I, too, am interested in description of Class A v A/B. In particular, what is the nature of crossover distortion or reduction in sound quality as an amp moves from Class A to B? I feel like I can hear it sometimes.
The difference if fairly simple. Class A amps run at full power all the time. Class B amps run 'on demand'.
Class AB amps usually run in class A at lowest power, then switch to class B at higher power.
Whether one is better than the other is hard to say, largely it depends on the individual design.
Personally I use 30wpc Class A tubes and it, to me, is far superior to anything I've owned in an AB design. But that's just my opinion.
As always it depends on which two amps you are talking about, not just what class they run in. I have had several Class A amps and countless AB amps; overall I think the Class A amps sound better, but I am not using one right now. Have one in mind I may try but I like my CJ 350. Not quite as sweet in the top end but close, with more bass than most A's outside of Krell and runs a lot cooler. Just one aspect of amplifier performance to consider. Bottom line, get what you think sounds right.
With a Plinius, which can be switched from A to A/B, A sounds more natural. Can't identify what crossover distortion sounds like because my ears are just not trained and tuned for that and most distortions tend to sound like any other frequency anomaly. I have heard some very convincing A/B amps, most of which I couldn't afford at the time.
No one is talking about the listening difference because the differences are so small with the latest crop of designs that most won't hear a difference , in fact many prefer the added dynamic's of A/B design's . Levinson was the first to shed itself of the class A circuit's , the 20.5 was the last I believe , the hundred pound hundred watt mono's ran as hot as large tube amp's . Even there reference amps of the late 90's were A/B designs . Class A amps were superior sonically 10 years ago or so but like so many things in audio the two designs have nearly reached convergence . I have a few friends that made up there minds years ago that class A is superior and won't change no mater what they hear .
The measurable difference between class A and AB is the crossover point. ie. In class A designs one tube/transistor amplifies the complete waveform. So bias current in transistor designs are at max levels all of the time. In a class B circuit, two transistors or tubes push/pull together to make the complete waveform. One does the (+) side and one does the (-) side of the waveform. That allows the bias current to be nearly zero at idle. The issue is the noise or not so perfect handoff of one transistor to the other as the waveform switches sides. One a scope, it can be seen as a discontinuity in the waveform at the zero line. Of course, todays amplifiers all but eliminate that crossover discontinuity. Older A/B amps had a brittle sound, an unpleasant edginess to my ears. I didn't like listening to them at very loud levels for long.
I recently compared 3 amplifiers, 1 Stereo, and 2 pairs of Monoblocks. The Stereo and 1 monoblock pair were reportedly standard class AB, with the stereo amp claiming "high-bias".
The 2nd Monoblock pair was reported to run class A up to a point.
The "class A/AB" monoblocks ran much hotter than the other amps, no surprise there.
In my case, I felt that at low listening levels the ClassA amp was in fact superior. I fell into using the term "inner detail". Whether that's the correct term from the Audiophile dictionary or not, I have no idea. I was absolutely convinced I could here more detail with the Class A/AB amps. I considered those monoblocks superior to both AB amps. At louder listening I felt this difference disappeared.
I consider myself NO golden ear and I don't subscribe to the tweek of the week, so the above is simply my 2cents worth on the subject.
I assume your question is directed primarily to SS amps, maybe including hybrids.
As also indicated by Stanwal, IMO the answer is; it depends on the amp.
There are excellent sounding Class AB amps and crappy sounding Class A amps. There are so many other factors involved such as the design, quality and size of the power supply; mosfet or bipolar output devices; the number, quality, and matching of the output devices; the use of negative feedback (although some respected designers use local negative feedback); input and output impedance as related to partnering equipment and speakers; power and ability to double down; and other critical design factors. As an example, the Belles 350A Reference amp is said to be a very good sounding amp by virtually every professional reviewer and most owners/listeners who have posted here - and that amp runs in Class B!
Class A amplification is not for everyone, no matter how good they sound. Class A amps are typically heavy, run very hot, use more energy, should not be left powered up all the time (as you can with cooler running Class AB amps) due to wear on internal parts from heat, and are generally more expensive to purchase per watt of power.
OTOH, if you have the ability to buy (and lift :>) quality Class A amplification, and you can deal with the issues listed above, there are some very good choices that many consider to be among the best in SS amplification. In my own experience with Class A, I have owned the Lamm M1.1 hybrids and two different Clayton models, each of which sounded very good to excellent. However, I need lots of power to achieve the level of dynamics I want from my speakers so the Lamm amps (at 100 wpc) would run out of gas at a certain level (although they sounded very good up to that point). My current Clayton M300 monoblocks with the most recent power supply upgrade are the best sounding amps I have owned. They do not run out of gas and offer 300wpc @ 8ohms and 600wpc @ 4ohms, all fully in Class A (when they are switched to high bias). They have a convenient high/low bias switch that can be set to low (200 wpc in Class A) for casual listening and still sound very good. Other Class A amps reported to sound very good to excellent include the Pass XA.5 series of amps (30 to 200 wpc in Class A), Boulders top amps, Lamm M1.2 Reference and 2.2, Krell class A amps, and more I have not listed. At the lower/mid price ranges, many have reported the Monarchy Class A amps to sound quite good, and Clayton also offers some amps in that range.
