuation.
Microphone wire comes in a wide
variety of diameters. Lavalier mics
require tiny, yet sturdy cables. Naturesound recording enthusiasts need
small cables that will roll up into the
compartment provided in their Nagra
tape recorders to conserve space.
Most microphone cables are about
the diameter of a normal pencil (1/4")
to provide the user with a reliable cable.
We have found that to present an
audience with totally no-hum, nobuzz, no-crackle sound systems
requires the use of quad (4-conductor)
microphone cables.
In situations such as TV studios
with huge hum fields created by TV
cameras and county fairs with lots of
stray radio frequency interference, 4conductor mic cables can lower hum
and noise up to 20 dB (20 decibels —
a lot) comparted to any two-conductor
XLR Female to XLR Male Balanced Lo-Z Microphone Cable
XLR Female to Balanced Mini (1/8”) Male Microphone Cable
XLR Female to Unbalanced 1/4” Phone Plug,
either Unbalanced Lo-Z or Hi-Z depending on how it is wired
microphone cable.
Why does quad mic cable work?
For vocalist only: spending
more on a great vocal mic
that sounds like you, and
picks up your tone and your
emotions, is something you
owe yourself and your audience.
Here’s the easiest way to think about it.
Balanced mic cables are quieter than
unbalanced mic cables because 1/2
of the signal travels on one of the
two conductors and they tend to
cancel out extraneous signals that
jump on both conductors. The tighter
the two conductors are twisted
together, and the shorter their twist,
the better the wire is at canceling out
noise. When two pairs of conducts are
twisted together (four conductors
total), this makes the conductors
much more tightly wound, and,
subsequently, ten times better at
defending against interference.
25’ Lo-Z microphone cables run in
price from about $15 to $75, depending upon the connectors and wire
used. The watertight cables used to film
the“Titanic”are worlds apart from the
“thrown-in-with-the-mic” cables given
away by retailers to “clinch the deal”.
For vocalists only
So, what kind of mic cables do I need:
Advanced
Pro Co Brand 25’ Model #
MSRP
Merlin
ME-25
$77.50
Ameriquad
AQ-25
$50.00
If you are an advanced play, look for a cable with:
a braided shield, 95% or better shield coverage, gold
contacts, Kevlar reinforced core and 4 conductor cable.
Intermediate Mastermike
M-25
$38.75
If you are an intermediate player, look for a cable with:
a braided shield, 90% + shield coverage and silver/gold
contacts.
Beginner
Excellines
EXM-25
$28.70
If you are a beginning player, look for a cable with:
a spiral shield, 70-90% shield coverage and silver/tin contacts.
Page 10
Your microphone is your instrument.
There are wireless mics now that
sound nearly as good as mics with
cables and allow you complete freedom of motion on stage. That is,
when they cost $3,000 each.
For the rest of you, spending more
on a great vocal mic that sounds like
you, and picks up your tone and your
emotions, is something you owe
yourself and your audience. It also
has to have great feedback rejection
if you are using a monitor system.
If you have spent the money to get
yourself a great microphone, get a
great cable to go with it, one that
transmits your sound and your emotions to your audience, without noise
and without adding any tone of its
own. We build cables that can to that.
They are called “Merlin”, and they are
truly magicians at work.
A LOT ABOUT SHIELDING
In this chapter, you will learn way
more about shielding than you need.
We’ll start with what shielding does.
Then we will discuss what makes one
shield better than another and talk
about the characteristics of each and
which is “best”.
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).
statically 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
Braided Shield
Spiral Shield
What makes one shield
better than another?
To be most effective the cable shield
is tied to a ground — usually 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 resistance — hence, more copper
equals better shielding. Above 100kHz
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, electro-
Foil Shield
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
during termination.
A serve shield, also know 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 a 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 spiralwrapped 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 termintate 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.
Page 11
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