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In order to grasp
a full comprehension of the concept of biamplification/
triamplification, it is necessary to spend some time discussing
exactly what a crossover is and the function of a crossover in the
system chain.
Crossovers
There are 2 types
of crossovers: passive and active. A passive crossover is a simple
network that handles high signal levels and is located in the system
chain after the amplifier and before the loudspeakers. It filters the
audio spectrum into separate bandwidths for the driver designed to
reproduce that specific frequency range. For example, the passive
high level crossover takes a signal and sends the low frequencies to
the woofer, mid frequencies to the midrange driver and the highs to
the tweeter. The number of sections it breaks the signal into
classifies the passive crossover. (2-way, 3-way, etc.) Technically
speaking, the passive crossover is composed of a number of band pass
filters. (This number corresponding to the amount of sections) In the
case of a 2-way high level passive crossover, a low pass (high cut)
filter slopes or rolls off the high frequencies and a high pass (low
cut) filter cuts out the low frequencies. The intersecting point
where the two frequency response curves meet is known as the
"crossover point." (See diagram below) At the crossover
point where the levels gradually decline to the center, the level is
lower than the initial level. Ideally the crossover is designed to
defeat this dip, when the two levels add together and compensate for
the 3 dB loss. (3 dB in this example) Since a 3 dB change is
equivalent to a doubled power increase, when the two levels overlap
and add together, the dip in the frequency response will be
eliminated. The rate that the level to each driver rolls off is
called the "slope" of the crossover. Common slopes in
crossovers are 6, 12, 18 and 24 dB per octave. The most popular are
12 & 18 dB per octave. Passive crossovers are normally enclosed
in loudspeaker cabinets. (Some are externally mounted)
An active
crossover is a complex network that handles low signal levels
(milliwatts) and is located before the amplifiers as opposed to after
the amplifiers such as the case of the passive crossover. The job of
the active crossover is to filter specific bandwidths and send them
to a number of amplifiers. (The number corresponding to the number of
sections) The active crossover employs the same basic principle as
the passive crossover in regards to band pass filters but also
includes a built-in line amplifier to compensate for the loss in
filter networks. Some systems combine active and passive crossovers
as well. For instance: given a 2-way active low level crossover is
used to filter the mids and high frequencies from the low
frequencies. The low frequencies would be filtered and directed to
the woofer. (Driven by one amplifier) The mids and highs would be
filtered by the active crossover and directed to the passive
crossover. There the appropriate bands would be sent to the drivers
designed to work with the specific frequency range they were given.
(Driven by a second amplifier)
What is
Bi-amping/Tri-amping and What are the Advantages?
Biamplification
(using a 2-way active crossover) and triamplification (use of a 3-way
active crossover) involves using a number of amplifiers, each
assigned to a certain frequency bandwidth through the use of an
active crossover. (Or many in larger applications) Passive and active
crossover combinations can even allow for 4-way triamplified systems
(the "bi" and "tri" do not indicate the number of
sections in the speaker, but refer to the number of amplifier
sections handling various frequency bandwidths.)
Bi-amping and
tri-amping is done for a number of various reasons:
1)To reduce
distortion- High-energy bass, especially in modern music requires a
considerable amount of amplifier power. Low frequencies use up most
of the amplifier power, leaving none for the high frequencies. When
an active crossover is used, the high frequencies are driven by a
separate amplifier, therefore eliminating distortion and increasing
the headroom of the system.
2)To increase
headroom- Bi-amping / tri-amping increases available headroom in the
system, (the difference between the clipping point and the nominal
level of a system) thus preventing severe distortion.
3)To increase
efficiency- Passive crossovers use resistors, capacitors and
inductors because they are designed to withstand the high voltage
output of the amplifier(s). These electronic devices consume power in
the system. Using an active crossover eliminates this problem,
therefore rendering the system more efficient. (Allows a higher sound
level for the same amount of power!)
There are also
side benefits for bi-amping/tri-amping as well. Distortion in the
system can wear away at the diaphragms in the speakers. A clipped (or
"unclean") signal stresses the speaker system by
overworking the diaphragm. The voice coils also start to burn out
when clipping occurs and it is also extremely hard on the amplifier.
By eliminating distortion through the use of bi-amping/tri-amping,
these conditions do not occur and your gear will last longer. If you
build your own speaker systems, you will need to include a passive
crossover in each cabinet, where in bi-amping/tri-amping, only one
active crossover is required, thus cutting costs. (Ok, so active
crossovers are more expensive, but in larger applications, one can
save a bundle by using an active crossover!) As well, when passive
crossovers are used, they increase the output impedance of the
amplifier and thus lower the amplifier's damping factor. (Load
impedance divided by amplifier's actual output impedance) In most
cases a higher damping factor improves sound quality, because it
allows for greater control over the speaker diaphragm. An active
crossover does not lower an amplifier's damping factor.
Please note that
bi-amping/tri-amping is a lot more expensive than using a
conventional "fully passive" full range system, and this
added expense and setup time may be weighted more of as a
disadvantage than it is worth for some DJ applications. My intent in
this article is to point out the advantages to bi-amplification for
those who do not employ this procedure in their setup.
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