Sonic difference between a class A amp and a class AB amp

All things being equal, a Class A amp will have less power and less harmonic distortion (at the power levels that it can produce) vs. a Class AB amp.

If your speakers are efficient enough so that the lack of power does not cause any problems for you then you should hear a cleaner, more "you are there" type of sound. But there are a lot of variables and things are never equal.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

it IS indeedreally hard to say, but I think one thing is fairly safe to say, posted on November 12, 2024 at 07:48:25

mhardy6647
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Class AB amplifiers are not at their best at very low output power levels. A Class A design will probably/usually sound better at reasonable listening levels on very high sensitivity loudspeakers than a "similar" (whatever that would mean, practically speaking) Class AB design. This is important for some of us ;) -- far less important for most people nowadays given modern trends in passive loudspeaker design.

A couple of notes.
As I implied above, there are subclasses of traditional Class AB operating points which will be more or less immune to crossover distortion than a "typical"/textbook Class AB1 amplifier design. In other words, it is really hard (impossible) to generalize very constructively as to an answer to Leo loves music's question. Sorry!
A push-pull amplifer can be designed to operate in Class A (both 'push' and 'pull' devices fully active and conducting/dissipating power - mostly as heat - all of the time), Class AB (intermediat bias/efficiency compared to Class A or Class B), or Class B (the 'pull' devices turned off when the 'push' devices are conducting -- the most efficient variant for an audio amplifier). A single-ended design must operate in Class A.
Tubes or transistors van be used as the active 'devices' in all cases.

Yamaha (for one) have made several integrated amplifiers cleverly designed and built to operate in either Class AB or Class A at the flip of a switch. Those amps give the fairly unique option of comparing operational topology of an amplifier with, in essence, "all else being equal", at the expense of far lower output power and much more heat produced (lower electrical efficiency) in Class A operation.

Here's an example of one: the CA-800 integrated amp from ca. 1975. Something like 45 wpc in Class AB; ca. 15 wpc in Class A.

all the best,
mrh

RE: Sonic difference between a class A amp and a class AB amp, posted on November 12, 2024 at 09:19:05

cawson@onetel.com
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I don't think you can generalise - and certainly you shouldn't - when deciding on an amp purchase. The brand house sound is likely to be just as important as the Class of amp.

I home auditioned about 10 amps before choosing one that suited my music tastes, speakers and listening toom - all factor much more important than Class.

I was expecting a Class A to win me over and indeed one (Accuphase A36) sounded great - others not so good - with my very high sensitivity speakers. However my mind was open as I listened at length to each and in the end, I chose a Class D one. Modern Class D will match or better just about any more costly A or AB, so don't dismiss them from your consideration.

RE: It's really hard to say, posted on November 12, 2024 at 16:49:57
Leo loves music Audiophile Posts: 397 Joined: October 14, 2021	In my systems, low bias class AB sounds hollow. Class A sounds solid. The later also offers more texture and details. Not to mention low distortion when music is loud and busy. Of course every system is different. Your mileage may vary. Too bad these class As consume too much electricity and generate too much heat.

RE: Sonic difference between a class A amp and a class AB amp, posted on November 12, 2024 at 18:28:01
Ozzie Audiophile Posts: 3910 Joined: August 1, 2002	Class AB Steinway Pure Class A Bosendorfer

have you tried a good Class D (PWM) implementation yet?, posted on November 13, 2024 at 05:52:21

mhardy6647
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E.g., Hypex?

all the best,
mrh

https://www.diyclassd.com/products/diy-components/amplifiers

OK- even that wasn't safe to say, posted on November 13, 2024 at 12:54:56
Ralph Manufacturer Posts: 4914 Location: Minnesota Joined: April 24, 2002	Class AB amplifiers are not at their best at very low output power levels. If the amp is set up right, at low levels is where it will be its best. If its not set up right, if there are crossover or notch artifacts, its not class AB, its class B. IOW a class AB amp has a bit of A operation at lower power levels.

The better the design of the amp and its execution, posted on November 13, 2024 at 12:59:26

Ralph
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the less class of operation matters. Class A is used to linearize the output section, handy when there's little or no feedback. These days with more advanced semiconducters available, there's far less need to do that.

Of course it matters how well the feedback is applied. If done incorrectly the feedback may well cause more problems than it solves (such as harshness).

This applies even if the amp is class D. IOW there are class D amps now that rival the best of class A amps.

