A audio amplifier is surprisingly complex, considering that most people use it to power and control the speakers in their home or car. If you are just getting started with an amplifier and speaker setup, there's nothing wrong with choosing some bookshelf speakers and a simple digital audio amplifier. Enjoying music does not have to be complex. However, amplifiers have a dozen differences beyond the inputs they use and how much power they send to your speakers.
To put it simply: songs are recorded by turning the vibrations in the microphones to an electrical signal. This signal becomes audio you can hear by vibrating the air (via your speakers) to create sound waves that your brain recognises as music. A digital amplifier increases the power used to play audio. Moving a speaker cone, especially in large speakers, requires a larger electrical current than the one created during recording. The amplifier increases the current without significantly distorting the sound, giving you more volume.
Different Input Types
Look for a digital amplifier that fits your specific audio setup. Some amplifiers, such as the Griffin Twenty and the Sonos CONNECT:AMP, are specific to certain digital protocols; in both cases, the amplifier is intended for wireless music management. The Twenty works specifically with Apple's AirPlay protocol, via an AirPort Express unit; the CONNECT has apps available for multiple operating systems and mobile platforms. If you want to go a bit lower tech you might consider an amp like the Lepai LP-2020A+, which uses both RCA and auxiliary input.
Power and Speaker Choice
The more powerful your amplifier, the louder you will be able to listen to music, so look at power ratings if you like to turn it up. This is expressed as watts per channel, usually in relation to the ohms. Most speakers are rated for eight ohms; look at the speaker impedance on the amplifier for more information how many ohms the amplifier can handle. The speakers are the most important aspect of your audio setup, possibly more so than your amplifier; even a great amplifier cannot make subpar speakers sound better. Hi-Fi speakers come in a variety of shapes and sizes, from bookshelf speakers to floor speakers.
Distortion and Other Considerations
While the goal of a perfect audio amplifier is to have zero distortion, this is not realistically possible; when looking for an amplifier, check the distortion. This specification is usually listed as THD (total harmonic distortion) and a percentage. For example, the Twenty lists 0.08 per cent THD, while the CONNECT:AMP lists 0.02 per cent THD and the Lepai lists 0.4 per cent. Keep in mind that when pushed hard, even the best amplifiers will have audible distortion. Look at frequency response as well, which measures the high and low frequencies that amplifier handles; 20 Hz to 20 kHz is the most common, thought this may vary between amplifiers.