Even though some audio sources have already a built-in amplifier, a separate amplifier gives you more flexibility for picking the model that best complements your speakers. There are some mini amplifier models available. These models are small enough to hide virtually anywhere.
Most people will look at amplifier wattage first when choosing an amplifier. Output wattage is the amount of power the amplifier is able to deliver to the speakers. Driving low-sensitivity speakers (sensitivity is expressed in dB/W) requires fairly high wattage. Also, driving outdoor speakers or speakers in a large room will require higher amplifier power. You shouldn't use an amplifier with significantly higher output power than your speakers can handle. Overdriving your speakers can do irreparable damage to your speakers.
But don't be overly concerned about wattage. An amp supplying 20 to 50 Watts will give you more than plenty of power to sufficiently drive a speaker in any but the largest room.
Amplifier output power is given as Watts rms (continuous) and Watts peak. The rms or continuous value tells you how much power the amplifier can deliver continuously while the peak power figure tells the maximum wattage that the amplifier can deliver for short periods of time. Most people will look at the rms power value when selecting an amplifier. However, music signals are anything but continuous. Therefore picking an amp that has sufficient peak power handling is just as important.
However, audio quality is just as important as having enough audio power. Therefore be sure to also look at factors related to the quality of the amplifier. One of these parameters is called total harmonic distortion or THD. No amplifier is absolutely perfect and the amplified signal will include some error components which will distort the signal to some degree. THD is either expressed in percent or dB. The amount of distortion varies between amplifier models. A smaller distortion means higher audio quality. Audiophile-grade amps will have distortion figures of at most 0.05% while consumer-grade amps have distortion of up to 10%.
Another specification is called signal-to-noise ratio. This number specifies how much undesired components such as hum and noise the amplifier will add to the audio signal. Consumer-grade amps will have a signal-to-noise ratio (SNR) of at least 80 dB. Higher-end amps will have an SNR of 100 dB or more. Audiophile amps will have an SNR of close to 120 dB.
Most of today's amplifiers are so-called Class-D amplifiers which means that they have higher power efficiency than Class-A or Class-AB amplifiers and therefore waste little energy as heat. Class-A and Class-AB amplifiers require a fair amount of heat sinking and therefore are typically fairly large and heavy. In contrast, amplifiers based on the Class-D technology are small and lightweight. Some of today's smallest mini amplifiers are no larger than a deck of cards. Many Class-D amplifiers, however, have higher distortion and a lower signal-to-noise ratio than Class-A or Class-AB amplifiers. Be sure to look closely at these numbers when picking a Class-D type amplifier.
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