He's not totally off
Tex-amp said:
I assume from this statement you have very little to no experience with them.
Just read an article written by a Bruno Putzeys. Here's what he had to say about these D amps: Class D designs are prone to distortion, mainly from imperfect power supply regulation and timing error. Since the output voltage of a Class "D" amplifier is directly purportional to the power supply voltage, any error in that voltage modulates the output voltage. Power supply diferences or variations in the amount of current drawn by the amplifier show up in the output as "distortion." Instabilities in the supply itself, such as "power line ripple", show up at the output as noise, or the dreadful
hum. Building a power supply so that voltages remain rock steady in spite of fluctuations in output current is a monumental and costly task.
The other source of distortion is timing error, due to variation in how long MOSFETs (metal-oxide semiconductor field-effect transistors - FYI) take to switch from on to off, which in turn depends on how much current the amp is being called on to deliver. This eror causes the output duty-cycle to differ from the input duty-cycle; the output signal's shape differs from the input signal's shape. Timing error causes distortion proportionaly to the duty-cycle error—the ratio of the timing uncertainty to a single switching period. The greater the timing uncertainty and the higher the switching frequency (the higher the frequency of the triangle wave in the PWM circuitry), the worse the distortion.
Frequency response is another performance issue for "Class D" amplification. The all-important
low-pass output filter, which recovers the original audio signal from the Pulse Width Modulation (pwm) waveform, is passive, and its frequency response is flat only when it drives a purely resistive load of a specified value. Since the impedance of real loudspeaker loads varies between 4 and 16 ohms at different frequencies, a solution is obviously needed.
Class D does not stand for digital. What distinguishes "Class D" amplifiers from all others is that their power transistors are always operated either fully on or fully off. Class D amplifier can reproduce only binary (two-valued) waves. So to use it to amplify analog music signals, those signals have to be
converted into a suitable on-off waveform. One way to do this is with pulse-width modulation - as stated above. In PWM, the amplitude of an analog input signal serves to control the average percentage of time the transistor spends turned fully on, known as its "duty cycle."
The PWM signal is generated by comparing the analog input signal with a "triangle" waveform—one that continuously sweeps linearly from a low to a high value and back. To do that, both signals are fed into an analog device whose output is
high whenever the analog signal has the higher instantaneous value, and
low when the opposite happens.
The output of this analog device, then, is a waveform that has the information of the original analog signal and yet switches between just two values. In a Class D amplifier, this "PWM" waveform acts as a binary control signal that switches the transistors
on and off depending on the amplitude of the analog input.
Changing the power supply voltages changes the amount of amplification.
If it seems complicated, it is. The best class D amps go into our subwoofers. They also go into car audio amps where ventilation is an issue. I don't think the home audio world is ready just yet for "Class D" providing all of the amplification needed to drive all of our frequencies to our satisfaction. Class A is still the best for lowest THD, and AB seems to be the best compromise of all options.
