A feedback controller in a PWM amplifier comprises a signal input for receiving a pulse width modulated (PWM) input signal (Vin) whose duty cycle represents a desired analogue output signal. A feedback loop filter 518 generates a filtered error signal (Vint) comprising a filtered representation of differences between the input signal (Vin) and a feedback signal (Vfb). A comparator (520) compares the filtered error signal with a reference to generate a provisional PWM switching control signal (C) for controlling the PWM amplifier (500). A pulse conditioner (532) receives both the provisional PWM switching control signal (C) and the PWM input signal (X=Vin) and outputs to the amplifier (500) a conditioned PWM switching control signal (Y), modified in accordance with predetermined constraints in relation to the PWM input signal. The pulse conditioner includes a single pulse function (534) whereby only a single pulse is permitted for every pulse in the input signal, thereby eliminating oscillations that can otherwise Occur under over-voltage supply conditions. Missing pulse and minimum pulse width functions (536, 538) can also be provided.