Amp/Preamp Asylum

Posted by Tre' (P) on February 04, 2022 at 07:27:41

In Reply to: RE: Sure, A/B amp usually have lower bias settings posted by Feanor on February 03, 2022 at 09:57:46

"A class A/B amp is a class A amp while operating under these conditions; when the power spikes above that limit, it operates in class B mode. This transition happens seamlessly and continually."

I guess it's a matter of semantics but in a proper Class A amplifier the output devices are biased halfway between saturation and cutoff (in the most linear region of the transfer curve). In a proper Class B amplifier the output devices are biased so they just reach cutoff with no signal present.

In a Class A/B amplifier neither of the two conditions above are meet.

Here's why is matters. Output devices are not perfectly linear. The most linear part is the center of the transfer curve. When the output device is biased right in the middle of this curve (in the most linear part of the curve) then the first watt (or so) is the cleanest. In a Class A/B amp the output devices are biased well below the most linear part of the transfer curve. The devices do not operate exclusively in the most linear part for the first watt or two like they do in a true Class A amplifier. So even though the output device don't reach cutoff (while the A/B amplifier is in the so called "Class A" mode) it's still not the same operating point as a true Class A amplifier would be.

I may be blind but I can't understand why people are not seeing my point.

Class A is a particular type of operation, Class B is a particular type of operation and Class A/B is yet a different type of operation, not a mix of A then B.

Tre'
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