Definition: A Class A amplifier means that the amplifying device (transistors or tubes) operates over the entire 360-degrees of the amplification cycle, amplifying both the positive and negative halves of the waveform. Class A amplifiers are generally considered to be the best sounding amplifiers because of their excellent signal linearity and the absence of crossover distortion, but are very inefficient, which means that a lot of the power generates heat not sound. Because of the higher cost of Class A amps, most amplifiers are Class A/B.
Examples:
Class A amplifiers are often preferred by audiophiles because of their linear sound quality.