Quick Guide to Headphone Amplifiers and Related Headphone Accessories

Yet for many, the term headphone accessories refers to nothing more than the standard assortment of 1/4- and 1/8-inch adapters and extension cables.

In reality, there is a lot more to the world of headphone accessories - from surround decoders and wireless add-on units to dedicated headphone amplifiers that can dramatically improve your listening experience.

Headphone accessories do not come cheap; they often cost several times more than the average headphone pair itself. A good quality headphone amplifier costs $200 to $300, while a high-end dedicated amplifier may cost close to a $1000!

We have already discussed in a rather comprehensive manner, the use of headphone surround sound decoders in our two-part Dolby Headphone technology guide; in a similar manner, we have also extensively covered the use of wired versus wireless units in our Wireless Headphone Basics article.

Headphone Amplifiers:

A separate dedicated headphone amplifier is the most important, and surely one of the most expensive headphone accessories. It is therefore no surprise that many find themselves in the dilemma of why should they spend money on a dedicated amplifier for headphone use when most audio gear come with a handy headphone jack on the front panel.

Beyer Dynamic A1 A high-end headphone amplifier with 0.001% THD at its rated output and an unweighted S/N ratio of 100dB for precise audio performance.

The truth is that manufacturers tend to regard headphone jacks and related electronics - especially on audio equipment designed for the home market, to be at best, an afterthought. There is the unfortunate misconception that headphones are nothing more than a toy!

The issue is not power but sound quality. The headphone output on home audio components is perfectly adequate to drive modern headphones at the maximum volume they can handle. However, little effort is put in designing the relevant headphone driving circuitry.

A headphone output might be nothing more than a couple of series resistors off the power amplifier's speaker terminals. But power amps are primarily designed to drive speakers rather then headphones. This configuration leads to higher noise and output distortion levels than when using small signal amps.

On some of the more expensive equipment, headphone outputs may have their own driver circuits integrated within the pre-amplifier or any other audio component. But even in these circumstances, sound quality is often an issue; as stated, hardly any design effort goes into the headphone driver stage in power systems designed primarily to drive speakers.

There is a logical reason behind this approach: Manufacturers know very well that many would simply buy an audio power product without every listening to the audio through the headphone jack.

Hence, if you want to enjoy the best audio performance from your headphone listening, the ideal approach is to consider investing in a dedicated headphone amplifier. A good quality headphone amplifier need not be very expensive. The FiiO E9 headphone amplifier featured on top of the page is selling on amazon at a reduced price of less than 130USD, this despite that it is one of the best around thanks to its solid built quality and excellent overall sound.

But pricing is also very much dependent on the supported features. While most headphone amplifiers include nothing more than a volume control and two pre-set gain settings, more advanced amplifiers may also include a built-in surround sound processor specifically designed for headphone listening - like Dolby Headphone - for a more spacious and comfortable listening experience.

In addition, a high-end headphone amplifier such as the Beyer Dynamic A1 Headphone Amplifier would cost more than a mid-range 7.1 home theater receiver!

Evaluating Headphone Amplifiers

The best way to evaluate a headphone amplifier is to try it out using your own pair of headphones.

As stated in our Headphones Basics & Buying Tips guide, the best tools to test your would-be audio purchase is nothing more than a music CD from your own collection that you know, and possibly an Audio test CD such as the Stereophile TEST CD; however, any audio test CD with an assortment of 'pink noise' tracks, frequency-sweeps and binaural tracks will do.

When looking for dedicated headphone amplifiers, there are a few basic parameters synonymous with amplifiers that one should be able to interpret to fully understand the capabilities on offer by the audio gear in question.

The good news is that today's headphones are much easier to drive than loudspeakers - power requirements are minimal - therefore minimum acceptable performance is easier to achieve.

The following amplifier specification guidelines should help you evaluate better headphone amplifier capabilities:

Power output: Modern dynamic headphones can reach maximum volume with only a few milliwatts (10-20mW) of power. Headphone amplifiers rated at over 500 milliwatts per channel should be able to provide more than adequate power to drive these headphones.

However, this all depends on the headphones impedance - high impedance headphones such as those used in professional studio applications require several Watts of power in comparison to the few milliwatts required by low impedance headsets.

