Class-AB Speaker Amplifiers, also called Linear Amplifiers, are Speaker Amplifiers that amplify the input signal as it is to drive the speaker. Generally, this method has an efficiency of less than 50% and generates a lot of heat. Class-D Speaker Amplifiers, also called Digital Amplifiers, are Speaker Amplifiers that convert (analog) input signals into pulses as output. This method achieves high efficiency and low power consumption by converting the output into PWM∗1 pulses.
PWM∗1 Abbreviation for Pulse Width Modulation. In the case of a class-D Speaker Amplifier, an LPF (low-pass filter) is attached to the output so that the carrier signal is removed and the audio signal can be extracted.