So you just have to play them and see what works for you.

point taken ;) (nt), posted on November 14, 2024 at 05:54:43
mhardy6647 Audiophile Posts: 16237 Location: New England Joined: October 12, 1999 Contributor Since: October 23, 2016	enn tee all the best, mrh

PS In my defense, I did put in a plug for "good Class D" :) (nt), posted on November 14, 2024 at 07:19:19
mhardy6647 Audiophile Posts: 16237 Location: New England Joined: October 12, 1999 Contributor Since: October 23, 2016	enn tee all the best, mrh

:) (nt), posted on November 14, 2024 at 08:25:00
Ralph Manufacturer Posts: 4914 Location: Minnesota Joined: April 24, 2002	-

RE: The better the design of the amp and its execution, posted on November 19, 2024 at 10:37:51
13th Duke of Wymbourne Audiophile Posts: 3005 Location: Orange Co., Ca Joined: September 19, 2001	Hi Ralph, could you expand on your sentence 'These days with more advanced semiconductors available, there's far less need to do that.' The distortion characteristics of transistors doesn't really change between process technologies so are you thinking of advancement as providing more gain-bandwidth allowing more feedback?

RE: The better the design of the amp and its execution, posted on November 19, 2024 at 10:48:29
Ralph Manufacturer Posts: 4914 Location: Minnesota Joined: April 24, 2002	are you thinking of advancement as providing more gain-bandwidth allowing more feedback? Yes, among other things. Its also possible to apply feedback in such a way that it is not distorted in the process of doing its job. Opamps are a good example of how that's done.

I am neither a technician, nor engineer, nor GEA, I'm merely..., posted on November 19, 2024 at 14:10:49

jeffreybehr
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...a musiclover who loves to throw money at his audio/video system.. And I've managed to build a spectacularly great-sounding system in only 65 years.

My recent struggles to find a great-sounding amp to drive my newish Sound Labs Majestic 745s caused me to listen more carefully than ever to find one.. The winner is Nelson Pass's class-A monoamp, the XA60.8.. Compared with Pass's class-A/B X260.8, a 260WPC-into-8 monoamp which was the first winner in this contest, the class-A amps are smoother and yet more resolving of everything in the signal.. I call that 'musicality' for short, and I'm VERY HAPPY with my class-A poweramps.

RE: The better the design of the amp and its execution, posted on November 19, 2024 at 17:23:54
sideliner Audiophile Posts: 213 Location: NYC Joined: August 22, 2013 Contributor Since: December 15, 2023	Its also possible to apply feedback in such a way that it is not distorted in the process of doing its job. Opamps are a good example of how that's done. Could you elaborate a bit on this

Sure, posted on November 20, 2024 at 08:28:31

Ralph
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Could you elaborate a bit on this

Opamps derive feedback from a Voltage divider network which mixes feedback with the incoming signal right at the input of the opamp.

Most audio amplifiers don't do this- instead apply the feedback to the cathode of the input tube, emitter of an input transistor (in the case of more primitive solid state amps) or to the gate or base of a semiconductor that is part of a differential input pair.

In all cases the device to which the feedback is applied is non-linear. So in the process of mixing with the audio signal, the feedback signal is distorted. This causes it to generate higher ordered harmonics and in some cases, IMD.

Resistors of course are a lot more linear.

Norman Crowhurst pointed out this problem 60 some years ago but didn't propose a solution. 15 years later Peter Baxandall also pointed it out but simply proposed 'more feedback' which really didn't work.

In either case if the feedback is wrapped around the amplifier as seen in opamp circuits, it will not be distorted so it can simply do its job.

Just so you know, posted on November 20, 2024 at 08:58:35

Ralph
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The Pass Labs X260.8 is a class AB amplifier.

It does not matter how much power the amp makes in its class A region. If it can make more power than that, its a class AB amp. From their website:

Like all of the class A/B .8's, this amplifier has some class A bias, rewarding its listeners with a whole new zenith of sound quality.

'Some class A bias' could mean anything- it might only make 1/2 Watt in class A or it could make 50 Watts. Either way its class AB. If the amp is properly designed, the class of operation (really important maybe 30 or 40 years ago) isn't important.

uhh...yes, I know the '260 is a class-AB amp.. The XA60.8 is..., posted on November 20, 2024 at 11:31:28
jeffreybehr Audiophile Posts: 5787 Location: Phoenix, Arizona Joined: December 10, 2004 Contributor Since: December 13, 2004	...the class-A amp.

