pure class A?

Oh yeah - more on "linearity"

If you look at the transfer curve of any device, they tend to be most linear (straight line) in the middle of the curve. That means that for a given input increase, the output changes the same amount for each increase/decrease.
(ie. "linear" - a 1:1 relationship being maintained)

At the ends of the curves (all the way on, and all the way off) the relationship is not so good. In fact the transfer curve looks like an "S."

What this means is that when almost off - at the bottom of the curve the device is not very linear - you need much more drive to get not so much output, UNTIL you get past the curve of the "S" and it gets linear.

So, two things are important here: 1) for class AB and B circuits, you're almost off (near "cut-off") so the curve to begin with is not linear. 2) Class A is in the middle of the most linear portion of the curve.

In class AB, the idea is usually to bias so that you start out just above the non-linear portion, so that all positive going drive sends you only up through the linear portion of the curve. Of course, when any one side of a Push-Pull pair is driven toward "off" it travels back down the non-linear area. Thus the gains do not sum perfectly, and the earlier comment about "crossover distortion" applies.

In class A, you can drive out of the linear region on peaks! This is *part* of the sound you get from pure triode Class A ZERO feedback amps. There is effectively some *compression* when driven hard into the ends of the linear region (more signal does not get you equally more output). Which in part accounts for why small pure triode amps can sound like they play louder than very linear solid state amps.

Feedback changes this, since it forces the input to be whatever is needed to keep things linear (in this example, more drive is required until a 1:1 gain relationship is acheived).

Here when I say "1:1" gain relationship it does not mean that the gain is unity, but if it is 1 input unit then you get out "N" output units, 5 input: 5N output.

Which is best? That's entirely unclear.

To add to the design mix - not all devices, tubes or transistors *ARE LINEAR* by themselves! Most are NOT. Most
are either entirely non linear everywhere or linear over only a small portion of their range. This is a large part of why feedback of all sorts is employed in practice. Keep in mind that the main reason a cathode follower is linear, is that it uses 100% feedback! :- )

Bear, your encore deserves another BRAVO!

This is my understanding of the subject.
Live music are complex vibration waves in air, an event which occur in space and time. Real live music can only happen as "pure class A" events in a mechanical sense. The pioneering audio amplifier engineers got it all right in the beginning. Back then there were mainly SE pure class A tube amps of very low power driving horn load or similar designs high efficiency speakers. (The success/shortcomings of such early speakers is another story for another A'gon thread.)

IMHO the reproduction of this real class A mechanical music event(recorded) can't get closer than through a pure class A electronic amplifier. I will not attempt to enter into the argument of "distortions" harmonic or others, of different types of amp designs, because honestly I cannot hear them. This is due, I can fairly suspect to the real event recording is not distortion free either, nor is my listening room perfect. So the minute amp distortions, added to the overwhelming inherent distortions, has become a non issue. In my own experience, my current SE pure Class A monoblocs provide that uncanny "breath" and "feel" in space and time which blew away all my previous equipment of various classes, AB & B etc. Ironically they all have more impressive distortion measurements in the catalogs than the Class A. Apparently the distortion numbers had not affect the way the "breath" and "feel" being reproduced. Don't take my word fully, go try out some pure Class A amp in your home setup to experience it. Sorry, you may need another set of speakers too.

I earn my bread as an electrical techy, but for my audiophile pursuits I will push aside my scopes and meters and use my ears. I wonder whether the audiophile equipment tradition had it all wrong, paying to much attention to measuring the unimportant parameter. I have no problem with the various class modes of amp operation, but today i take a break cos the posts above done a wonder job. I will attempt it in lay terms.

IMHO classes AB,B,C etc will always be commercial or technical compromises. Some came into scene to drive low efficiency modern speakers. Some such designs rightly belong in professional audio or even public announcing systems, not home audio. Chopping pure signal apart, reflecting mirror images of it, massaging it with some feedback at some point, and then reconstitute it for consumption. This is not real milk.

IMO pure Class A, warts and all is the way to go for the purist audiophile.