Class A Power in A/B amplifiers?

There really is no such class as Class AB, it is used to describe the mode of operation when a Class A amp hits a low impedance and one of the pairs of output transistors stops conducting.

This statement is false. Class AB is defined as any push-pull amplifier which is class A at lower power (which is done to prevent artifacts at the zero crossing point) and class B at higher power. The *amount* of class A power is undefined.
Types of amplifier class:
Class A1, A2 and A3 (the latter recently develeped)
Class AB (which includes class AB1 and AB2)Class B
Class C (only used in RF power amplifiers)
Class D (doesn’t refer to digital; ’D’ was simply the next letter- the first commercial class D amps were made in the 1960s)
Class E (switching RF amplifier)
Class F (harmonically tuned RF amplifier)
Class G (modified A or AB amp; IIRC first used by Hitachi)
and on and on...

Might be helfpul to understand how A and B differ, and that transistors are not perfectly linear. It takes a little current before they wake up and can act in a linear fashion. This is what causes the notch distortion during the crossing of 0V.

In Class A, both legs start at full on. Since they are balanced this ressults in 0V at the output, and maximum power draw through what are essentially to closed switches.

In Class B both are off, again, achieving 0V at the output and minimum power draw through essentially open switches.

The bias, or quiescent current, in an AB amplifier keeps both legs on enough to avoid the dreaded notch distortion without consuming 100% of the power in a true class A amplifier. The difference is that eventually as the output voltage shifts far enough away from 0Vthat one leg will shut mostly off, switching to B. When the voltage goes back towards 0V that leg now turns on, with some inherent distortion as they attempt to reach equilibrium.

Personally I find the arguments for/against Class A on technical terms alone lack overwhelming evidence. I've heard Class A I liked and Class A I really did not.  Same for A/B amplifiers.  So while it's fun to look at how engineers have take on the same challenge via a variety of methods I always find my ears better at determining what I am going to want to listen to than amplifier classes.

Personally I find the arguments for/against Class A on technical terms alone lack overwhelming evidence.

@erik_squires The arguments are twofold for which there is a lot of evidence if not proof. The first is that you can bias the device in its most linear region. Given that most amps tube or solid state can't really be given the feedback they really need, this is a useful means of allegedly not having to have so much feedback to build a linear amplifier because distortion is lower to start with.


The second issue is that you get more accurate even ordered harmonic cancellation when both output devices are operating in the A region. With the even orders cancelled, there is less overall distortion and so the odd orders tend to be at a lower value (this particular aspect is more useful if the amplifier employs the push-pull/differential operation from input to output). This again is helping for the fact that its likely that insufficient feedback will be applied to the circuit.

The first is that you can bias the device in its most linear region.

This is correct, and why the "better" higher power Class-A’s use up to 24 sometimes 48 even 64 complimentary outputs as in the Threshold Stasis SA/12E monoblocks, so as to only use the "most linear part" of each transistors curve. https://ibb.co/d7BKwzL

The SA/12e, being fully class-A from input through to and including output stage, draws its maximum power from the line at idle—about 1000W for the pair.

https://www.stereophile.com/content/threshold-stasis-sa12e-power-amplifier-page-2

Cheers George

Hi @atmasphere

Your technical explanations are, as usual, spot on, but since I failed to explain what I meant by by "proof" I am afraid you made a counter argument to a point I did not mean to make.

What I meant to say that my own ears have yet to discern a preference of Class A over Class A/B or even D which is even moderately consistent. The behavior of class A circuits vs. A/B in technical terms is more or less well understood, as you show in your rebuttal to my post, but not in the realm of personal experience.

To me, there is no proof class A is consistently or even broadly better than any other class of amplifier when each class is well executed.

Yes, some stellar Class A amplifiers exist. Do they make me go "wow this is what Class A means?" No too many other middle of the road or poor sounding Class A amps out there.   At best, I could be convinced that class A amps are an acquired taste which some develop and then never grow tired of.  Fair enough! It's a happy man who finds what makes them happy early in life!

Best,

Erik