ADDITION NUMBER FIVE SYSTEM INTERCONNECTION

ADDITION NUMBER FIVE SYSTEM INTERCONNECTION
ADDITION NUMBER FIVE
SYSTEM INTERCONNECTION
INTERCONNECTING THE SYSTEM
The importance of proper interconnection of components in
a system can hardly be over-emphasized. Not only must the
wire or cable be an appropriate type, but the compatibility
of component input and output parameters must be
considered.
In this supplement we will discuss such
parameters as signal level, impedance, and balanced versus
unbalanced terminations. Our approach is to begin with the
lowest electrical signal level and follow it through the line
level stages to the power amplifier and speaker circuits. A
short glossary of definitions appears at the end of this
supplement to assist in reading this material.
LOW LEVEL CONNECTIONS
Microphone level circuits are potentially the most prone to
hum pick up and noise interference because of the low signal
voltages involved.
Therefore, microphone cables and
connectors must be shielded to prevent electrostatic pickup.
Also, dynamic microphones, because they use a coil of wire,
are sensitive to external magnetic fields, such as those
produced by transformers, motors, fluorescent lamp ballasts,
and high current wiring. If operation in the vicinity of such
devices is anticipated, a model with a hum bucking coil or
special shielding should be used.
Magnetic phonograph
cartridges, guitar and piano pickups also use coils of wire and
may require physical separation or shielding to prevent hum
pickup.
Attenuators
Although the voltage levels in low impedance microphone
circuits often run in a low millivolt or microvolt region, it is
possible to encounter substantially higher levels, even exceeding one volt, when a high output microphone is used in
extremely loud sound fields. This might occur, for example,
when a condenser microphone is used for close pickup of a
rock vocal or trumpet soloist. The high level microphone
output may overload the input of the microphone mixer or
preamplifier. To eliminate this problem, a built-in or external attenuator can be used. These attenuators usually allow a
selection of 10, 20, 30 or even 40 dB signal reduction. It
should be noted that some external attenuators cannot be
used with a phantom powered microphone. See Figure 1.
MICROPHONE
PHANTOM
POWER
SUPPLY
ATTENUATOR
( OPTIONAL)
Input overload creates raspy distortion when severe, but it is
more difficult to detect if marginal, especially under live P.A.
conditions. To assist in determining overload, some equipment is provided with LED input overload indicators. When
using such equipment, simply insert enough attenuation so
that the LED seldom flashes under the loudest signal
conditions.
Impedance
Modern microphones can be segregated into two general
classifications: high and low impedance types. High-Z
microphone impedances lie in the range of 10,000 to 40,000
ohms, and are typically around 20,000 ohms. Low-Z values
range from 50 to 500 ohms, and are frequently a nominal
150 ohms. Because transformers with very low DC resistance
are often an integral part of the microphone, it is not meaningful to attempt to measure microphone internal impedance
with an ohmmeter.
An exact match between microphone impedance and its
associated equipment is not necessary. However, high-Z and
low-Z microphones must be connected to separate inputs;
and, where required, matching transformers are available to
convert from high to low, or low to high. Such transformers
may be plug-in options provided on the electronics, or be
add-on, in-cable types.
To minimize hum pickup, these
transformers must be designed with adequate magnetic
shielding, and be positioned well away from motors, power
transformers, and other equipment generating large AC
High-Z microphones have become less
magnetic fields.
popular in recent years because cable lengths greater than ten
or fifteen feet cause a reduction of high frequency response;
and, in addition, high-Z connections have somewhat greater
susceptibility to hum and noise pickup, even though a good
grade of low capacitance, single conductor, shielded cable is
used. To avoid ground-loop noise and hum, the housing and
stand of a high-Z microphone should not be allowed to come
into contact with other grounded items (conduit, metal
flooring, other equipment, etc.).
Low impedance microphones are recommended for all
permanent installations and wherever cable lengths must
exceed fifteen feet (see Chart I). Two conductor shielded
cable is required for the hookup. The shield, which totally
surrounds the signal conductors, may consist of a thin,
metallized foil and "drain" wire running the length of the
cable, or alternately, a braided mesh woven of fine wires may
be used. The braided construction has a longer flex life, so is
preferred in applications involving frequent cable movement.
Avoid the use of a widely spaced mesh or a spirally wrapped
shield, since they are more prone to interference pickup,
particularly SCR hash from lighting circuits. Stretchable coil
cords, such as those used on guitar pickups, are frequent
offenders in this respect.
