PWM-input, class-D amplifiers that do not require an external clock are known as conventional amplifiers (refer to Patent document 1, for example).
In the amplifier disclosed in Patent document 1, a difference signal between a negative feedback signal from a power stage and a PWM input signal passes through two integrators and is thereby demodulated sufficiently. PWM modulation is realized without using an external clock by comparing, with a comparator, the sufficiently demodulated difference signal with a clock signal that is taken out of the first integrator and in which PWM harmonics remain.