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A touch of Class: Class A vs. A/B

stevenrberg

So I've read things on the difference between Class A amps and Class A/B amps that are written in terms of how the electronics work (here at the Sweetwater web site), and in terms of how the harmonics work (on the Mesa-Boogie web site), what I really need are examples -- Can people name some guitar riffs on record or streaming from web sites that were recorded through Class A amps, and others that were recorded through A/B???
December 14, 2009 @04:46pm
Guitarplayer

The "class" of operation of an amplifier (in my humble opinion) has less to do with tone than feel. A VOX AC30, Mesa Boogie Lonestar Special, Fender Tweed Champ and Marshall Class 5 are all "class A" amplifiers but they could not have more different tonal characteristics! There are SO many factors other than simply the class of amp operation that will have a larger effect on your tone (the tone stack and where in the circuit it's located, how much gain the circuit has, how the amp was voiced by the designer, and especially what speaker is in the cabinet) that to make a blanket statement that all class A amps sound a certain way is simply not accurate. You can have a class A and a class AB amp that "sound" virtually identical. There is much more difference between the two in how the amp "feels" rather than tone. What I have found is that the dynamic response, or how the amp responds to your touch is much more, well, dynamic on a class A amp than class AB amp. You can play a single channel class A amp or a class AB and go from clean to overdrive sounds simply by varying how hard you play but the class A amp generally will have a wider variation between those two sounds than a comparable AB amp.
Hope this helps. :)
December 15, 2009 @03:26pm
jdesola

I was going to try to answer this post yesterday but knew it would be a mouthful...
Props to 'Guitarplayer' for a great explanation of tone vs feel. That's good stuff right there.
December 15, 2009 @05:24pm
stevenrberg

Thanks, that's very helpful. If the manufacturer doesn't choose to spell it out in promotional material, is there any way of knowing whether a particular amp model is A or AB?
December 16, 2009 @02:09am
DAS

If they don't talk about it you should assume AB. A costs quite a bit more to manufacturer, and as you can see it's widely assumed to be "better" (though that's debatable in some cases), so they'll usually talk about it if it's class A.
December 16, 2009 @02:27pm
Guitarplayer

I was going to try to answer this post yesterday but knew it would be a mouthful...
Props to 'Guitarplayer' for a great explanation of tone vs feel. That's good stuff right there.

Thanks Jd!
December 18, 2009 @06:26pm
5454stevef

Thanks, that's very helpful. If the manufacturer doesn't choose to spell it out in promotional material, is there any way of knowing whether a particular amp model is A or AB?

Even if the manufacturer does choose to specify operating class, you may or may not get the straight scoop. There has always been a fair amount of liberty taken with jargon in the musical instrument and amplifier industry (Anyone remember "Music Power" and "IPP"?)
Amps such as the Vox AC30 are commonly said to be Class A designs.
For a thorough exploration of whether the Vox AC30 is Class A or not, check out Randall Aiken's paper on the topic. www.aikenamps.com
One clue you can look for in determining this kind of thing is the amount of power the amp is rated for. Class A designs typically produce substantially less power than Class AB designs using the same tubes.
Does this matter? Not to me. It's still one of the best sounding amps in the world. And if it were Class A it would only put out about 20 watts. I'd rather have the extra oomph.
SF
December 18, 2009 @11:57pm
jdesola

Even the "oomph" or output isn't an equal comparison. A 5w Class A amp puts out an output similar to a 15-30w Class AB (comparative numbers, not accurate). There are too many variables to truly compare tube amps. The circuitry and voltage at the tubes and transformers are so infinitely variable, you really have to know your stuff.
Once you go to solid state, wattage means nothing as far as comparison. I feel like I know a lot about this type of thing and I've just scratched the surface.
December 19, 2009 @04:19pm
stevenrberg