PREAMPLIFIER
OR MIXER
WITHOUT PHANTOM
POWER
RIGHT
Microphone
mpedance
(ohms)
Cable Length (feet)
Causing 1 db Loss
at 10 KHz
50
920
150
310
250
1 90
I
MICROPHONE
ATTENUATOR
WRONG
FIGURE 1 - Phantom Powering
CHART 1 - Cable Capacity Loading
Phantom Connection
Some types of microphones require power to operate internal electronic circuitry. Such power may be supplied by
integral or external batteries, or it may be provided from the
following mixer or preamp by feeding the required DC back
over the same cable conductors which carry the signal (see
Figure 2). This is called a "phantom" connection. If required
by the microphone, and no provision is made in the following equipment, a separate phantom power supply module
may be placed in series with the mic cable. Although the
balanced nature of the phantom supply minimizes interaction (and noise) between the DC and signal, a severe
transient can be generated during insertion and removal of
connectors in the circuit, so this should not be attempted on
a live channel.
LINE LEVEL CONNECTIONS
The output of a typical microphone mixer usually provides a
signal level on the order of one volt, which is considered to
be `line level." If such equipment has VU meters, they are
often calibrated so that zero VU reading corresponds to an
output level of +4 dBm (decibels relative to one milliwatt).
Frequently, several pieces of line level equipment may be
interconnected to yield the desired system performance. For
example, a microphone mixer, equalizer and low level crossover may all be connected in series. Often these units are
designed with a gain of unity between their input and output
and have compatiable level requirements. This is not always
true and it is essential that the equipment specifications be
checked to assure compatible input and output levels.
Another consideration regarding compatibility of units
relates to input and output impedances. It is particularly
i mportant to connect each piece of equipment to its
Recommended load
recommended load impedance.
i mpedance, as its name implies, is that value which the device
should "see" when looking forward into the circuit it is
driving. Usually a minimum recommended load will be
specified, meaning that the unit will operate properly as
long as it sees a load of any impedance above this limit.
FIGURE 2 - Phantom Connections
Connectors
Usually, 1/4 inch phone plugs are used for high-Z microphone connections, and 3-pin XL style connectors are used
for low impedance balanced lines. By convention, the cable
ground braid, or shield, is always connected to pin 1 or to
the phone plug shell. On XL connections, pin 2 is the inphase terminal (positive pressure produces positive voltage on
pin 2), in accordance with EIA standard RS-221-A. These
same connectors are also popular for line level use, and the
same comments on polarity apply. Figure 3 illustrates some
of the most popular connectors suitable for low level and line
level connections.
The internal impedance of an active device is not necessarily
related to its load. It is that value which we would "see"
looking back into the output terminals of the device. Internal
i mpedance tells us, among other things, how much the gain
will vary with changes in the external load. As stated above,
active line level devices do not operate properly below a
minimum load impedance, a value often above the device's
internal impedance.
Balanced Versus Unbalanced
Another consideration when interconnecting electronic
components is whether their inputs and outputs are balanced
XL CONNECTORS
For balanced or unbalanced
l ow or level circuits. Provides
l atching action and excellent
cable strain and flex relief, for
heavy duty use.
PHONE PLUGS
Two conductor for unbalanced
circuits. Three conductor for
balanced or dual (stereo)
unbalanced circuits.
3CONDUCTOR
RCA-TYPE PHONO PLUG
For use on low or line level
unbalanced circuits.
FIGURE 3 - Industry Standard Connectors
FIGURE 4 - Balanced Versus Unbalanced Connections
or unbalanced with respect to ground. A balanced system is
usually preferred, particularly in more complex and sophisticated systems, due primarily to its superior freedom from
interference and avoidance of ground loops. This is achieved
by separating the signal and its return from the ground and
shielding both paths. Figure 4 shows typical input and ouput
variations.
Unbalanced systems utilize the ground for a signal return and
require only two terminals, variously referred to as "hot"
and "ground", "high" and "low", or "+" and ` - . Balanced
connections require three terminals, with ground being
separate from the high and low. In preparing balanced cables,
it is wise to make an ohmmeter check to insure that neither
signal lead has been inadvertently shorted to the cable shield
or the shell of either connector. The positive and negative
terminology often used on balanced units refers to the
polarity of a signal at any instant, allowing the user to tell if
a signal is being reversed in phase in the equipment. This
becomes important in multiple channel systems where the
same phasing between reproducers must be maintained.
SIGNAL PATH
A- Unbalanced ouput to
unbalanced input. Single
conductor shielded cable.
B- Unbalanced output to
balanced input. Single
conductor shielded cable.
C- Unbalanced output to
balanced input. Two
conductor shielded cable.