If I were to generalize based on my experience, in an attempt to answer your question, I would say well designed Class A amps, when compared to well designed AB amps, offer sweeter sounding high frequencies with less tendency towards grain or harshness, seductive midranges that are generally full and rich sounding with excellent harmonics, as well as being more dimensional (although tubes and hybrids do dimensional the best IME), extended decays, full, meaty bass, and a warmth and/or organic nature that adds a level of realism to the listening experience. The better/best Class AB amps can provide these things also, to varying degrees, and tend to sound different, with sharper leading edges (not necessarily in a bad way), more detail at the frequency extremes, better bass dynamics, and perhaps a touch of dryness in the midrange (in comparison). Of course there are exceptions.
If you're not one of the elitist that wont have anything but a class A amp, suggest you try a Magtech amp by Sanders Sound Systems. Wont cost you anything to try it, has a lifetime warranty, reasonable priced, delivers 500 watts per channel at 8 ohms and 900 watts at 4 ohms and and will deliver down to 1/4 ohm and finally runs absolutely cool. Has a 30 day no risk trial. What does one have to lose? Numerous reviews on this amp. See the Sanders Sound Systems website. No, I am not a dealer. I am just a very satisfied customer. IMHO an incredible amp.
everyone is talking about design difference. No one is talking about difference when listening. Can you hear the difference!???
no one ever talks about differences during actual use, because, there are none. As is the case with most 'issues' on this forum. But, since no one has mentioned hemorrhaging ears yet to support their argument, things are improving.
Rok2id, did you read the last paragraph of my post??? The only thing addressed in that paragraph is "difference when listening." That is the best I can explain it based on my experience owning three different Class A SS amps. To compare, the Class AB SS amps I have owned include McCormack DNA500, Cary 500MB, Herron M1, BAT VK500, McCormack DNA2 and Classe DR25.
Soundlock, why would a preference for Class A amplification make someone an "elitist?" I fail to understand why topics cannot be discussed on their merits, technical attributes, or on the basis of what folks hear in their systems and personal preference, without resorting to labeling.
"If you're not one of the elitist that wont have anything but a class A amp, suggest you try a Magtech amp by Sanders Sound Systems.
Has a 30 day no risk trial. What does one have to lose?"
Just to clarify, the Magtech is not a class A amp (you didn't state it was but kinda sounded like you did).
What one has to lose is the same thing one has to lose with just about every other piece of gear - Time, hassle, etc.
btw: The write up on the Magtech sounds a lot like the Sunfire amps by Bob Carver with the "Tracking Down Converter" power supplies, although there isn't enough info in the Magtech write-up to understand the technology used:
Mithch2: "If I were to generalize based on my experience, in an attempt to answer your question, I would say well designed Class A amps, when compared to well designed AB amps, offer sweeter sounding high frequencies with less tendency towards grain or harshness, seductive midranges that are generally full and rich sounding with excellent harmonics, as well as being more dimensional..."
TY, Mitch--I could not have written it better nor agreed more. To my 67-year-old (non-Golden) ears, the really good Class-A amps sound just a little bit smoother--less edgy, grainy--but still highy detailed.
I've had (and LOVED) Vandersteen 5As for months. Altho they're relatively insensitive at c. 87dB, a few weeks ago I bought and started using on the 5As two pairs of twenty-year-old Marantz MA-24s.
They're rated at 30 into 8 and 60 into 4, and with 4 amps driving the 5As' 6-to-4-Ohm impedance, combined maximum continuous power is around 200. That's plenty enough for me, even in my largish--c. 3200CF--room. These amps never sound gritty, edgy, grainy--and are always detailed and musical. They are indeed full-Class-A and do get hot, but that's certainly manageable in my room.
Class-A is a design tool that I appreciate and can afford, and my Class-A amps sound fabulous to me. Other amps sound great, too, and I don't argue with anyone who chooses not to buy Class-A amps. We're blessed with LOTS of fine-sounding amp to choose from.
I find that class A amplifiers play bass better as well as being smoother in the mids and highs. They can also be more detailed as they make less distortion.
BTW/FWIW there is no such thing as A/AB. That would simply be an AB amplifier. If you see such designations they are marketing terms, not technical terms.
Liguy, I have to disagree, despite Nelson's paper. The designation of 'AB' **already** "denotes that the amplifier operates in Class A up to a certain power level then switches over to Class B. This switch point is determined by the bias.", as you put it above.
IOW, Class AB already meant this; it could be class A to 0.5 watts or class A to 35 watts with clipping at 40 watts, either way that is what AB is... Putting another 'A' in front of it thus has no meaning, and is not found in the textbooks. That's why I say its a marketing term.
Newer Class Ds done well may be the emerging holy grail for most in terms of overall performance, efficiency, and cost of ownership. Maybe even just on performance alone, the rest aside.
I am not a Class A/B fan anymore in general.
Class A is good but practical for most only in certain cases.
I've had older Class G rail switching amps also. Those were smaller and more efficient also but also very mediocre with a compromised design, like much Class A/B perhaps.
Mapman, I suspect that that is sort of true, at least I have hope for it but have yet to see a class D amp come anywhere close to what I expect out of an amp. I think the technology has more to offer than we have seen available so far though.
Joenies, when you read that TAS article, you may also want to read this: