pure class A?

Audiofile9, what you describe is known as a push-pull arrangement in terms of one output device "handing off" to another device as the signal changes polarity. These types of devices, especially SS models, are typically sold or marketed as "complimentary pairs", etc.. This somewhat assures near identical operating conditions for each polarity of the cycle. This is very similar to the way that running matched pairs in parallel assures that they share the load equally. Sean
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Sean, your the man. I've read numerous descriptions on the differences of "class", but yours is the most lucid and succint yet. Thanks.

Unsound, right on! Sean, thanks for the clarification. If you would be so generous, perhaps you could explain the difference between "Single Ended" and "Class A" (and how "push pull" relates)? I've always been fuzzy about the differences there. Perhaps like many others, I assumed Class B was synonomous with PushPull, etc. Have seen the many threads that touch upon these subjects, but have not found any bearing your excellent analogy as above.

Audiofile9 makes a very good point (request). I once recall someone telling me that while you may understand something, the true test is being able to explain it in simple terms to others, so that they understand it. That said, I felt I understood class B vs push pull, but clearly Sean's articulate analogy shows he REALLY understands this concept. And while I also feel I understand "single ended", "triode", "pentode", and "OTL" I don't think I could do the explaination justice--but hope that Sean will.

All "single ended" amplifiers are by definition Class A,
if they are high-fidelity amplifiers.

All that single ended means is that the amplifying element
(tube or transistor) is alone, and not in push-pull. So,
this means that it is starting out, when doing *nothing*
at all "halfway" on. In other words, biased Class A.

By being halfway on, it can go all the way ON,and all the
way off, this being the AC signal that it can produce. If you are sitting there thinking, 'hey, if it's on halfway all the time, that's wasting a lot of power' then you've got it right. Class A single ended amps tend to waste about 75% of
their power as heat, not audio output.

If there is an advantage to Class A amps of any type it is that at low levels the inherent linearity of the output stage is better than a class AB or class B which has some non-linearity happening as one set of output devices switches all the way *off* as the signal (AC) swings through "zero" going from + to - and back to + again (through zero twice there).

The linearity issue is small, as is the deviation from linear gain - you can think of it as a dip or peak in the frequency response of a crossover, not summing quite right. But of course, this is not frequency dependent. It is however usually *bias* dependent. Since by definition all distortion is deviation from ideal gain, non-linearity in the "crossover" region is measured as distortion.

Doug Self has an excellent discussion of this on his website and in his book, although I may not agree with all of his conclusions on the subject, his technical work is impeccable. He has a "blameless" Class B design that measures better than most Class AB designs...

But that's not single ended. Single-ended is what is in most
tube preamps, the classic cascade circuit. A series of simple class A stages one run into the next.

Single ended class A tubes require a transformer in place of the resistor used in preamps at the plate, or else a plate resistor or choke and a cap blocking the DC to a transformer (a popular idea these days). In any event the output is created by sinking current through the tube, causing a voltage drop in the resistor, or through the transformer (more or less).

Push-pull is in effect replacing the resistor with an active device that amplifies the signal but *inverted* in polarity - so that when one side is ON, the other is OFF.

Two guys on opposite ends of a big old saw - one pushes, the other pulls (in phase output, out of phase input)! That's
push-pull.

Single ended - one guy on one side of the saw - pushes and
pulls all the time.

Single ended, parallel- two guys on the same side of the saw.

Push-pull class B - One guy pulls, then the other guy pulls - or you can think of it as one guy pushes and pulls *exactly* 1/2 way,
then the other guy completes the cycle, pulling and pushing back 1/2 way (a more accurate description...).

Class AB - as with class B, but in the middle, their efforts
overlap a little bit.

Class H? guys with jack hammers, pound the damn thing from one side to the other with monster pulses... or is that class G... I forget.

_-_-bear