Headphone amplifier
Encyclopedia
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
designed particularly to drive headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
instead of loudspeakers. Most commonly they are found embedded in electronic devices such as integrated amplifier
Integrated amplifier
An integrated amplifier is an electronic device containing an audio preamplifier and power amplifier in one unit, as opposed to separating the two. Most modern audio amplifiers are integrated and have several inputs for devices such as CD players, DVD players, and auxiliary sources...
s, portable music players and televisions, but standalone units are not uncommon.
Consumer headphone amplifiers
Headphone amplifiers as referred to here are commercially available separate devices, sold to a niche audiophileAudiophile
An audiophile is a person who enjoys listening to recorded music, usually in a home. Some audiophiles are more interested in collecting and listening to music, while others are more interested in collecting and listening to audio components, whose "sound quality" they consider as important as the...
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
Stax Earspeakers
Stax Ltd. is a Japanese company that makes High-end audio equipment. Stax is best known for their electrostatic and electret headphones, called "earspeakers". Electrostatic headphones work just like electrostatic loudspeakers on a smaller scale.-History:Stax Ltd. was founded in 1938...
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
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
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.
Output Impedance
Many headphone amplifiers have an output impedanceOutput impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
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. http://www.benchmarkmedia.com/products/headphoneamps.html
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
Damping factor
In audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
. 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
Total harmonic distortion
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency...
, frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
, IMD, output power
Audio power
Audio power is the electrical power transferred from an audio amplifier to a loudspeaker, measured in watts. The electrical power delivered to the loudspeaker, together with its sensitivity, determines the sound power level generated .Amplifiers are limited in the electrical energy they can...
, 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 (Do It Yourself) 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 CmoyCmoy
A CMoy is a pocket headphone amplifier originally designed by Chu Moy on the Headphone Resource Site and public discussion Forums.The headphone amplifier is designed around single or dual-channel op-amps such as Burr-Brown's OPA2134 or OPA2132PA, however, a wide variety of op-amps have been...
design, arguably one of the most popular headphone amplifier designs available. The simplicity of the Cmoy
Cmoy
A CMoy is a pocket headphone amplifier originally designed by Chu Moy on the Headphone Resource Site and public discussion Forums.The headphone amplifier is designed around single or dual-channel op-amps such as Burr-Brown's OPA2134 or OPA2132PA, however, a wide variety of op-amps have been...
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
CrossfeedingCrossfeed
Crossfeed is the process of blending the left and right channels of a stereo audio recording. It is generally used to reduce the extreme channel separation often featured in early stereo recordings , or to make audio played through headphones sound more natural, as when listening to a pair of...
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.
Professional audio (pro-audio) headphone amplifiers
Headphone amps with sub-mixing capabilities allow the listener to adjust, mix and monitor audio signals coming from multiple sources at 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 at 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 simultaneous sub-mixing and distribution capabilities.