Amplifiers in this term are Headphone amplifiers in common.
A headphone amplifier is an audio amplifier designed particularly to drive headphones instead of loudspeakers. Most commonly they are found embedded in electronic devices such as integrated amplifiers, portable music players and televisions, but standalone units are not uncommon.
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 greater audio clarity than the 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.
Effectively, a headphone amplifier is a small power amplifier that can be connected to a standard headphone jack or (usually) the line output of an audio source. The headphone amplifier improves the sound quality by increasing the amount of power available to move the transducer, increasing the control that the source has over just where the transducer is in space.
Many headphone amplifiers have an output impedance in the range of 20 – 50 Ohms. The 1996 IEC 61938 standard recommended an output impedance of 120 Ohms. The standard included a note that “For most types of headphones, the source impedance has very little effect on the performance.” In 2008 Stereophile published an article that showed that a 120-Ohm output impedance could cause a 5-dB error in frequency response with certain headphones. The article concludes that the effect of output impedance on frequency response is “non-trivial”. Some newer headphone amplifiers have output impedances that are less than one Ohm.
Low output impedance can also reduce distortion by improving the control that the source has over just where the transducer is in space. This is often expressed as damping factor. For example, a 32 Ω headphone driven by a popular DIY headphone amp with a <1 Ω output impedance (the Gilmore Dynamic) would have a damping factor of >32, whereas the same headphone driven with an iPod (5 Ω output impedance) would have a damping factor of just 6.4. If the 120 ohms recommendation is applied, the damping factor would be 0.26.
Of course, output impedance is not the only specification relevant to choosing a headphone amplifier — THD, frequency response, IMD, output power, minimum load impedance, and other measurements are also significant.
The DIY approach
For those who are more electronically inclined, the low-power and fairly simple nature of the headphone amplifier has made it a very popular DIY project. There are a great many designs for headphone amplifiers posted on the Internet varying considerably in complexity and cost. The cardinal example is the simple opamp-based Cmoy design, arguably one of the most popular headphone amplifier designs available. The simplicity of the Cmoy makes it an easy build, while it can be made small enough to fit inside a tin of breath mints (including batteries).
Crossfeed and other audio processing
Crossfeeding blends the left and right stereo channels slightly, reducing the extreme channel separation which is characteristic of headphone listening and is known to cause headaches in a small fraction of listeners. Crossfeed is used to compensate for extreme separation of sound sources in older stereo recordings. Crossfeed also improves the soundstage characteristics and makes the music sound more natural, as if one was listening to a pair of speakers. While some swear by crossfeed, many prefer amplifiers without it. The introduction of DSP technology led a number of manufacturers to introduce amplifiers with ‘headphone virtualization’ features. In principle, the DSP chips allow the two-driver headphone to simulate a full Dolby 5.1 (or more) surround system. When the sounds from the two headphone drivers mix, they create the phase difference the brain uses to locate the source of a sound. Through most headphones, because the right and left channels do not combine as they do with crossfeed, the illusion of sound directionality is created.
Taken from : Wikipedia.org