Preferred circuit.
"D- Balanced output to
unbalanced input. Single
conductor shielded cable.
Circuit F, showing a completely balanced input and output
provides the best interconnection and should always be
selected, if compatible with the equipment involved.
It is desirable to place all line level equipment at one physical
location, such as in one or more adjacent equipment racks,
thus minimizing connecting cable lengths and providing good
grounding. This is helpful in minimizing interference such as
SCR hash and RFI (see "Definitions").
One or two-conductor (as required) shielded cable should
normally be used for all line level interconnections. However,
because the voltage level is typically 40 dB greater than in a
low-Z microphone circuit, noise pickup is somewhat less
troublesome. In fact, unshielded terminal strips can be
employed as connectors, particularly when fed from a low
internal impedance device. The presence of large grounded
areas of metal in the vicinity, such as the chassis, adjacent
equipment, equipment racks, etc., also help to minimize any
electrostatic interference pickup by exposed signal terminals.
Interference Pickup
Interference from the outside enviroment (as opposed to
hum and hiss generated within the equipment) may include
hum (usually picked up from nearby power wiring) and radio
frequency interference such as AM or FM broadcasting,
citizens band transmitters, television sync buzz, or X-ray
diathermy equipment. Also, it may include the sharp buzz
generated by nearby lighting control dimmers, i.e. SCR hash.
A system may pick up interference in three ways: 1 ) electrostatic coupling, 2) electromagnetic induction, and 3) ground
l oop conduction.
Two conductors in space exhibit an electrical capacitance
between them, which increases as their area becomes greater
or as they move closer together. Electrostatic pickup occurs
by means of this capacitance (capacity coupling, if you will).
It can be largely eliminated by placing a grounded metallic
shield between them, or around either one.
`E- Balanced output to
unbalanced input. Two
conductor shielded cable.
Perferred over D above.
F- Balanced output to
balanced input. Two
conductor shielded cable.
" Use dashed line only if output is
floating. Consult installation manual.
FIGURE 5 - Interconnecting Circuits
Some equipment is designed to allow either balanced or
Unbalanced operation. In other instances, transformers may
be added as an outboard module or as a plug-in option.
Figure 5 illustrates some typical connections. If hum is
i ntroduced using circuit 13 (as when the two pieces of equipment are separated by a long distance), circuit C can be
used to avoid noise from ground currents in the shield.
Similarly, circuit E may be used to advantage, but only if the
output is not referenced to ground (for example, an
ungrounded transformer secondary). Note that in circuits C
and I:, the "low" signal wire and the shield braid must be
tied together only Lit one end, as shown. Otherwise, ground
and signal currents will be combined in the signal path
return- defeating the advantage of these circuits.
When two insulated wires are run next to each other for a
distance, and an AC current is caused to flow through one of
thetas, an AC voltage will appear across the ends of the other
wire. This effect is called electromagnetic induction. The
amount of energy transferred is a function of' the mutual
i nductance between the wires, which increases with closer
spacing between wires or with greater length. When the wires
are wound into coils, the mutual inductance is further
i ncreased. This is why signal transformers are so susceptable
to external magnetic fields. To reduce the coupling into (or
from) a wire or coil, a magnetic shield must be employed.
Such a shield is made of a magnetic material such as soft
iron, mu-metal, or a number of special alloys Inanufactured
for the purpose. The shield should enclose the part, forming
a closed cylinder or box around it. If this magnetic shield is
grounded, it may also serve as an electrostatic shield at the
same time. Magnetic shielding is ordinarily required around
all microphone and line level signal transformers. Even so, it
may be necessary to separate them from equipment produc
ing high magnetic hum fields. II' such a problem exists Ill an
equipment rack, try increasing the spacing between low level
and high power equipment. Fven five inches can often make
a substantial improvement. For the same reason, avoid
running signal and power cables in the same bundle or
conduit.
CHART 2 - 2-Wire Copper Cable Lengths For 0.5 dB Loss in SPL
Load Impedance
Unlike microphone circuits, a power amplifier should be
matched to its rated load impedance by a factor of two to
one, or closer. Too high a load impedance will simply reduce
the amount of power which the amplifier can supply. Too
low an impedance may not only reduce available power, but
may cause protective devices (fuses, thermal cut-offs, etc.) to
open, interrupting system operation. Increased distortion
can also result. Some amplifiers provide multiple taps to
match a variety of load impedances, while others are
designed for only one, or a narrow range of values. The
i mpedance of a typical transducer varies widely with respect
to frequency, but the nominal rating is usually close to the
minimum value obtained within its normal operating range.
