An audio amplifier that works in the digital domain. It generates the equivalent analog output for the speakers by using pulse width modulation (PWM) or pulse density modulation (PDM) rather than the traditional digital-to-analog conversion. See PWM
Less Heat than Analog
Because pulse modulation output signals are either on or off, Class D amplifiers produce far less heat than analog amplifiers. Reaching efficiencies greater than 90% compared to only 50% for analog, they are widely used for every amplification requirement from cellphone speakers to high-end stereos.
Digital and Analog
Class D was not coined for "digital;" it was the next letter after Class C. However, it does produce a "digital-like" output because the signals are generated by turning a switch fully on or off. But it is not technically digital because the output is not digital data. It is a modulated audio signal that is feeding analog speakers and is equivalent to the output of a traditional analog amplifier. Some call this a "synthesized analog" output. See amplifier classes