Headphone amplifier
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[edit] Consumer headphone amplifiers
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.
[edit] How they work
Effectively, a headphone amplifier is a small power amplifier which 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. This is often expressed as damping factor. For example, a 32 Ω headphone (such as the Grado line) 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. Of course, damping factor is not the only specification relevant to choosing a headphone amplifier — harmonic distortion, frequency response, IMD, and a myriad of other measurements are also significant.
[edit] 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!).
The more complex, higher-fidelity headphone amplifier designs are exemplified by those of Phillip LaRocco and Kevin Gilmore. Mr. LaRocco is notable for the introduction of the buffered-opamp topology to the DIY headphone amplifier community. Dr. Gilmore's designs are notable for their resemblance to high-end high-power amplifiers and their adherence to purist ideas. Both Mr. LaRocco and Dr. Gilmore's designs are commercially available for those unable or unwilling to build them.
[edit] Crossfeed and other audio processing
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). 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.
[edit] Professional audio (pro-audio) headphone amplifiers
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.
[edit] Cautionary note
Using headphones at high volume can lead to considerable ear damage. In noisy surroundings, it is tempting to turn up the volume while wearing headphones — this is very harmful to the ears. It is therefore advisable to use headphones at low to moderate levels and in quiet surroundings.
It should be mentioned also that around 100 dB SPL sound level is perceived as not particularly loud so playing real loud is approx. 105-120 dB which is not good for the hearing in the long run. Let the ears rest often from high level listening.
When it comes to noisy environment less than 85 dB is considered as safe so consider what is said above.