Class A is not Always better IMHO, posted on November 20, 2024 at 13:19:30

AbeCollins
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If your speakers require a decent amount of power to 'come alive' a higher power Class AB amp will likely be a better choice than a marginally powerful-enough Class A amp. Ever wonder why so many modestly powerful Class A amps come up for sale used? My theory is that people try them only to discover they don't sound so great, and that's probably because they lack sufficient power for their speakers. If you are hell bent on going Class A be prepared to fork over enough cash for a really hefty one with a good amount of power.

Benn there. Done that! My Pass Labs X150.5 150wpc Class AB amp was so much better sounding than my 30wpc Pass Labs amps. And my Tannoy speakers aren't THAT demanding yet 30wpc didn't quite cut it.

At least in my setup I found the Pass Labs Aleph 3 Class A amp to be (yes!) very smooth but lacking in dynamic punch.

RE: I am neither a technician, nor engineer, nor GEA, I'm merely..., posted on November 20, 2024 at 15:12:01
Leo loves music Audiophile Posts: 397 Joined: October 14, 2021	Totally agree with your observation of XA amps. I have Plinius class A and Cary class A amps. I love them.

Sure?, posted on November 21, 2024 at 11:52:03

13th Duke of Wymbourne
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I would say that almost all solid state amplifiers follow the Lin topology (input differential Gm stage - transimpedance stage - output buffer/follower) which is, basically, the same as used by op amps. So, the feedback signal will connect to the input gate/base of the input diff pair just the same in audio power amps as in op amps.

I think this idea that feedback intermodulates with the signal in the amplifier is specious. As we know, the closed loop performance is given by A/(1+AB) and as A becomes larger and larger A effectively drops out of the equation and performance is defined by the feedback network, usually resistors. A portion of the output is subtracted from the input signal to create the error signal - this is what is amplified by A. The error signal is pre-distorted and amplified so that distortion at the output is minimized. IMO, it is a red herring to think of what is passing through the amplifier - yes, it is not 'pure' but it is exactly what is required to make the output 'pure'. The only thing we care about what goes on inside A is that the small signal gain remains high and there is no saturation in either the time domain or amplitude domain.

I don't know Norman Crowhurst's writings but I do know Baxandall's analysis of applying feedback to a pure second-order device. The way to think about what is happening is that the feedback is straightening the very curved transfer curve. Even if you have a transfer characteristic that looks straight visually it will still have some curvature and wiggles that equate to high order terms in the power series expansion of that characteristic and they create high(er) order distortion. You can play with this in MS Excel by plotting a straight line and doing a power series fit that will have only the linear term. Then add points to make subtle deviations to that line and the power series fit will then calculate the added high order terms. Only a perfectly straight line produces no distortion and there is no perfectly straight transfer characteristic. Just as there is no perfectly square law characteristic to start with.
In Baxandall's think-piece, second-order and total distortion decreases as feedback is increased but all other terms increase until you reach an inflection point (approx 20dB loop gain) and increasing feedback above that point then reduces all distortion components. This is explained in Bruno Putzey's 'F-Word' article. The worst thing would be to apply only 'a little' feedback because of the widely held view that 'a lot' is bad but puts you right at that inflection point where the higher harmonics are their worst! Nelson Pass has written of the Baxandall example that if the purely second harmonic is not objectionable then why apply any feedback if you need to apply a lot to lower the high order harmonics back to where they are with no feedback - though he also writes about using 'a little' feedback in his amplifiers? Maybe hedging his bets.

I'll leave the subjective impacts of distortion spectra aside because it's, er, subjective and I see that amplifiers of all different topologies are highly regarded so, maybe, non of this really matters?

Yes., posted on November 21, 2024 at 13:03:02

Ralph
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Yes, many amplifiers are very similar to opamp design- nearly the same. But the way feedback is applied is different. In most power amps the differential input is used for the input signal on one side and the feedback is applied to the other input of that differential amplifier. So the feedback signal is mixed with the incoming signal within the differential stage itself.

In an opamp, the inverting input usually has both the audio and feedback applied to it; the gain of the opamp thus defined by the ratio of the input resistance (in series with the input signal) and the feedback resistor. If a single-ended connection, the non-inverting input is tied to ground. That seemingly small difference is important as the differential input circuit of a power amp isn't linear and the coupling between the devices is usually degenerated slightly to prevent distortion driving the next stage. By contrast, the resistor divider network of an opamp's feedback is considerably more linear!

