Explain Class A amp to non audiophile friends

In Class A, the bias current is turned on to the transistors all of the time. Meaning that the transistors are always on and ready to reproduce the signal. In a Class AB amp, the signal triggers the current to the transistors, turning the transistors on when louder passages are coming and shutting bias off when signal quites down. The end result is that in a Class AB amp the current to the transistors is turned on and off, thereby sparing electricity. Those that enjoy Class A sound say that it's due to not hearing the byproduct of what they call switching disorder. That in essence is the sound of transistors turning on and off.

The down side to Class A is that the amp runs very hot when bias current is on all the time, which also increases the electric bill.

First you have to think about the waveform of a sound, it has a peak and a trough and it cycles from peak to trough over and over at a certain frequency.

With a Class A, the amplifing device is drawing more than enough current at all times throughout the whole cyle and is being called on to amplify the whole cycle from peak through trough. There can be one or more amplifying devices -- a tube, a transistor, etc.

With a Class B there has to be at least two amplifying devices per channel -- a pair of transistors or tubes (or two pair or three pair etc).

The signal is split going into the output devices so that one device is amplifying the peak of the waveform and one is amplifying the trough of the waveform. When one is working the other is switched off. This results in crossover distortion at the point when the devices switch.

In Class AB it's like Class B but the "resting" device isn't ever fully switched off. This reduces the crossover distortion.

http://sound.westhost.com/class-a.htm

Is the crossover distortion of class AB amplifiers actually audible?

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.

Charles1dad, Yes it is. You have to keep in mind though that the ear hears many distortions as tonality issues. Also, the presence of distortion tends to obscure detail so in the range where the distortion appears the amp will likely sound harsher and less detailed.

In Class AB it's like Class B but the "resting" device isn't ever fully switched off. This reduces the crossover distortion.

The above statement is incorrect. If the device does not go into cutoff it is by definition a class A device. If it is class AB, the output tube or transistor will stop conducting during part of the waveform.

OK, here's an explanation for your non-audiophile friends & for you:
class-A amplifiers are a class of power amplifiers where the output transistors (or output stage) driving the loud speakers are always on & are always dissipating the maximum wattage that the power amplifier is rated for. Due to this fact, the heat sinks of class-A power amplifiers get very hot - 54C is considered the norm. Class-A power amplifiers actually run a bit cooler when the music is playing because a significant portion of the output wattage is delivered to the loudspeaker (rather than being dissipated as heat when no music is playing). The advantage of class-A power amplifiers is that, since the output stage is always on at full power, there is minimal amount of signal distortion (THD).

in a class-AB power amplifier, the output stage driving the louspeaker is biased at a very low level in comparison to a class-A power amp such that the output stage is considered to be weakly on (as compared to being full-power on in class-A and completely off in a class-B power amp). The reason for having the output stage weakly on is to avoid what is called as "cross-over distortion". Cross-over distortion occurs when the power amp needs to follow a music signal that is going from a positive cycle to a negative cycle or vice-versa and some time is required for the transistors servicing the negative cycle to turn on (pos cycle --> neg cycle). The advantage of a class-AB power amp is that it consumes much less power during idle & remains warm to the touch (as opposed to hot/scalding to the touch for a class-A power amp). The cross-over distortion is minimized in a class-AB power amp but I believe that it is not eliminated.

There are several well-known amp designers who subscribe to the view that a class-AB power amp can sound just as rewarding as a class-A power amp & that it is all in the implementation.

Hope that this helps.....

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

here's my attempt at a semi-laymen view of the issue.

with Class A, a single output device (eg, transistor) produces the full output wave. That is, both + and - side of the sound wave.

with Class AB, the + side of the wave, and - side are produced by 2 separate output devices.

The result is Class A generally produces more coherent sound because the 2 waves in Class AB don't always match up perfectly resulting in "crossover distortion". Further, Class A is less efficient, and generates more heat because of the way the output devices need to be biased to operate in Class A. Conversely, Class AB is more efficient to operate, and to manufacture (all other things being equal).

Don't explain, as there is nothing non audiophile's will ever understand about your obsession. Non audiophile's can only understand what they hear, better or worse. So, leave them with A is better, then let them hear so.

Thanks Atmasphere,
It would seem(at least in theory) that class A operation although much less efficient is a more pure and less 'altered' form of signal propagation.Implimentation probably has much to do with the final sound quality when comparing class A vs class AB.

The last responder got it right technically speaking. I see confusion regarding Class A and Class A/B all the time. Some believe that Class A means the bias is such that the output (could be any devices in the amp) are turned on all the time. This is techically true. But, others don't understand that there is class A and class A. One is the output devices (or pre-drivers), input devices, etc. can be biased such that they are always "on". This could be the equivalent of a 100 wpc amp biased "on" at 2watts. it is still class A because the drivers are "always on". However, there is the other class A, where it is biased "on" at rated output power. This means that without an input signal present the output drivers are biased "on" at sufficient current that the drivers total output produces the current for 100 watts. This means that you really need a huge, beefed up power supply with expensive large transformers, etc. and expensive heat sinking. So, there is class A and class A. Class A/B is as discussed previously. So, like any electronic device, especially amps, you really have to read between the lines on the specs, to get the real truth. I have seen amps touted as class A at 150wpc, which were totally untrue. Because without large transformers, and stupidly large and expensive heat sinks, there is no way on this planet that they would be biased at 150wpc class A. Maybe 2wpc class A, but not the other. The old saying is still true. you can really tell the quality by how much it weighs. Get on Nelson Pass' web site on the DIY site, he really gets into detail to explain Class A and heatsinking and power supply design to the lay person. This is Engineering 101. There is no magic here. it is what it is. lots of manufacturers outright lie or stretch the truth regarding specs. others, such as Nelson Pass tell it like it really is. Seriously, his explainations are really good for non-Engineers to really understand amp design.

enjoy

here's my attempt at a semi-laymen view of the issue.

with Class A, a single output device (eg, transistor) produces the full output wave. That is, both + and - side of the sound wave.

with Class AB, the + side of the wave, and - side are produced by 2 separate output devices.

The result is Class A generally produces more coherent sound because the 2 waves in Class AB don't always match up perfectly resulting in "crossover distortion". Further, Class A is less efficient, and generates more heat because of the way the output devices need to be biased to operate in Class A. Conversely, Class AB is more efficient to operate, and to manufacture (all other things being equal).

LOL! Reading over this thread I think that Buconero117 has the best explanation, that being no explanation.
I very, very rarely try to explain any of my "crazy" hobby to "normal" people. I find it frustrating, much like trying to teach a pig to sing.
It only wastes my time and annoys the pig.

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.

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.

How about, Class A is like a lightbulb that is on all the time, but increases in brightness as the music demands more power, whereas Class AB, the lightbulb is being switched on and off as needed.

Hi all ! Here is my standard answer....like driving a car , in class AB you drive around and vary speed with the gas pedal . In class A you hold the gas pedal to the floor and use the clutch to vary your speed . Not perfect , but effective .