Normally all amplifiers are analogue based where the input music or frequency is amplified in accordance with the same pattern that's being fed at the input.
Since a music may largely have exponentially rising and falling contents and also frequencies accompanied with all sorts of amplitudes causes heating up of the devices.
This happens because BJTs and mosfets do not "like" transitional inputs where the signal do not have sudden rise and fall rather gradually transits across the points where the devices are neither fully ON or OFF, this causes a lot of heat generation and power loss
In a class D type of amplifier, the music input is compared with high frequency triangle waves and converted into a PWM "language" at the output. The PWM content stores all the information of the music and translates it back into the connected loudspeaker in an amplified manner.
However since the PWMs will consist of non-exponential pulses where the pulses are in the form rectangular pillars switching ON/OFF suddenly without transitions can result in significant distortions at the output.
In order to smooth out the above issue, a low pass filter is generally incorporated wherein the spikes are smoothed to generate a reasonably good and clear amplified replication.
The proposed design of a class D digital amplifier circuit utilizes the famous 555 IC for the intended comparisons.
Instead of the PWM method here we use an alternative mode called the PPM or pulse position modulation which may be considered as good as a PWM.
PPM is also known as pulse density modulation due to the specific nature of its functioning.
Here the modulation input is compared with high frequency triangle waves and the output is optimized by varying the position or the density of the generated/compared pulse output.
As can be seen in the below class D amplifier circuit design, the IC 555 is configured as a standard astable MV mode, where the resistors Ra, Rb and C determine the frequency of the triangle waves generated at pin6/7 of the IC.
The above high frequency triangle waves are compared with the music input applied at the control input pin5 of the IC.
Here the low voltage music signal is first amplified to some optimal voltage level and then applied at the control input pin#5 of the IC555.
This results in the discussed PPM output at pin#3 of the IC. This is amplified by T1 to a high current output and is fed to a loudspeaker for the required class D type amplification.
The audio trafo does a couple of interesting functions, it amplifies the output for the LS and also to an extent smooths out the harmonics which are normally a part of all class D type amplifier circuits.
A filter capacitor (non-polar) may be tried across the LS for obtaining cleaner sound outputs.