While a monophonic, or single-channel, power amplifier takes one line-level signal, like the signal from a Blu-ray™ player, and increases its power so that it can drive a speaker, a three-channel amplifier magnifies three signals at once. The three-channel amplifier occupies a middle space between a two-channel stereo amplifier and a five- or seven-channel surround sound amplifier. It brings the cost and space benefits of having a single component that drives multiple channels while not driving so many channels as to cause performance to diminish.
Like any external power amplifier, a three-channel amplifier should offer better sound than amplifiers built into a receiver. Three-channel power amplifier components are designed to do only one thing — take small signals and make them larger — and are usually built for this one task. To this end, they have purpose-built power supplies which can generate large quantities of clean power to drive speakers to high levels as cleanly as possible. Their three channels of amplification allow them to work with a two-channel stereo amplifier in a five-channel surround sound system.
The most typical use of a three-channel amplifier is in a home theater system that already has a receiver. By connecting the receiver's preamplifier-out terminals for the left, center, and right front channels to the amplifier, the most musically important channels get the benefit of the external three-channel amplifier. The receiver then only has to drive the rear left and right surround channels. This makes it easier for the receiver to deliver its rated power because its power supply is not being required to reproduce five channels of sound. Also, since most receivers have sound quality that is inferior to external power amplifiers, this set-up ensures that the only channels that are handled by the receiver are those which are least active and important.
Three-channel amplifiers have uses beyond home theater applications, though. Some audio enthusiasts believe that separately amplifying every driver in their speakers improves sound quality. Speakers with a bass driver, mid-frequency driver, and high-frequency driver require a single three-channel amplifier to "tri-amp" them. An audio system configured in this way requires two three-channel amplifiers to drive a stereo pair of speakers. Many high-end car audio systems use these amplifiers for component speaker systems which separate bass, mid-range and high-frequency drivers.