Okay, here's a different viewpoint, in favor of Class A having a different (and better) sound, from a review of a class A amp on premierguitar.com:
"What makes the Class-A amp desirable for many guitarists is the way the output tube distorts. Class-A output distortion strongly favors the even order distortion overtones, while class AB favors the odd order overtones. Even order distortion is far more pleasing to the ear in most situations and is commonly described as having a “singing” quality."
Is there anything to this?
December 29, 2009 @07:07pm
jdesola

Eric Johnson claims to be able to hear the difference in brand of battery installed in a stompbox.
There's a lot of analysis in to harmonic overtones and which are more pleasing and other this and thats. I don't think that you'll ever get an answer to "which is better?" overall. Now, you should be able to answer "which is better for you?" or which is better for a certain application.
I've read a lot of the same types of remarks you're researching and listened for the differences. I have a Class AB 50w amp and a Class A 5w amp. They are very different and Class has the least effect on the sound, in my opinion.
They both 'sing' and they both generate complex harmonics and overtones when driven hard.
>What is your goal in answering this question?
If it is to choose between the purchase of a Class A or AB amp you should move towards finding amps in your price range, with features you like, and compare them personally. I was deciding between the Laney Lionheart (Class A, 20w) and the Vox AC50CP (Class AB, 50w) when I last bought an amp. I chose based on features and 'bang for the buck', not Class.
If you just want to be knowledgeable in the subject, I can understand that. These things are very specific to the individual. Blues players may favor Class A and tell you to keep away from those terrible "odd order overtones" while classic rock players will swear by their Vox, Mesa, and Marshall amps.
Fun topic though!
December 29, 2009 @09:00pm
5454stevef

Okay, here's a different viewpoint, in favor of Class A having a different (and better) sound, from a review of a class A amp on premierguitar.com:
"What makes the Class-A amp desirable for many guitarists is the way the output tube distorts. Class-A output distortion strongly favors the even order distortion overtones, while class AB favors the odd order overtones. Even order distortion is far more pleasing to the ear in most situations and is commonly described as having a “singing” quality."
Is there anything to this?

This is from their review of the 57 Champ re-issue, I believe. A great example of what I was bitching about in my earlier post... : )
It's partly true, partly hokum. The part that's true is that the Champ is a Class A amp - the reason for this is it has only one output tube, which means it has to operate in Class A by definition, unless you like the sound of having half your waveform chopped off. The part that's hokum is the part about the Class A amp "favoring even-order harmonics". Operating class is nothing more than the way the tubes are biased. Class A means tubes conduct throughout 100% of their duty cycle. That's all.
The thing that makes the Champ's output distortion different from, say, a Bassman is that the Champ is single-ended (one tube) and the Bassman is push-pull (two tubes operating in a complementary pair, wired to opposite ends of a center-tapped transformer). In the push-pull circuit the second harmonic distortion of the two tubes are mostly canceled in the output transformer. This was the main incentive behind development of the push-pull amplifier circuit that has been in use since at least 1930.
So the idea that the Champ has more "2nd harmonics" in its distortion because it's Class A is misleading - if it has, which is debatable, it is because it's single-ended rather than push-pull. Coincidentally, single-ended amps are all Class A, but not all Class A amps are single-ended.
You can make a Class A push-pull amp easily enough. It will behave similarly in terms of distortion to an AB amp because they're wired the same with respect to the output transformer.
The reason I would argue that amplifier class is not very significant in terms of one being "better" than the other is that most amps are chock full of single ended (and thus Class A) operating tubes in their preamps. A lot of the distortion you hear when you crank an amp comes from these tubes rather than the output tubes, therefore there are plenty of 2nd harmonics being generated that are not being cancelled, more than enough to make an amp "sing". I've had plenty of push-pull AB amps that "sing" like crazy, and it is mostly because they play really loud and have high-gain preamp circuits.
A quick recap of this mostly tedious topic:
All single ended amps are Class A by definition. (Champ, etc. - anything with only one output tube, even (gasp!) the $100 Epiphone Valve Jr., so it's not limited to the high end.) Back in Leo Fender's day no one would have thought of using operating class as a marketing buzzword, it's just silly.
Not all Class A amps are single-ended - you can make a Class A push-pull amp by having two tubes operating thru 100% of their duty cycle in Class A.
Significant trade-off in power output, heat and tube life though, which is why you don't see that many. They also cost more, mostly because the output transformer has to be bigger.
All Class AB amps are push-pull, it's inherent in how they work. You can bias a Class AB push-pull amp really hot, very close to Class A but it's not a good idea, lots of amps have been destroyed doing this. It's an old joke that "it never sounded better, right up to the point where the smoke came out"...
Many of the most highly regarded tube amps in the world operate in Class AB2. It's their careful design, quality components, and robust construction that makes them great, not what operating class they are.
SF
December 31, 2009 @05:10am
stevenrberg