Make sure that the amplifier can deliver adequate output for the impedance of the intended headphones. Unlike power amplifiers which are typically rated for a 4- or 8-Ohm load, there is no typical standard impedance with headphone amplifiers. As always, the easiest way to check is to plug in the headphones and listen - whatever the volume setting is, there should always be enough gain to spare.

Distortion: This refers mainly to Total Harmonic Distortion (THD) figures, but at times, it also includes Intermodulation Distortion (IMD) as well. Whatever distortion levels that are being quoted, these should always be less than 1% at the rated output.

S/N ratio: This is the Signal-to-Noise ratio and indicates how quiet the amplifier is when there is no input signal. Basically, it reflects the level of internal noise generated by the electronics inside the amplifier itself.

This is one of the most important parameters to watch when it comes to dedicated headphone amplifiers. The natural close coupling that exists between headphones and the ears tends to make equipment noise even more noticeable as there is practically no ambient noise to camouflage equipment generated noise.

This ratio is measured in decibels (dB) and is specified as either A-weighted or unweighted. A-weighted is better than unweighted as it takes into account the human ear sensitivity. Whatever is being quoted, it should be greater than 80 dB.

Sometimes, it is expressed as a negative number; what is important to remember here is that the bigger number (ignoring the negative sign) the quieter and therefore the better the equipment is. Thus, an S/N ratio of 90dB is better than one of 80dB.

Output impedance: In the majority of cases, this represents the impedance of headphones that the amp is rated to drive i.e. the load impedance; at times, it is also used to specify the output impedance of the amplifier itself. Both are measured in ohms.

The rated load indicates the range of impedances that the amplifier can drive to full power. Keep in mind here that the use of headphones with a higher impedance than the specified load impedance of the amplifier results in a lower power output.

Damping factor: This is more of an issue when driving complex loads such as low impedance speakers rather than headphones. The damping factor is a stability indicator and reflects the amplifier's ability to drive a specific load.

Headphones are essentially resistive loads whose response varies little over frequency. The larger the damping factor, the better is the amplifier response over load variations with frequency.

Feel the Bass!

Irrespective of whether one makes use of dedicated headphone amplifiers or not, one of the main problems with headphone listening is the lack of impact that bass notes leave on the listener in view that the physical sensation of bass is missing. Expressed differently, the part of the sound that is 'felt', also referred to as 'tactile sound', is missing.

As expressed in our guide to Tactile Sound in Home Entertainment, airborne vibration is not the only way sound reaches our ears. Sound energy is also perceived through various pathways within our body - mainly through muscle and deep tissue, the sense of touch (skin sensation), and also through bone conduction - in particular the skull. Most humans can feel sounds in the range 10Hz to 800Hz. 

In a speaker system, subwoofers are used to add a solid foundation to the bass by generating high sound pressure levels (SPLs). Unfortunately sustained exposure to high SPLs is dangerous to our hearing.

The use of what are referred to as tactile transducers - or vibration devices - can help provide an even deeper bass sensation without driving the subwoofer to dangerous SPLs. Tactile transducers come in various forms, including compact bass shakers like the inexpensive Aura AST-2B-4 Pro Bass Shaker featured here; this is more suitable for the 'individual' overstuffed home theater seat. More powerful tactile transducers like the ButtKicker BK-LFE low frequency effects system can handle heavier structures and are often mounted on floor boards. Obviously, the use of tactile transducers implies you will have to make use of the seat or room within which the tactile transducer is fitted to feel the effect during your headphone listening.

The Aura AST-2B-4 Pro Bass Shaker This inexpensive Aura bass shaker delivers 50W of bass power to your seat for a shaking experience!

Does this represent any constraint?

Most probably not, especially if what you are after is to use your home theater late at night without bothering your neighbors. However, it all depends on what are your headphone listening requirements. One thing you can be certain though...

The combined effect of tactile transducers and Dolby Headphone technology simply transforms ordinary headphone listening into a great immersive and spacious experience that is very much close to that enjoyed through a home theater speaker setup!

For more information on the use of tactile transducers in home entertainment, we suggest going through our guide to Tactile Sound in Home Entertainment.