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The difference between low and high level
crossovers
Crossover networks can be placed in either of two locations
in the signal path of a sound system. The most common
loca-
tion for a crossover in consumer "hi-fi" speakers or packaged
speaker systems is inside the speaker enclosure. In this case,
the crossover network divides the output of the power ampli-
fier just before it is fed to the individual drivers. Such cross-
over
networks
are called
"high
level"
since they divide
speaker-level signals. They are also called "passive" because
they require no power source to operate.
The circuit of a passive, high-level crossover network typical-
ly consists
of inductors
(coils)
and
capacitors,
but
no
transistors or integrated circuits. When high power operation
. is involved, the coils in the passive crossover can heat up, in-
crease their resistance, and create noticeable power loss to
the drivers; larger, more expensive coils help, but don't elimi-
nate the losses. Good quality, passive high-level crossovers
can cost several hundred dollars, weigh 10 to 25 pounds each,
and one such unit wil! be required for each speaker enclosure.
The other focation for a crossover network is just before the
power amplifiers. In this case, the crossover network divides
the output of the console (or other line-level signal source)
just before it is fed to two or more channels of power ampli-
fier, which, in turn, feed the speakers (drivers) in different
bands
of a speaker
system. Such
crossover
networks
are
called "low level" since they operate with line-level signals
rather than with the output of power amplifiers.
While low-level crossovers can be passive, most are "active,"
which means they require a source of power for their internal
circuitry. Since the expression
"active, low-level crossover
network"
is awkward,
the term
"electronic crossover"
is
more widely used.
When fed by alow-level crossvoer network (passive or active),
each power amplifier is used to drive just one band of fre-
quencies — one type of driver. In a two-way speaker system,
for example,
one amplifier is fed bass frequencies, and its
output goes to the woofers, while another amplifier is fed
higher frequencies,
and
its output
goes to high frequency
compression drivers,
This approach, using a low-level crossover to feed different
bands
of frequencies to two or more
amplifiers
is called
"multi-amping." Specifically, a two-way multi-amped system
is said to be "biamplified," a three-way system is "triampli-
fied," and so on. There are numerous advantages to multi-
amped systems, as compared to systems where a single ampli-
fier is fed full range signals and a high-level, passibe crossover
network divides the amplifier output; the sound quality can
be improved,
overall
amplifier power requirements can be
less for a given amount of headroom, and total system cost
and weight can be reduced. These benefits are especially im-
portant with large-scale sound systems.
The pros and cons of low level and high
level crossovers
Passive, low-level crossovers are constructed with small induc-
tors and capacitors in a metal box with connectors on it. The
major drawback of passive low-level crossovers is their fixed
characteristics;
neither
the
crossover
frequency
nor
the
"slope" (the rate at which the internal filters rol! in and out,
in dB per octave) can be changed easily, unless many more
components are added, which eliminates the cost advantage
of the passive circuit. Passive low-level crossovers are used
primarily where they are designed to complement a specific
speaker system; since exact characteristics are known, no ad-
justability is required.
More often, however, low-level crossovers rely upon active
electronic circuits to serve as filters. Because an IC, a capaci-
tor and a resistor (plus a few peripheral components)
can
serve the same electronic function as a inductor, electronic
crossover networks do not need to use inductors. Without
inductors,
it becomes
feasible
to provide
an
inexpensive
control that alters the value of the electronic "inductor equi-
valent" circuit. This makes it relatively simple to adjust the
crossover frequency, as well as to alter the slope or steepness
of the filters.
i
Large, low-loss inductors of copper wire (the type used in
good high-level crossover)
are very expensive. This is why
'smaller, iron-core copper inductors or air-core copper induc-
tors of marginal size are often used in commercial
high-
level crossovers — they save money. Such inductors produce
greater distortion at high power levels. Worse yet, if the sig-
nal level exceeds that for which the crossover was designed,
the inductors can saturate; they stop being inductors, cross-
over characteristics are suddenly and drastically altered, and
the result may be severe distortion, and even "blown" drivers.
Using a low-level crossover (passive or active) instead of a
high-level crossover
network avoids these potential sources
of distortion and driver failure. Moreover, by using a low-
level crossover, the series resistance of the large inductors in a
high-level crossover is removed from the signal path to the
drivers
so the
amplifier output
impedance
is no
longer
"swamped" and the effective damping factor increases.
26
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