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Why Bi-amp?

Chad Oslach - DJzone staff writer

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.


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. I hope it was useful for you!





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