A headphone amplifier is an alternative to a standard home hi-fi amplifier or multimedia speaker setup for those that use headphones a lot, the main advantage of a headphone amp versus a full variant is that since the power requirements of a headphone are quite small, the designer can usually spend quite a bit more on the amplification components than he can when designing a normal amplifier, usually resulting in a pure Class A output stage which is something that is impossible to attain with a more powerful amplifiers except at exorbitant prices. In other words for something like 100 to 300 € you can get a sound quality that really approaches high end ideals.
Equally, a well designed headphone amplifier should also drive headphones that cheaper amps and systems have problems with and have a reasonable damping factor which is often more important for headphones than it is for loudspeakers, potentially resulting in a vastly improved sound. And please note that a headphone amplifier is not to be called a Head Amplifier and while Headphone distribution amplifiers perform the same basic function as a normal headphone amp their design is quite different.
1388 Technology Makes an interesting and unusual devise called the Bit 88, it's a headphone amplifier and signal enhancer, the signal enhancement side has both a valve based "enhancer" and more conventional SRS based IC's.
Boostaroo Makes a product that they call a headphone amp but is actually a signal booster and splitter, useful nevertheless but not for purists.
CEC Has a lovely little full class A amp called HD-51 that is unusual in that it has 2 output channels each with it's own volume control and has balanced input in addition the usual RCA's, also unusually since the circuit used was a fairly powerful or 8w the designers added loudspeaker outputs, very odd for a headphone amp, but it means that if you have very efficient or small easily driven speakers it can utilise them. This model is designed for the Japanese market in mind and I have not seen it for sale in the US or UK but a few shops are starting to carry it on mainland Europe since this unusual feature set is just what some people are looking for and it also has an unusual and reportedly good sounding amplifier topology.
Fostex Makes the PH-50 a 5 channel amp that sports balanced inputs.
Heed Audio Makes the keenly priced CanAmp, it's actually quite flexible, not only can it work with almost any headphone, have a pure class A topology (with an output power of a whopping 1w > 8ohms) but it also has a line output meaning that it can be used as a preamp or as a tape output in an amplifierless setup.
Portable headphone amplifiers are designed to solve two problems, firstly the limited amount of gain (volume) most modern portable players put out, especially those sold in the European Union, and secondly the fact that the puny amplifiers in most portable devices have problems handling the impedance characteristics of hi-fi and professional headphones. Although more and more budget and lower midrange head & ear phones are these day designed to mate with portable players the upper crust models, classic designs and pro models are not and represent difficult load for the portable player, conversely the modern phones have a tendency to sound weak on normal headphone outputs because of their low impedance. Many of the portable headphone amps are actually boosters rather than proper amps per se and contain no real buffering, therefore do not help with impedance issues.
A portable amp made by the South Korean company Audinst, fairly keenly priced unit although you should take note since they primarily sell through a direct sales model that the prices they advertise exclude import duties and sales taxes if you live in a country that has such. The AMP-HP is tiny and light at only 60x80x20 and 134 grams but still sports a built in rechargeable battery that is good for 10 hours use according to the manufacturer, and comes with a charger and cables for standard 3.5mm plugs and one specifically for iPods. Note that unlike the Headphone/DAC combis that the company sells, this unit is frequently sold cheaper in their eBay store than in their online shop, and like some of their full size amps they are only optimised for headphone with impedances up to about 300 ohms meaning that pro 600 ohm models may not sound at their best. Other notable features are a socketed user replaceable op-amp.
Discontinued Portable headphone amplifiers
"Hi-Fi hodetelefonforsterker" A device made by a Norwegian gentleman that we never managed to find the name of, the only thing we could dig up was that his auction handle which was "SH4DRY" and he was from a small village called Spikkestad that is to be found in the county of Buskerud in southern Norway. He made a portable headphone amplifier which he sold through the Norwegian QXL auction site in the 2007/8 timeframe, this amp came completely unmarked in a black ABS project box that you will find in any DIY electronic store all over the world and the only name given was "Hi-Fi hodetelefonforsterker" which translates simply as "Hi-Fi headphone amplifier".
Despite looking terribly DIY, complete with the sort of volume knob and rocker switch that PAIA Electronics used to ship with their kits in the 70's, and despite at first glance appearing to be a buffer amp this is actually a proper headphone amp with quite an interesting specifications, on paper at the least better than most factory made portable amps. It was designed to be used both as a home hi-fi headphone amplifier and was supplied with a PSU for such usage, and also as a battery driven portable amp, powered then presumably by a 9v PP3 battery and for that use came supplied with a belt clip.
Unlike most portable amps it can handle just about any headphone with impedances at up to 600 ohm and here on the right is reproduced a graph showing the effective output performance of the unit at various impedance points, the blue line represents it working on mains power and the green one on batteries. The most interesting thing about the HFH was simply the rather keen price of 600 NKR, which at the time translated to something like 50 - 60 € this made it an unqualified bargain but despite this there only seem to have been made somewhere between 60 and 100 units before the maker gave up.
The Hi-Fi hodetelefonforsterker technical specifications are: Inputs: 3.5mm stereo jack on a ca. 80cm long cable. Outputs: 3.5mm socket. Max output level: 3.2Vrms Ω. Output power: 150mW into 64Ω per channel. Distortion: 0.1% at max volume. Output inmpedance: 25 to 600 Ω. Frequency response: 5hz to 110kHz. Size: 5 x 3 x 10 cm (Width x Height x Depth). Weight: (inc. battery) 160 grams. Included accessories: Belt clip and power supply unit. RRP: 600 NKR.
