This paper reports on a novel voltage-mode class-S power amplifier for the 450 MHz band, based on GaN–HEMT monolithic microwave integrated circuits (MMICs). It achieves a peak output power of 3.4 W for a single tone at 400 MHz, encoded in standard band-pass delta-sigma modulation with 1.6 Gbps sampling frequency. The corresponding efficiency is 38%, peaking at 52% for 0.5 W output power. In order to demonstrate the influence of coding efficiency, additional measurements using a periodic square-wave signal were performed (class-D operation), which yield a maximum output power of 7 W with 64% efficiency. To the author's knowledge, these are the best results in this frequency range achieved so far with the voltage-mode class-S configuration. The paper discusses behavior at power back-off and the influence of the loss mechanisms.