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What Is a Power Amplifier?

By W. Joyner
Updated May 16, 2024
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A power amplifier is an electronic device that receives an electrical signal and reprocesses it to amplify, or increase, its power. The boost in power is achieved by significantly increasing the input signal’s voltage. A power amplifier is used to power an output source, such as a stereo speaker, a relay or a motor.

One of the most common functions for a power amplifier is in audio applications. An audio amplifier is designed to deliver a signal to a speaker. In large systems, the electrical signal usually comes from an audio mixing board or a preamp component. Power amplifiers are also found in small systems, such as typical home stereo setups.

The three main components of power amplifiers are the power supply, the input stage and the output stage. The power supply receives alternating current (AC) from an electrical outlet and converts it to direct current (DC). The power supply sends the DC signal to the input stage, where it is processed and prepared for the output stage. The signal is then transferred to the output stage, where the actual amplification of the signal takes place. The output stage is connected to the speaker.

Power amplifiers are rated in terms of power units known as watts. The rating is referred to as the amplifier's power rating. The power rating reflects the maximum output that the power amplifier is capable of producing.

The majority of modern power amplifiers are transistor-based models. There are amplifiers available that use vacuum tubes for their internal circuitry. Transistor-type amplifiers are more commonplace because they are more efficient and more cost effective. They also generally are smaller than their tube-style counterparts and therefore more easily transported.

Power amplifiers are divided into categories known as classes. An amplifier’s class is a reflection of the design of its circuitry. In audio applications, the class of the amp is determined by its output stage.

Most power amplifiers used in audio systems are of the A, B or AB classes. Class A amplifiers are distinguished for having very low levels of distortion. A class B amp usually costs less than a class A amp and generally is more efficient in overall operation. The quality of the output is not as good, however, particularly at low signal levels.

The class AB amplifiers are an attempt to combine the best of both worlds. They are less expensive than class A models but provide a signal that is superior that that provided by class B amps. Class AB is possibly the most widely used amp in home stereo systems.

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Discussion Comments
By Melonlity — On Jan 13, 2014

Here's something I didn't know about power amps -- one is almost necessary if you're running a record player (old school, I know) into a modern surround sound system. It seems stereo receivers built with record players in mind boosted those signals for you, but modern surround sound receivers do not. Luckily, a good amp for your record player is small, unobtrusive and doesn't cost much.

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