Looking at the circuit diagram it follows the classic op-amp with a long-tailed pair of NPN transistors driving a PNP gain stage and finally a complimentary emitter follower as an output buffer.

but suffers from an output impedance too high to drive a set of headphones. An NE5532 steps in for the op-amp buffer role, making for a very simple circuit. Power comes from a set of four 18650 ...

Rather than having an external boost converter to produce enough volts, this design adds just two mosfets and an inductor to the Class-D output structure (see diagram ... and TU Delft presented a ...

Hats off to to EPC because the PWM is not an off-the-shelf IC but a self-oscillating design (~600kHz) put together from op-amps and comparators ... over-temperature and over-current. Find the product ...