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Headphone amplifier
A headphone amplifier is a low-powered audio amplifier designed particularly to drive headphones instead of loudspeakers. Most commonly they are found embedded in electronic devices that have a headphone jack, such as integrated amplifiers, portable music players (e.g., iPods), and televisions, but standalone units are used, especially in music studios and in audiophile markets. Headphone amplifiers are available in consumer grade models and professional audio models, which are used in recording studios.
==Consumer models==
Headphone amplifiers as referred to here are commercially available separate devices, sold to a niche audiophile market. These devices allow for higher possible volumes and superior current capacity compared to smaller, cheaper amplifiers that are used in most audio players. In the case of the extremely high-end electrostatic headphones, such as the Stax SR-007, a specialized electrostatic headphone amplifier or transformer step-up box and power amplifier is required to use the headphones, as only a dedicated electrostatic headphone amplifier or transformer can provide the voltage levels necessary to drive the headphones. Most headphone amplifiers provide power between 10 mW and 2 W depending on the specific headphone being used and the design of the amplifier. Certain high power designs can provide up to 6W of power into low impedance loads, although the benefit of such power output with headphones is unclear, as the few orthodynamic headphones that have sufficiently low sensitivities to function with such power levels will reach dangerously high volume levels with such amplifiers.
Effectively, a headphone amplifier is a small power amplifier that can be connected to a standard headphone jack or the line output of an audio source. Electrically, a headphone amplifier can be thought of as an amplifier that presents a very high input impedance (ideally infinite) and presents a lower output impedance (ideally zero) and larger range of output voltages (ideally infinite). This allows headphones of a low sensitivity to be driven louder as a result of the extra voltage provided by the amplifier. There are potential fidelity gains if headphones are driven with lower distortion than using a headphone amplifier integrated into a general purpose audio product. In practice, this most often occurs when using low impedance headphones with consumer electronics with insufficiently low output impedance (see impedance discussion below).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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