Important noise sources affecting power output waveforms of digitally controlled switch-mode (Class-D) converters/amplifiers are investigated with a 400 V hardware demonstrator utilizing gallium nitride (GaN) power transistors. First, the influence of the open-loop gains of the cascaded feedback control system on the load current signal-to-noise ratio (SNR), which is a key metric in low-noise, nanometer-precision mechatronic positioning applications, is demonstrated. With a PWM frequency of 200 kHz, the high achievable controller gains enable unprecedented SNR values in excess of 105 dB (DC–10 kHz). Next, it is shown how a real-time Kalman filter, calculated at a rate of 100 kHz and used to attenuate sensor noise, increases the amplifier output SNR by more than 10 dB. Furthermore, digital delta-sigma (noise shaping) modulation improves the SNR by 5 dB, while not affecting distortion.