A digital amp is a class D amp that takes normal PCM signals and converts it to PWM in software.....then into a class D output stage. Some will just say its a class D amp with a digital input only. However, most of the manufacturers of this type of amp call them digital amps........including Technics and Lyngdorf. How you do the software conversion of PCM to PWM is crucial. Tact was the first company to make such an amp back in the 90s. In their white paper they stated that converting PCM to PWM was inherently non linear.....and that is why it took them so long to do the software. We now have much greater computing power and people have been writing audiophile code for years......but there is always room for improvement.
Because it converts the digits directly to PWM (analog) then there is no need for a "normal" dac......and, if you use excellent digital attenuation......no need for preamp or other kinds of analog volume controls. You might call it a "power dac". Tact actually lowered the output power voltage to reduce the gain in their amp they made in the 90s. Now we have seriously good digital attenuation and we now work with higher bits then back then.