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The phrase external amplifier has two connected uses. The first is to describe an audio system using passive speakers, which do not have built-in amplification. This means the audio signal must be amplified by a separate device before being carried to the speaker. The second use is to refer to a device that carries and processes the audio signal from multiple audio-visual products such as DVD players or televisions and relays it to speakers. Although such devices usually include an external amplifier, the device itself is usually referred to as a receiver.
The need for an external amplifier comes about because there are two types of loudspeakers, active and passive. An active speaker, sometimes known as powered or self-powered, has a built-in amplifier. This means it can be connected directly to an audio source such as a record player. The main benefits are that the speaker has a more powerful output and can cope with a wider range of volume. The biggest drawbacks are that such speakers need their own dedicated power cable and that they tend to be much heavier. They are also be too expensive for home and non-professional settings.
Most home audio systems and small-scale public address systems instead use passive speakers. These require a separate amplifier that amplifies the audio before it is passed on to the speaker. With hi-fi systems, this is usually built into the unit that houses the various components such as CD player, tape deck and radio. With home cinema, the external amplifier is usually part of a separate device, a receiver, that allows the user to switch between the audio signals of home entertainment equipment such as televisions, cable boxes or games consoles.
Strictly speaking, there is some confusion between the concept of passive and active speakers, and the concept of powered or non-powered speakers. This confusion can mean the uses of the various phrases involved is not always accurate. The confusion lies in the fact that there are two separate operations that can be carried out either in the speaker or in a separate device.
The first of these operations is the amplification of the sound, which is covered by the terms powered and unpowered. The second operation is the splitting of the audio signal into separate frequencies, which are produced by separate parts of the speaker, often known as the tweeter and the woofer. An active speaker system splits the signal before it reaches the speakers, while a passive speaker system leaves the splitting to the speaker itself. It is possible to have a speaker which is powered but passive: the most common example is the small speaker systems used for listening to portable music players without headphones.