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 wired or wireless units in our Headphone Basics article.

The issue is not power but sound quality. The headphone output of 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 the question remains as to their quality. 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 the best audio performance, the ideal approach is to consider investing a dedicated headphone amplifier.

Some of headphone amplifiers come with built-in spatial processors - also referred to as surround sound processors using Dolby Headphone technology.

Evaluating Headphone Amplifiers

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

And 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,  and possibly an Audio test CD.

Good inexpensive audio test CDs include Stereophile TEST CD #2 and TEST CD #3, 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 spec's  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 pro applications would typically 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%.

• 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 headphone listening. 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 either 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,  a 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. What is important to remember here is that the use of headphones with a high impedance than the specified load impedance of the amplifier would result 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.

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.