Amplifier
When an amplifier tries to make the sound louder than it can, it adds distortion to the sound. Some amplifiers are made to add controlled distortion. Distortion from transistors sounds different than distortion from tubes. Distortion from tubes is often said to be more musical. Because of all this, more expensive amplifiers often add controlled distortion with tubes. Many of these amplifiers use transistors for a "clean" sound (without distortion).