Class D (PWM) can be easily constructed without negative feedback at all. NGF improves linearity, bandwidth and output impedance like in any other class of amplification.
While global negative feedback in class A, AB is going over many stages of amplification, that create delay (thus producing TIM distortions), class D amplifiers have only one stage - a modulator that drive output switches (Mosfets). This modulator converts voltage to duty cycle of the output frequency. In simplest case it can be created using ramp generator and comparator but current modules resemble more of Delta-Sigma A/D converters. In short one analog quantity (voltage) is converted to another analog quantity (duty cycle) to end up with voltage again by obtaining average value of duty cycle. It is usually done by common mode choke and capacitors (Zobel Network), leaving about 1% of switching noise on the speaker wires. Amount of output power is controlled simply by setting amplitude of switched DC voltage. Frequency of this remaining noise is too low for the speaker cables to become antenna for electromagnetic coupling, but direct capacitive coupling is still possible.
As for the type of power supply - first generations of class D amp also has SMPS power supplies REF1000 being one example and my Rowland 102 being another. Ice power modules were available from the start in two different varieties - with or without SMPS. These switching supplies used to operate around 50-100kHz to preserve efficiency, but newer designed by Rowland run at 1MHz to make filtering easier (very difficult to design). I would say that good efficiency, very quiet SMPS operating at 1MHz able to deliver close to 1kW is a masterpiece.
One advantage of SMPS powered amplifier is often ability to operate at universal voltage and to tolerate any amount of DC. In fact my amplifier can be supplied by DC only up to 400V. Another advantage is regulation. SMPS have line and load regulation. Amplifier with SMPS keeps composure during power peaks since voltage does not sag, like it often happens in linear power supplies.
While global negative feedback in class A, AB is going over many stages of amplification, that create delay (thus producing TIM distortions), class D amplifiers have only one stage - a modulator that drive output switches (Mosfets). This modulator converts voltage to duty cycle of the output frequency. In simplest case it can be created using ramp generator and comparator but current modules resemble more of Delta-Sigma A/D converters. In short one analog quantity (voltage) is converted to another analog quantity (duty cycle) to end up with voltage again by obtaining average value of duty cycle. It is usually done by common mode choke and capacitors (Zobel Network), leaving about 1% of switching noise on the speaker wires. Amount of output power is controlled simply by setting amplitude of switched DC voltage. Frequency of this remaining noise is too low for the speaker cables to become antenna for electromagnetic coupling, but direct capacitive coupling is still possible.
As for the type of power supply - first generations of class D amp also has SMPS power supplies REF1000 being one example and my Rowland 102 being another. Ice power modules were available from the start in two different varieties - with or without SMPS. These switching supplies used to operate around 50-100kHz to preserve efficiency, but newer designed by Rowland run at 1MHz to make filtering easier (very difficult to design). I would say that good efficiency, very quiet SMPS operating at 1MHz able to deliver close to 1kW is a masterpiece.
One advantage of SMPS powered amplifier is often ability to operate at universal voltage and to tolerate any amount of DC. In fact my amplifier can be supplied by DC only up to 400V. Another advantage is regulation. SMPS have line and load regulation. Amplifier with SMPS keeps composure during power peaks since voltage does not sag, like it often happens in linear power supplies.