When more than one transducer is to be driven from a single
amplifier, the effective load impedance may be altered, as
shown in Figure 7. Parallel connection of two eight ohm
transducers would match a four ohm amplifier output, for
example. The series connection should be used only with
identical units, which are to be driven at equal power levels.
Polarity
It is important that the loudspeaker systems in a multiple
array be connected in phase with each other. If one or more
units is reversed in polarity from the others, frequency
response, polar pattern, and/or sound levels may be adversely
affected.
Output Impedance Matching Transformers
Some installations require a very large number of speakers to
be operated at a relatively low level. In such applications,
the 70 volt speaker line can be utilized to advantage, and is
connected as shown in Figure 8. The amplifier is designed so
that it will deliver 70 volts (RMS) of signal when adjusted for
full power output. A transformer, with multiple taps to
adjust level, is associated with each speaker. Often, these
taps are labeled directly in watts corresponding to maximum
amplifier output (70 V RMS). In this way, the level of each
speaker may be independently adjusted. Each transformer
need handle only the power drawn by its associated speaker.
There is no limit to the number of speakers which can be so
connected, as long as the sum of their power settings does
not exceed the amplifier's rating. Not all power amplifiers
operate satisfactorily into a transformer load. Therefore,
unless a 70 volt output is specifically provided, the manufacturer should be consulted as to its suitability for this
application.
Z = Impedance of each
transducer
FIGURE 8 -Typical 70 Volt Line Connection
N = Number of transducers
SERIES - PARALLEL CONNECTION
( FOR FOUR TRANSDUCERS)
FIGURE 7 - Loudspeaker Connections
Page Five
DEFINITIONS
Decibel (dB)
Fundamehtally, a unit of loudness. However, it is convenient
to express ratios of electrical voltage, current, or power in
terms of dB. For example, a 6 dB voltage increase to a
speaker voice coil would produce a 6 dB increase in sound
level, regardless of the starting point.
A 3 dB increase
represents the doubling of power, and a 6 dB increase, a
doubling of voltage. Voltage gain of equipment is often
expressed in dB.
dBv
The electrical voltage level compared to a one volt reference
level. Thus, -6 dBv corresponds to 1/2 volt, +6 dBv to 2
volts, or +12 dBv to 4 volts.
dBm
The electrical power level compared to a reference level of
one milliwatt. If dBm is used to indicate a voltage, the
For
circuit impedance must be stated or understood.
example, in a 600 ohm circuit, 1 mw = 0 dB = .775 volts.
Thus, at 600 ohms, a level expressed in dBm is always 2.2 dB
greater than if expressed in dBv.
Active Devices
Devices requiring operating power (battery or other) in
addition to the signal. Usually contain transistors, tubes, or
IC's. Includes amplifiers, mixers, equalizers, etc.
Passive Devices
Devices requiring only signal power, and containing only
resistors, capacitors, transformers, etc. Includes dynamic
microphones, high level crossovers, speakers, etc.
Impedance
In an alternating current circuit, the ratio of the voltage to
the resulting current which it causes to flow. The counterpart of resistance in a DC circuit.
Ground Loop
A condition existing when components in a system are tied
to each other or to ground with more than the minimum
number of wires to accomplish the connection. The duplicate
paths can form a loop, which may allow circulating interference currents to flow, resulting in possible hum and noise.
RFI
Radio Frequency Interference. High frequency signals may
enter at various points in the PA system. They include AM
and FM broadcasts, television (continuous buzz), citizens
band, nearby radar, and diathermy.
SCR Hash
Silicon controlled rectifiers, triacs, and various other semiconductors are finding increased application in light dimming
and motor speed control. They generate extremely sharp
wavefronts which, like RFI, can result in audible background
buzz in sound systems if proper precautions are not exercised.
Signal
An AC voltage or current, representing the desired input to
the system. The signal is amplified and conditioned by
various system components, and appears at the output.
NOTE TO THE READER
We hope this addition of the "PA Bible" has shed some light
on the diverse subject of interconnections. We appreciate
receiving your comments and suggestions. Send them to
E-V "PA Bible"
Electro-Voice, Inc.
600 Cecil Street
Buchanan, Michigan 49107
600 Cecil Street, Buchanan, Michigan 49107
8234 Doe Avenue, Visalia, California 93277
Electro-Voice Div., 345 Herbert Street,
Gananoque, Ontario - Electro-Voice, S.A.,
Romerstrasse 3, 2560 Nidau, Switzerland
Litho in U.S.A.