So yes, I'm sure.

RE: Sure, posted on November 21, 2024 at 18:32:51
sideliner Audiophile Posts: 213 Location: NYC Joined: August 22, 2013 Contributor Since: December 15, 2023	Thanks for the explanation, I can understand the logic behind it.

RE: Yes., posted on November 22, 2024 at 10:20:01

13th Duke of Wymbourne
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IIUC, you are contrasting non-inverting feedback with inverting feedback. You can do either with an op amp or with a Lin-style audio power amp. I get the point you are making that for the inverting feedback case the input differential amplifier appears 'within' the feedback loop but appears 'outside' the feedback loop for the inverting feedback case. I say appears because there may be no significant difference as the feedback will drive the input differential signal to be the same for both cases with the same closed-loop gain.
Diff pairs, whether FET of BJT are the most linear amplifier configurations for small signals. If you apply a lot of feedback the input differential input should be tiny. Degenerating the diff pair is usually done to cope with transient overload of the error signal - the appendix in Putzeys' 'F-Word' article explains this really well.

Yes., posted on November 22, 2024 at 10:40:28

Ralph
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Putzey's explanation is excellent.

But it also points to one of the reasons using the feedback internally isn't the best move. That's because for the feedback to do its best job, the other transistor of the differential pair has to be coupled as tightly to the transistor receiving the feedback as possible. It isn't due to the degeneration. This ignores of course the simple fact that the two devices aren't linear.

Best, IMO/IME to avoid that problem altogether and simply use both inputs to the differential pair as inputs to the amp- and then mix the feedback externally rather than internally. According to Baxandall this should help things out.

Of course we've not talked about the implications of insufficient Gain Bandwidth Product on distortion but Bruno covered that pretty well.

RE: Sonic difference between a class A amp and a class AB amp, posted on November 22, 2024 at 18:36:34
Cpwill Audiophile Posts: 1122 Location: DC Joined: December 22, 2003	But neither the Steinway nor Bosendorfer will fit in my living room. Is a Fazioli F308 a Class D? Ciao, "Anyone who understands jazz knows that you can't understand it. It's too complicated. That's what's so simple about it." - Yogi Berra. Cpwill

RE: Yes., posted on November 23, 2024 at 17:32:35

13th Duke of Wymbourne
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A test for the curious would be to take an op-amp and configure it in non-inverting mode with gain X and then re-configure to inverting mode, again with gain X, and compare the output spectra.
I still think, to a first order, the differential input voltage will be the same so the distortion will be the same but, to the second order, non-inverting does have a large common-mode on both inputs (i.e. the input signal swing) whereas the inverting effectively has no common-mode swing. So, CMRR could have an effect.

'Putzey's explanation is excellent' - I had another read through and agree very much and that is a good place to end.

You can't compare using an opamp, posted on November 25, 2024 at 09:41:13
Ralph Manufacturer Posts: 4914 Location: Minnesota Joined: April 24, 2002	Power amplifiers have internal feedback. Opamps do not. That is the point I'm making. When the feedback is distorted by the feedback node, it can't do its job properly. The feedback isn't distorted nearly so much when mixed externally.

That's why I suggested we end with the last post, posted on November 26, 2024 at 14:25:32
13th Duke of Wymbourne Audiophile Posts: 3005 Location: Orange Co., Ca Joined: September 19, 2001	I don't disagree with you as such but I don't see insight in your position. I hope my reasoning is laid out clearly enough so that any flaws are apparent. I don't have access to SPICE at present but I will simulate these cases hopefully in the not too distant future.

RE: uhh...yes, I know the '260 is a class-AB amp.. The XA60.8 is..., posted on November 27, 2024 at 06:25:14
Bill the K Audiophile Posts: 8512 Joined: June 3, 2006	How bad is Class D in comparison?Do audiophiles talk about Class D in the same breath as A and AB? Bill

RE: uhh...yes, I know the '260 is a class-AB amp.. The XA60.8 is..., posted on November 27, 2024 at 07:00:26
Leo loves music Audiophile Posts: 397 Joined: October 14, 2021	I once listened to a 300K system driven by a pair class D monoblocks. I have to say I was not impressed. I currently have Crown XLS 2502 class D amps. I would say they are better than expected. At $600 each, a good value.

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Sonic difference between a class A amp and a class AB amp

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