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A, AB, B, C, D has nothing to do with the word 'digital'.

The Class "D" amplifier term is an extension of the class A,AB,B and C terms related to how much of the wave the output devices are conducting. Class D being something like bipolar class C used in RF transmitters.

The pulse width modulator at the core of class D is neither a voltage nor current amplifier, it is a duty cycle modulator that controls the "on" period of the output switch and for an audio signal it is a switch that connects the output to the positive and then negative supply Voltage. The audio signal looks like pulses that are positive or negative and with no signal, there is still some "on time" for both voltages while full amplitude means the pulses have very little "off time".
An easy way to describe this process is having a battery, light bulb and a switch in series. IF you opened and closed the switch say 25 times per second with a 50/50% duty cycle, the bulb is getting about half the battery V.


Just like the D to A process, a low pass filter is required at the output AND the higher the switching frequency, the easier that filter is to make. Since this is not a Voltage or current amplifier, the output Voltage must be derived and to close the loop around the process only a first order filter is used to preserve the Bode criteria for stability.

Modern class D amplifiers have a switch rate typically several hundred KHz some in the low MHz range, in the 90's PWM amplifiers were common driving servomotors but the upper audio limit was under 1-2 KHz with a switch F 30-50KHz.

All other performance things being equal, the switching process has a large difference so far as the output devices and what
s called safe operating area, a map describing what combination of voltage and current is "safe". For the switching amp, reactive loads are not a problem, for linear output stages death occurs if you go beyond the SOA.

Also another difference that in some cases is huge is the power strategy. OLD style amplifiers were rated to put out say 50 Watts RMS with a sine wave signal and that meant that driven to say 55 Watts they were clipped or distorted.

Loudspeakers (oddly) had a different strategy for the measurement / rating since music is very different than a sine wave, this alternate method is used that uses "noise". Currently, this is shaped pink noise that has a 10dB peak to average ratio. That means if you connected an RMS voltmeter to the amplifier, the amplifier's RMS average or heating value is 1/10 the largest peaks in that signal.

So if one had a loudspeaker rated at 50 Watts RMS, you actually need an amplifier that can put out the entire test signal including peaks of 500 Watts (10dB over the average).

The next thing will sound crazy unless you were aware of it but music is actually harder to produce than the pink noise signal. Even compressed FM has a peak to average of 10dB or more and +20dB (+20dB = 100X) is not uncommon.

There is a test signal that is closer to music called "M" noise, this has peaks that are +17dB over the average level (in sound level, it is the average level a sound level meter shows and what we interpret as loudness)
To pass that signal in it's entirety, you need an amplifier that can put out 50 times more than the average power level.

Everyone knows what clipping sounds like and how digital clipping sounds different BUT until clipping lasts long enough, you can't hear it as a flaw, but you can hear the difference when you take it away, the unclipped version is more dynamic in an A vs B comparison.
Back when an oscilloscope was a common tool, one could often see clipping on the output long before you could hear it.

But if one gets rid of that instantaneous clipping and produced more or all of the music's dynamics that is audible.

So, many commercial class D amplifiers are designed around what's required for music not a sine wave rating and it can be hard to sus that out looking at data sheets.
I am familiar with some of these modern Class D amplifiers. These do what they claim with a music signal. They can put out 20KW peaks while not tripping a breaker and do not violate physics, they are designed around what the music signal asks for.

https://linea-research.co.uk/44m-series/

There is a reason people have high efficiency speakers in the home, it's easier to produce more of the dynamics with any amplifier.


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