Thanks everybody who contributed a lot of thoughtful responses, the kind of thing that makes this such a great forum!
December 31, 2009 @05:20pm
willqen

This is from their review of the 57 Champ re-issue, I believe. A great example of what I was bitching about in my earlier post... : )
It's partly true, partly hokum. The part that's true is that the Champ is a Class A amp - the reason for this is it has only one output tube, which means it has to operate in Class A by definition, unless you like the sound of having half your waveform chopped off. The part that's hokum is the part about the Class A amp "favoring even-order harmonics". Operating class is nothing more than the way the tubes are biased. Class A means tubes conduct throughout 100% of their duty cycle. That's all.
The thing that makes the Champ's output distortion different from, say, a Bassman is that the Champ is single-ended (one tube) and the Bassman is push-pull (two tubes operating in a complementary pair, wired to opposite ends of a center-tapped transformer). In the push-pull circuit the second harmonic distortion of the two tubes are mostly canceled in the output transformer. This was the main incentive behind development of the push-pull amplifier circuit that has been in use since at least 1930.
So the idea that the Champ has more "2nd harmonics" in its distortion because it's Class A is misleading - if it has, which is debatable, it is because it's single-ended rather than push-pull. Coincidentally, single-ended amps are all Class A, but not all Class A amps are single-ended.
You can make a Class A push-pull amp easily enough. It will behave similarly in terms of distortion to an AB amp because they're wired the same with respect to the output transformer.
The reason I would argue that amplifier class is not very significant in terms of one being "better" than the other is that most amps are chock full of single ended (and thus Class A) operating tubes in their preamps. A lot of the distortion you hear when you crank an amp comes from these tubes rather than the output tubes, therefore there are plenty of 2nd harmonics being generated that are not being cancelled, more than enough to make an amp "sing". I've had plenty of push-pull AB amps that "sing" like crazy, and it is mostly because they play really loud and have high-gain preamp circuits.
A quick recap of this mostly tedious topic:
All single ended amps are Class A by definition. (Champ, etc. - anything with only one output tube, even (gasp!) the $100 Epiphone Valve Jr., so it's not limited to the high end.) Back in Leo Fender's day no one would have thought of using operating class as a marketing buzzword, it's just silly.
Not all Class A amps are single-ended - you can make a Class A push-pull amp by having two tubes operating thru 100% of their duty cycle in Class A.
Significant trade-off in power output, heat and tube life though, which is why you don't see that many. They also cost more, mostly because the output transformer has to be bigger.
All Class AB amps are push-pull, it's inherent in how they work. You can bias a Class AB push-pull amp really hot, very close to Class A but it's not a good idea, lots of amps have been destroyed doing this. It's an old joke that "it never sounded better, right up to the point where the smoke came out"...
Many of the most highly regarded tube amps in the world operate in Class AB2. It's their careful design, quality components, and robust construction that makes them great, not what operating class they are.
SF

