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Basic differences between AB and D amplifiers with different loads?


Major Contributor
Aug 8, 2020
North Carolinia
Having built amplifiers and speakers, I understand some of the generalities of how loads effect the amplifier. Mostly in stability, but some in "signature" Not for class D. The goal is of course, no discernable signature, and no load induced issues.
I noticed one company giving all their specs into 4 Ohms. While this is good, not having the 8 Ohm spec leaves me wondering.

Typically in an AB amp, distortion rises with current, so 4 Ohms has more distortion than 8.
Typically, an AB will get unstable at low Ohms.
Typically, a well designed with sufficient power supply AB will produce almost double the power into 4 as 8.
Typically, we can get about 50W per pair of outputs in AB safely
Typically, if you double the number of outputs, you double the bias current. Things get hot quick.
Typically, the feedback is taken before the output filters. Curious. I see both opportunity and difficulty with stability.
Typically, big phase angles on the load can cause instabilities as well as high currents and distortion
Typically, noise is a floor and does not change much with input level ( should it be listed in mV, not just SNR?)
Typically, we need quite a bit more bandwidth to minimize the various distortions, Whatever the catch phrase distortion is today.

Class D is different. How does it react? What can the "standard" tests as currently done tell us how, or more importantly, if, it will have a sound of it's own when driving a real load? I can look at an AB amplifier test and look at the schematic and get a reasonably good idea how clean it could be. Not a clue on D.
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