The capacitor is to remove the "Cannot find solution".
The driving mechanism (the transistor with the transformer and resistor) may be very unrealistic, but it's simply what I found easiest to design. Below 45V, the THD (excluding noise) is at most a few uV because the oscilloscope just gives a flat line. If you remove the entire class-A section leaving just the class-AB section, its THD (excluding noise) is a 5-10mV third harmonic distortion at 40V and 10mV fifth harmonic distortion at 45V. If you short the two diodes the class-AB section will turn into pure unbiased class-B, but the distortion is still much lower than if it wasn't class-H (tested and proven when examining the modified version of this circuit very closely and at high frequencies so that it is visible on the oscilloscope).
