Explain Class A amp to non audiophile friends

A couple of non-electronic analogies perhaps:

A complete duty cycle of waveform is 360 degrees, remember 10th grade geometey?

1) Imagine hand cranking a wheel.

With one crank and one (or two) hand(s) cranking all the way around with one crank this is Class A (powering 360 of the duty cycle with one or more devices (hands) to make the complete waveform).

Now imagine a crank on either side of the wheel at 180 degrees apart, but in this case using two hands, you can only push each hand for one half of the rotation, pushing off from one to the other to make the wheel rotate with each hand stopping to push the very instant the other takes over. This is Class B (180 degrees of powered rotation per hand, and not more, transferring the other the very instant the other stops working, of the duty cycle X 2 to make the complete 360 degree waveform).

Now imagine that, using two hands, as the other begins to take over, the first still provides a little assistance until the other is able to "take over" and complete the one half cycle push. This is Class AB (180+ of the duty cycle, dependent on the bias, X2, to complete the duty cycle to complete the waveform, there is a bit of "hand off" from one hand to the other before ceasing to powere the half cycle. You cna see why this is the most efficient because each hand gets to assist the other during the power on phase and gets to "rest" for most of the one half cycle. Walking is also like Class AB.

Another analogy is bicycling without clips on the pedals where you're cycling by pushing only from one foot to the other (Class B)during the push phase only and the other where you are using clips or clipless pedals, so that each foot helps all the way around, but during the power portion of the stroke, one foot dominates over the other, and then trades off to the other and so forth (Class AB).

If each foot could power exactly the same amount all the way around this would be Class A and could be performed with one or two(or more!)feet, but each foot would power the complete 360 cycle evenly and all the time.

Well the whole goal of running class AB instead of Class B is to pretty much eliminate crossover distortion by judicious biasing of the output devices and usually also the use of negative feedback. I'm not sure if it can be eliminated to the point where it never appears on a 'scope under any circumstances, probably not would be my guess, though it is worse at lower power, better at higher power. Still, that's a great question for a circuit designer with real tech knowledge. Here's a link to a bit of an explanation of crossover distortion: http://www.electronics-tutorials.ws/amplifier/amp_7.html

I guess the crossover distortion in a Class B or Class AB amp is measured as part of the overall THD spec, so I'm sure, like any departure from linearity, it's audible in some form or another, but it may be so small as to be a tiny component of the sound. You know, no amp is perfectly linear. I'm sure there are single-ended Class A amps with higher THD levels than some well designed push-pull AB amps -- it's just different distortion. With circuit design there are always compromises, it's always a matter of picking your poison.

What is the difference in a single ended solid state class A amp and one that is not single ended?

What is the difference in a single ended solid state class A amp and one that is not single ended?

A single-ended class A transistor amplifier will be very inefficient. It will also be higher distortion, as the 2nd transistor of a push-pull class A design will help cancel certain distortions, in particular even-ordered harmonics.

This is true of SET vs push-pull tube amps as well. However I would expect an SET to manage better than a SE transistor amp as triodes are inherently more linear. Note also that coupling a single-ended transistor amplifier to the speaker has some challenges which are likely best met by the use of an output transformer. It is for this reason that single-ended transistor amplifiers will be extremely rare.

One thing that has not been mentioned about push-pull and class A is that its not entirely about crossover distortion! If you read between the lines my explanation above points to it- by having two output devices track together in opposition, you have distortion cancellation all the way up to the full power of the amp. In a class AB situation this is not true- so they tend to have more distortion at higher power levels (outside of the 'A' region). This can cause the amp to sound good at low volumes but get harsher at higher volumes. Many of you may think that all amps do that but you would be mistaken :)

Now here are some other facts about class A:

It can be used single-ended or push-pull. It can also be tube or solid state. Because class A is more linear, it also makes possible the ability to run the amp without feedback. This is true whether single-ended or push-pull, and/or whether tube or transistor.

(In the world of tubes there are also two variants of class A, A2 and recently an A3 design has been introduced by Electra-Print of Las Vegas. Neither A2 nor A3 can be executed in a semiconductor embodiment as the output devices would be destroyed in the process, but tubes can and do allow this operation with good reliability. A2 and A3 describe amplifiers wherein the output devices do conduct during 100% of the waveform at full power, but have operating perimeters set up in such a way that considerably more power is available, although there may be considerably greater requirements to drive the output tubes with linearity. Fisher made a class A2 amplifier called the A-50 back the 1950s, Atma-Sphere makes Class A2 OTLs right now and Electra-Print of Las Vegas offers a class A3 SET amplifier.)

Since feedback is associated with odd ordered harmonics (which the ear finds quite unpleasant), there have been a lot of designs showing up in the last 10-15 years that are class A/zero feedback in an effort to make the amplifier sound more like real music. This is the impetus behind the SET movement, Nelson Pass' '1st Watt' amplifiers, the Ayre amplifier, certain triode push-pull amplifiers (VAC makes some if memory serves) and our own amplifiers (OTLs).

However class A is a tool in a designer's kit, so to speak, and like many other aspects of amplifier design, are not the make or break indication of the sound of the amplifier. However, it *is* usually a good sign :) that things will go well.

I want to thank everyone for the above explanations of how a Class A amplifier works. I explained Class A to someone at my gym today and he got. A miracle! Thanks again to everyone. Great job.