Small sized, efficient, stereo, audio-amp modules using the PWM (pulse width modulation)
or class D method are now cheaply available.
e.g. $1 for 2 x 3 watt (PAM8403) and $2.15 for 2 x 10 watt (PAM8610) 3Q 2018
A Class D amp works by taking the analogue input signal and creating a PWM
(pulse width modulation) replica of it.
This is essentially a train of pulses, which correspond to the amplitude
and frequency of the input signal.
In its most basic form, a comparator circuit is used to match the input signal with the PWM signal.
The PWM signal is then amplified by an output stage operating in switch mode.
So there are two states, on or off, at very high speed, corresponding to the PWM pulses.
A linear amplifier�s output stages, by comparison, see a continuous waveform and,
to avoid distortion, are on for more than half the waveform (Class A/B)
or for the complete waveform (Class A), greatly reducing efficiency and generating heat.
The amplified PWM waveform is low pass filtered to recover the audio waveform
and eliminate spurious ultrasonic noise before outputting it to the speakers.
This process seems digital but is in fact analogue in nature.
The signal is not �digitised�, or assigned a numerical value;
the PWM pulse train is an �analogue� of the input audio signal.
The Sinclair X10 (c1965) used the PWM or class D method of audio amplification,
but from some reports, many units sold weren't too reliable.
(possibly due to shortcomings of the semiconductor devices available at that time?)
Links:
 Sinclair X10 audio amplifier |
 Sinclair X20 audio amplifier |
|
Class D Amplifiers - Lenard Audio Institute |
Class D Amplifiers - Maxim Integrated Products, Inc. |