This is a nice thread, hence a small contribution to the Class D information seekers and long with when Class D will have some synergy with the system.
The largest characteristic of the Class D is the low output impedance. This allows a higher damping factor and in turn more control of the speaker.
Synergy 1: If the speaker cannot handle the damping, it will cause break up, distortion and hence won't work. Typically softer cones and larger cones will have this issue. This is not limited to woofers only.
Class D is high frequency modulation or switching which is then filtered to recover the original signal. So band bass filters in essence, high frequency requirements and so on. Most Class D run around 400-450khz. This is much higher then the normal audio band of 20khz. Design, layout, placement, even length of similar runs will create RF noise and distortions. So design of the board, and chassis is very important.
The high switching speed places a requirement on the transistors to switch fast. However as with all transistors, there is a lag between the command to switch and the actual full switch. This is call the dead time. This has been reduced over the years with faster transistors and also limited with feedback. But it is fast. Hence the apparent speed of Class D also.
Synergy 2: The speakers should be able to hand the speed. Otherwise breakup etc. Bi-amping with a tube/classA (for illustration purposes only) on top and Class D at the bottom causes different speeds, creating a bit of coherency distortion, if there is a word for this.
Most Class D amps use switching power supplies. These have higher instantaneous power delivery. Hence can delivery a very high current and voltage in a small package. There is no large reservoir so the draw on the mains is almost as instantaneous as the draw from the amp.
Synergy 3: Power delivery which will include power cords. Line conditioners and other filtering devices, including power cords can limit and clearly slow/change the sound of Class D amps. It is best, IMHO, to remove all conditioners and any high filtering devices but you can listen to the differences to determine what suits you and your system the best.
Class D is quite efficient, however it has to constantly filter the carrier signal and hence the coil gets very hot. This is one of the main sources of heat in the system. The transistors are on or off, so very little heat here. While class D is claimed to run cool, it still requires sufficient venting for the coil to dissipate the filtered energy. So it takes a while to warm up and if not vented, can have thermal runaway and early failures. The heat build up is most when playing, not in idle. For example the VERITAS Monoblocks takes up 20 watts in idle, about the power of 2 LED light bulb or low power CFL. Certainly a lot less then a incandescent bulb at 70 watts
Class D tends to be fast, unless slowed intentionally or with a tube (which helps with dead time distortion also).
Synergy 4: personal preferences.
Class D with recent large improvements is catching on in audiophile world. There will always be multiple technologies, as there will be multiple personal preferences. The audiophile world continues to improve in the reproduction of the audio signal, and more rapidly it seems these days. We are luckly.
“In times of change, learners inherit the earth, while the learned find themselves beautifully equipped to deal with a world that no longer exists. ” Eric Hoffer