Audinst HUD-MX2 Integrated DAC/DD and headphone amplifier Made by the South Korean company Audinst, this is a small USB and optical S/PDIF (TosLink) capable digital to analogue and digital to digital converter with an integrated headphone amplifier. It looks identical to the older USB only Audisnt HUD-MX1 from the same company but is actually built on completely different technology, with the MX1 being based around a Wolfson chip we are more used to seeing in portable audio products, while the MX2 is actually built around a VIA Technologies codec/USB interface chip and a Burr-Brown DAC chip that we usually associate more with computer peripherals, but has the added bonus of having better USB Audio Class B compatibility which should mean that most phone and tablet OS's should be able to use it without a driver. The Audinst DAC is probably best suited as a desktop PC converter, acting as headphone amp and a frontend to a PC loudspeaker systems although it is perfectly capable of operating as a standalone DAC. It should be noted though that even if the unit can operate at up to 192kHz using the optical input, it is limited to 24 bits/96kHz if the USB is used, and furthermore an 88.2 kHz sampling rate is only possible via the USB port if a specific Windows driver is used that supports them as the generic driver that comes with the OS only supports multiplies of 48kHz (96kHz etc).
This means that if you intend to use the unit with a PC and have an optical output on it you may want to look into driving the DAC from the computers optical port rather than the USB, not only does it mean that you can utilise 192kHz, it also means you can electrically isolate the DAC from the electrically noisy USB connection, the neat thing about the unit is that you can have both interfaces, if the USB connected to the computer it controls the volume of the MX-2's output even when you are utilising the SP/DIF input. In addition to the usual headphone (3.5 and 6.3mm) and cinch line outputs the MX2 features an optical SP/DIF output that is capable of either copying the optical in stream to a third device, converting the stream from the USB input into an optical signal or if a MS Windows specific driver is installed can send a bit-stream such as a Dolby AC3 signal trough the DAC and forward it to a device that can utilise and convert such a signal but has no USB interface, like a home theatre amplifier. Using the driver also allows you to control the Qsound functions of the converter chip although that is a bit on the gimmickry side.
The quoted specification of the converter in the HUD-MX2 is excellent and for the headphone amp section is fairly good, they are however not quite up to the specs of a separate headphone amplifier although much better than what you would find in a normal amp. As the unit is shipped it is only optimised for headphones with impedances up to about 300 ohms as standard but can accommodate 300 to 600 ohm headphones by allowing people to disable gain stages via jumpers on the PCB. Other notable features are that two of the three primary op-amp's on the MX2 are socketed and thus user replaceable in theory, although you will need enough knowledge to judge which op-amps are fully compatible with the ones that are in the unit, but the company also sells op-amp upgrades via its eBay store.
Borbely Audio The company had 2 very well regarded kits available, so well regarded in fact that even though expensive at the time they sold fairly well but are almost impossible to get hold of on the second hand market. The best known is the EB804/421 that was introduced in 1998 or 9 and has a Dual Mono hybrid MOS-Fet/Valve output stage that is fully Class A, the model had a few minor refinements through its lifetime with the later models being the more desirable ones.
The other model was a refinement of the 804/421 known as EB602/210, it shared the same topology and basic design but instead of a hybrid it used an all FET output stage, people usually agreed on it having considerably more resolution than the 804/421 even though some preferred the sound of the 804 valve hybrid, Borbely amplifier designs tended to be highly damped which could be a minus in power amplifiers but it suited these headphone amps to a T. Both models cost the same, they retailed for 428 euros excluding VAT and case in 2010, or 478 euros if a Teflon based PCB was chosen, or 510/569 including VAT, a case vas available for 155 euros ex-VAT. Both models were discontinued in 2010 when the company closed down. It should also be noted that some of the Borbely pre-amp kits had unusually good headphone outs.
A headphone distribution amp is an utility product intended to drive a number of headphones, usually in a some sort of a professional situation such as a recording studio or a language teaching lab etc., the emphasis in the design of a distribution amp is usually more on cost per channel than on sound quality although there are some excellent quality distribution amps out there designed for use in recording studios.
Discontinued headphone distribution amps & systems
A portable, battery driven distribution amplifier with 3 electrically isolated outputs intended to be used in film and TV location work. Made by USA based film audio specialist Professional Sound Corp. (PSC) and was commonly referred to by the company simply as the HDA. Housed in a ruggedized brushed aluminium case, driven by a standard 9v PP3 battery which had a quoted lifetime of only about 6 hours in this unit, but that should be enough for its intended application. Introduced around 1990 and was taken off the market in ca. 2005.
The PSC Headphone Distribution Amplifier technical specifications are: Inputs: 6.35mm stereo male jack on a 45 cm long cable. Outputs: 3x stereo female 6.35mm jacks. Input impedance: 600 Ohms. Output inmpedance: 25 ~ 600 Ohms. Optimal otputs impedance: 50 Ohms. Frequency response: 20Hz ~ 20kHz. Size: 16.5 x 4 x 6 cm (Width x Height x Depth). Weight: 0.25 kg. Power requirements: 9v PP3 battery. Battery life: 6 hours.