A tube amplifier is an electronic amplifier that uses vacuum tubes to amplify an audio signal. Also known as a valve amplifier, a tube amplifier is a type of analog amplifier that has been used in audio applications for over a century. Tube amplifiers are known for their warm, rich sound, and are popular among audiophiles and music enthusiasts who value the "tube sound" over the more accurate but clinical sound of solid-state amplifiers. They are used in both home and professional audio systems, and are often used to power high-end speakers or headphones. Tube amplifiers can be designed for different types of audio inputs, such as phono, CD, or digital sources, and can range in power output from a few watts to several hundred watts.