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. Traditionally, linear amplifiers, which are commonly based on power operational amplifiers or discrete devices, are preferred for these tasks due to their inherently low noise over wide frequency ranges, combined with low output impedances [10]. Noise and linearity can be further improved with closed-loop feedback systems that can be implemented in the analog or digital domain, whereas the former also features intrinsic low noise due to the absence of (digital) amplitude quantization [11], [12]. However, linear amplifiers collectively suffer from limited efficiencies and limited output power levels. Hybrid amplifiers strive to alleviate these restrictions by combining linear amplifiers with high-power, efficient switch-mode converters [13], [14]. Nonetheless, such topologies increase the complexity of the electrical circuit and, if utilized, the feedback control system, as the fundamentally different characteristics of the linear and switched conversion stages, with respect to their dynamics and input/output impedances, have to be analyzed and stabilized individually, which increases development effort and cost.