Audio power amplifier circuits that are arranged to operate in either a class AB or a class D operating mode. A drive circuit is configured to share control between a class AB driver and a class D modulator/driver circuit. The input signal level is monitored to determine their levels. When the input signal level is below a minimum signal level, the audio power amplifier circuits are operated in a standby mode. The power amplifier circuits are operated in a class AB mode when the input signal levels are in a defined operating range that exceeds the minimum signal level. When the input signal levels exceed a maximum signal threshold, the power amplifier circuits are operated in a class D mode. Hysterisis can be employed to minimize oscillation conditions about any one particular trip-point for the operating modes.