An analog amplifier is a device that takes a low-power analog signal as an input and outputs a more powerful version of the same signal. This type of device is frequently used in audio applications such as personal media players and home theater systems. The audio amplifier generates the actual signal that drives the headphones or speakers that make the sound.
Every device that outputs sound has an amplifier of some sort, whether it is analog or digital. A portable media player, for instance, contains a digital media source, digital-to-analog converter (DAC) and analog amplifier all in one device. A home theater system might have a separate digital source and a receiver, which combines a DAC and an analog amplifier. High-end systems can separate out all of the stages, featuring a separate analog power amplifier that simply takes a line-level signal and amplifies it to drive the speaker system.
One way for a person to enhance the performance of any audio system is to upgrade the amplifier. For a home stereo system, this typically happens by adding an external power amplifier. External power amplifiers typically have better power supplies, allowing them to deliver more power to the speakers. Separating the power amplifier from the rest of the system's power chain also reduces the risk of noise and interference with the lower power line-level signals that come from source components. For personal audio, external analog headphone amplifiers are excellent for improving sound quality through providing higher quantities of clean power to the headphones.
An alternative to the analog amplifier is the digital amplifier, which gained popularity over the first decade of the 21st century even though the underlying technology dates to the 1950s. More correctly known as Class D amplifiers, these circuits — unlike pure analog amplifiers — can take a purely digital signal and amplify it, generating an analog speaker-level output. On the other hand, with an analog-to-digital converter, they also can accept analog signals and amplify them. The key benefit of the digital amplifier over an analog amplifier is that it is more efficient, allowing a less-expensive digital circuit to produce as much amplification as a more elaborate and expensive Class AB analog amplifier. The historical drawback of the Class D amplifier has been that its sound quality is below the standards of analog amplification, although the technology has gradually improved.
Although digital amplifiers might seem like the wave of the future, analog amplifiers have unique benefits. They can natively work with analog sources as well as with digital sources that have a DAC, allowing for simpler circuit paths. They also provide the best quality amplification and sound.