Headphone amps differ from headphones because they contain a power amplifier. Power amplifiers take a guitar signal and increase the power of the signal. This power increase also serves to increase the volume to the point it reaches audible levels once sent to the speaker. Regular headphones do not have a power amplifier. The actual power amplification must occur in another device before the audio signal is sent to a pair of regular headphones.
There are two major reasons that a headphone amplifier would be used. Audiophiles and sound engineers will frequently use headphone amps in place of regular headphones. The sound quality from headphone amps is usually better because the power amplification can strengthen the signal. Musicians that play electrical instruments use headphone amps as portable practice amps. The size convenience is useful for a traveling musician that doesn’t want to carry around a much bulkier practice amp.
Headphone amps do not sound as good as an equivalent quality, loudspeaker amp. Higher quality headphone amps might sound better, but this usually requires a substantial quality gap between the two types of amplifiers. Headphone amps have restrictions in their design that loudspeaker amps do not, which decreases the overall audio experience.
Whether or not a person should get headphone amplifiers is largely based on their needs. Most musicians that specialize in electrical instruments should get a pair. The practice versions are inexpensive to purchase. These headphone amps are quite useful to have if a musician needs to practice quietly or while traveling. The issue for audiophiles and sound engineers is a bit more complex. Headphone amplifiers cost more than normal headphones and do not always provide better sound quality. Higher quality sound equipment often does not benefit from the extra power amplification. Sound engineers with a large number of headphones set up simultaneously will often use them to decrease the drain regular headphones would have on the audio signal. Audiophiles will need to test them with their equipment setup in order to determine if there is an improvement.
The headphone amplifiers used by audiophiles and sound engineers normally are much more expensive than a musician headphone amplifier. This variant has a rack mounted box, which headphones are plugged into. The main concern of this is signal quality. This causes these units to be more expensive, but provide better sound. Musician headphone amps are more focused on the convenience factor. These are smaller and only expected to provide decent enough sound to allow the musician to practice.