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A plate amplifier is a power amplifier that is built on a flat board for installation inside of a speaker. Like any power amplifier, a plate amplifier takes line-level signals and increases their power level to be able to drive a speaker. Unlike a standard musical amplifier, many plate amps have limited frequency response, befitting their typical use as a subwoofer amplifier.
Plate amplifiers typically have a face plate with inputs, outputs, controls, and a power connection, along with exposed components on their other side. They are usually designed to be mounted inside the enclosure of a subwoofer, which is a specialized speaker designed to reproduce bass and subsonic frequencies. While store-bought subwoofers include their own amplifiers, hobbyists who build their own subwoofers cut a hole in the back panel of their unit and screw the plate amplifier into it. This hides the plate's exposed components while leaving its controls and inputs available. Typically the only connections on the interior side of the plate amplifier is a pair of speaker terminals which are used to connect the amplifier's main output to the subwoofer driver.
Power ratings for plate amplifiers vary greatly. While most of them are relatively powerful due to the large quantities of power necessary to drive a subwoofer, their output power can range from around 80 to 500 watts in output power. One unifying feature is that while musical amplifiers attempt to reproduce a frequency range from 20 to 20,000 hertz so that they can cover the entire range of human hearing and music tones, plate amplifiers usually roll off their response on frequencies over 200 hertz. This limits them to use with subwoofers, since they typically do not reproduce frequencies above 100 to 150 hertz.
Most plate amplifiers also contain a range of inputs and controls which are more elaborate than a typical power amplifier. They have crossovers, which serve to strip higher frequencies out of the signals that they amplify and usually allow the end user to vary the crossover's active point. Plate amplifiers can typically accept both line-level and speaker-level signals and can then send those signals back to a component or speaker with the bass portion removed. Many plate amplifiers also contain phase or delay controls, allowing the bass signal to be altered to better match up with a given speaker system. Finally, they contain volume controls which allow their output level to be modified to better blend with the sound system's response.
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