A headphone amplifier is a kind of 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.
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).
Crossfeeding, blends the left and right stereo channels slightly, reducing the extreme channel separation characteristic of headphone listening that is known to cause headaches in a small fraction of listeners (and compensating for extreme separation in older stereo releases). 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-speaker headphone to simulate a full Dolby 5.1 (or more) surround system. This feature has not caught on in high-end audio circles.
In the pro-audio terminology a headphone amplifier is a device that allows multiple headsets to be connected to one or more audio sources in the same time in order to monitor progress of a recording session or a live performance.
Headphone amps with sub-mixing capabilities allow the listener to adjust, mix and monitor audio signals coming from multiple sources in the same time. This kind of headphone amp is often utilized during recording sessions to sub-mix playback of individual stem-mixes or instruments coming from a mixing board or a playback device. In many cases the listeners have their own sets of controls allowing them to adjust various aspects of the mix and individual and global parameters such as, channel level, global loudness, bass and treble.
Distribution headphone amplifiers are specialized headphone amps allowing a single signal to be fed to multiple headsets or multiple groups of multiple headsets in the same time. Many distribution headphone amps, like the one shown here, can be cascaded by connecting the audio input of one of the amps to the cascading output, marked "THRU", of another amp.
There are also available various other combinations of pro-audio headphone amps with sub-mixing and distribution capabilities in the same time.