The effects of key sources of total harmonic distortion (THD) in power output signals of digitally controlled switch-mode (Class-D) converters/amplifiers are analyzed. Extensive measurements with a 400 V amplifier prototype, based on gallium nitride (GaN) power transistors, support the investigations. First, the semiconductor loss model and a comprehensive circuit simulation of the converter with its closed-loop feedback system is presented to provide insights on distortion caused by junction temperature variation of the power transistors. Half-bridge interlock time is identified as a significant source of nonlinearity and hence, three simple and effective methods to reduce its deteriorating effect on THD are presented. Another important contribution to linearity arises from the closed-loop feedback controllers, which benefit from small delays and/or high converter switching frequencies. It is also shown how a Kalman filter, which can be used to significantly reduce converter output noise, deteriorates the THD due to its linear system model. Finally, a method to reduce harmonic distortion and other disturbances caused by a non-ideal DC supply is also demonstrated. By rigorously eliminating distortion sources and applying the presented compensation methods, amplifier output current THD values below −100 dB (0.001%) are achieved and experimentally verified in wide load current ranges.