Bi-amplification isn't much of a word to begin a column. It's not as short as stereo or snappy as quadrophonic; it threatens a contorted technical explanation that'll elicit groans from the bravest reader; and it sounds expensive.

The traditional audio entertainment formats are monophonic, stereophonic, and quadrophonic. Each is associated with the number of speakers used to reproduce the sound: mono retrieves the signal from a singlechannel record, amplifies it with a power amplifier that drives one speaker; stereo retrieves two signals from a two-channel record, amplifies each of the signals with independent amplifiers, each of which drives one speaker; quad records have four channels, quad amps have four power sections, and four speakers are used to reproduce the total quad sound.

The catch with all this classical simplicity is that inside each speaker is more than one speaker. Each speaker in a hi-fi system is a box containing one or more component speakers. The typical "speaker" contains a bass speaker to reproduce the low frequencies, a mid-range speaker to reproduce the middle freqnencies, and a tweeter to reproduce the high frequencies. A cross-over network is located, at the speaker's connecting terminals. The signal from the power amp is fed to the cross-over which divides the signal into three and sends a portion of the signal to each of the three internal speakers.

The cross-over network is a series of filters which limit the frequencies delivered to each speaker. The cross-over filters out low frequencies from the signal sent to the* tweeter, very high frequencies from the signal sent to the mid-range speaker, and high frequencies from the signal sent to the bass speaker. _

A speaker isn't a wooden box that talks. It translates electronic impulses corresponding to portions of the audio frequency into waves of air which we hear as sound. The audio frequency is between 30 and 15,000Hz. Bass sounds are from about 30 to 400 cycles per second, and high frequencies are above that to the point where they are so high pitched as to be inaudible/The signal from the power amplifier contains all the bass, mid-range, and treble signals of that channel. The speaker's cross-over network divides them so a portion can drive each of the internal component speakers.

Bi-amplification puts the cross-over network at an earlier stage in the process. Instead of amplifying the total signal and then dividing it at the speaker, bi-amping divides the signal before it is amplified. A stereo system is a record player, a stereo pre-amp and power amp (usually integrated into one component), and two speaker boxes. A bi-amped stereo system is a record player, a stereo pre-amp, a cross-over network, two stereo power amps, and two speaker boxes. The signal is retrieved from the record and fed to the pre-amp in both cases. The pre-amp allows the user to shape the sound with bass, treble, and volume controls..

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In sterio the signal is fed from the pre-amp" to the power amps, each channel being amplified by a single power amp! In bi-amping the signal from the pre-amp goes to the crossover network which divides each channel's signal into two parts: bass frequencies and mid-range to treble frequencies. Each of these two signals is fed to a seperate power amplifier, so a bi-amped stereo system requires four power amps: 1) left channel bass frequencies;2) left channel mid-fange and up frequencies; 3) right channel bass frequencies; 4) right channel mid? range and up frequencies. The output from these amps is fed to the associated speakers. The bass signals for the left and right channels are fed directly to the bass speakers of the system, the higher frequencies are fed to the midrange speakers and tweeters.

The reappearance of mid-range is the result. Because the bass is not distorting the smaller; high frequency speakers, but is instead pumping along out of its own speaker from its own amp, the bi-amped system produces a much clearer, more panoramic sound.

It's almost as expensive to set-up a£ a quad system, though it's still stereo. The initial extra investment to upgrade a stereo system by bi-amping as an additional stereo power amp ($300 up), a sensitive cross-over network ($70 to $400), and modifications on the speaker boxes^to feed the bass signal direct to the bass speaker ($20 or so)..

Bi-amplification is an important step towards the absolute sound. Used with the proper components, bi-amping increases the clarity and spread of the reproduced sound. It suggests that stereo, in this and more advanced states, cap provide more audio magic than quadrophonic. The basic idea is like the Sensurround effect used in theaters during the showing of Earthquake. The bass frequencies, which are more p5umping air vibrations and bottomy thunder than they are noise, are amplified seperately, often into bass speakers designed to just blast out bass. The mid-range and trdble are then left to provide the more audible, listenable rather than felt portions of the sound.