Form 1919-026
Page Six
System Grounding
The term "ground" refers to conductors with a common
potential. Usually, it refers to any connection to the chassis
or external metal cabinet of a piece of equipment, with the
ground terminals on each piece of equipment tied together
forming a common "system ground." An "earth" ground is
the zero or reference potential of moist earth as measured in
a system of conductors buried in the earth. The metal pipes
of a water supply system usually provide an excellent earth
ground.
The ground, or third, wire of a power line also provides an
approximation of an earth ground. However, due to ground
currents flowing through the finite resistance of such wires,
no two points are usually at exactly the same potential. For
this reason, ideally, the system ground should be tied to an
earth ground only at one point. In an enviroment where
strong RF signals are present, additional care in grounding
The sequence in which these
may prove beneficial.
connections are made can be important in preventing the
formation of a ground loop, which may introduce hum and
noise into the system.
Various safety requirements mandate that most professional
equipment be supplied with a three wire power line cord.
This connects each of the various chassis to the power line
ground, and largely eliminates the possibility of incurring a
potentially lethal shock when touching a defective piece of
equipment and a true ground at the same time. Unfortunately, it can also complicate the elimination of ground loops.
This subject becomes quite involved, and will be discussed in
a later supplement.
Crossovers
It is sometimes advantageous to divide an electrical signal
into two (or more) frequency bands and connect each band
to a transducer optimized for a limited frequency range. The
device which divides the signal into these appropriate
frequency bands is called a crossover. A crossover may be
connected between the power amplifier and reproducers as
shown in Figure 6. Passive circuitry, consisting chiefly of
capacitors and inductors, is employed in a high level crossover, so named because of its circuit location following the
power amplifier. In some instances, it may be built into the
transducer or into the system enclosure.
Low level crossovers, on the other hand, may be either active
or passive, and are wired as shown in Figure 6. The low level
crossover system offers a number of advantages over high
level crossovers, but requires additional power amplifiers.
The term "low level" indicates a crossover operating at line
level in front of the power amplifiers.
POWER AMPLIFIER CONNECTIONS
The signal input to most present day power amplifiers is
considered to be line level, requiring in the region of 1/2 to 2
volts to drive the unit to its full output. Power amplifier
output levels are much greater than those at line level; and,
in addition, the impedance is considerably lower. These
factors combine to eliminate the need for shielding of the
output circuit wiring. To avoid excessive power losses,
however, a much heavier gauge of wiring must be employed.
Two distribution methods are in use to deliver the audio
power from the amplifier to the speakers. Where it is
necessary to adjust the comparative loudness level of a
number of speakers, a 70 volt line distribution system is
popular. Let us first consider the other method, however, in
which a direct connection from amplifier to voice coil is
used. Since the impedance of a horn driver or speaker is
typically only eight ohms, any small resistance (impedance)
in the wiring, which is effectively in series with the load, will
result in a voltage drop and hence a power loss. Unlike most
transducers, the impedance of this wiring is almost identical
with its DC resistance, so these terms may be used some-what
interchangeably. Also, readings from an accurate low-range
ohmmeter are valid for checking losses. To measure wiring
resistance, remove system power, place a heavy jumper wire
across the load, and after disconnecting at least one lead
from the amplifier output, measure the resistance at the
amplifier end of the wiring. As an extreme example, wiring
having a 3.3 ohm resistance, driving an 8 ohm load, will rob
half (3 dB) of the amplifier power output. This power loss
shows up as a slight heating of the wires. A more reasonable
design would allow a power loss of 1/2 dB (approximately
12%). Total resistance with an 8 ohm load would then be
held to less than 1/2 ohm.
Wiring resistance should vary with load impedance, with a 4
ohm circuit requiring heavier conductors, and with proportionately relaxed requirements for 16 ohms. A wire table
(Chart 2) tells us that for a distance of forty-eight feet
between amplifier and a 4 ohm load, 14 gauge wire is
required. This wire may be either solid or stranded copper,
depending upon flexibility requirements. Of course, halving
the distance will also cut resistance in half, just as doubling
it will increase resistance by a factor of two. In calculating
resistance using Chart 2, do not forget that since there are
two wires in the circuit, the total wire length is twice the
distance between the components. For permanent installations, 12-2 or 14-2 house wiring is popular, while the flexibility of 14 or 16 gauge "zip cord" is favored for portable
setups.
FIGURE 6 - Crossovers
Obviously, the terminals used in these low impedance circuits
must not contribute appreciable resistance. For permanent
installations, solder lugs, binding posts, and heavy screw
terminals are ideal. For portable equipment, banana plugs
and phone plugs can also be used.
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