Hello,
You are partially right. That is, of course, simply MY OPINION. First off preamps are usually class "A" due to their circuit designs. They are usually Linear. The signal is not split into upper/lower, positive/negative, etc. Or at least I have not heard of any preamp circuit designs that are, or are identified as being class "AB".
Secondly; all sounds in nature (this includes the plucking of a string) are created using a fundamental tone in combination with a series overtones, or as they are sometimes described: Harmonics. In a series of natural occurring overtones the first overtone is usually called the second harmonic. No need to go into why at this point. Obviously since it is the "2cd" harmonic it is held to be an "even" order harmonic. Scientific tests (scientific meaning with a control group and other devices used to keep the test accurate) indeed show that humans prefer their sounds containing even order harmonics rather than not containing them. In other words even order harmonics, including and especially the 2cd harmonic, are pleasing to the human ear. No assumptions here, this has been tested to death and it is accurate.
Since certain power amp designs do split the signal at various points, such as the dividing point between the upper and lower part of a sound wave, ie., positive and negative, these designs do exclude, or remove, some or all even order harmonics, especially the all-important "2cd" harmonic. Class AB is one of these designs.
Class A (especially, or only, single ended?? I'm not sure!!) on the other hand, is a Linear power amp design and amplifies the entire signal. It does not divide the signal, or remove harmonics including the important "2cd" harmonic. The reason the 2cd harmonic is important is that it is almost always reproduced an octave above the fundamental tone or pitch. This is one of the few harmonics that is consonant. In other words it adds to the pleasing quality of a particular sound, pitch, or tone, rather than detracting from it the way odd order harmonics do. Odd order harmonics sound "harsh" or are displeasing, because they tend to be dissonant. they do not form what most humans would consider pleasing intervals with the fundamental tone or pitch.
This is why, or most of why, Class "A" power amp sections using valves, or tubes, are considered to sound more pleasing, all other things being equal, than class "AB" power amps. This is especially (always?) true in "single ended" Valve power amps, or sections.
It is a subjective phenomenon, and one of many parts of a signal chain, that make it sound pleasing, or just plain good, or great!!
Others would include using valves, what type of preamp, tone stack, or rectification. The type and brand of speaker, and cabinet, the quality of valves and other electric parts, etc., etc.
I happen to believe that it is the artist himself, or herself, that plays the biggest part in shaping the quality of his or her sound, or tone. However don't discount the value to your own tone of the operating class of the power section in a particular amp. It can make a significant difference. This is where I disagree with the gentleman I've quoted, but disagree in degree only!!!
This is a rather subjective question, that will have to be decided by the individual. No consistent "rule of thumb" can be developed for this question because of it's subjectivity. It is much like asking which piece of art is better; Vincent Van Gogh's classic paintings or Monet's impressionistic ones.
Good single ended class A amps sound great!! Poorly designed and cheap single ended class A amps don't necessarily sound any better than any other cheap, poorly designed amp.
Just my VERY subjective opinion.
Try out an amp if you can before you buy it. Or better yet, buy it from Sweetwater or a place like it where you can return it if you aren't satisfied with it. Relying on a spec, however true it is, that says an amp is single ended class A, or any other impressive sounding spec, is always a gamble and seldom will you get what you want, or need.
You are much better off relying on your own good judgement and ears!!
November 19, 2013 @10:57pm
willqen

Hello,
You are partially right. That is, of course, simply MY OPINION. First off preamps are usually class "A" due to their circuit designs. They are usually Linear. The signal is not split into upper/lower, positive/negative, etc. because there is no transformer in the signal path to split it. Or at least I have not heard of any preamp circuit designs that are, or are identified as being class "AB".
Secondly; all sounds in nature (this includes the plucking of a string) are created using a fundamental tone in combination with a series overtones, or as they are sometimes described: Harmonics. In a series of natural occurring overtones the first overtone is usually called the second harmonic. No need to go into why at this point. Obviously since it is the "2cd" harmonic it is held to be an "even" order harmonic. Scientific tests (scientific meaning with a control group and other devices used to keep the test accurate) indeed show that humans prefer their sounds containing even order harmonics rather than not containing them. In other words even order harmonics, including and especially the 2cd harmonic, are pleasing to the human ear. No assumptions here, this has been tested to death and it is accurate.
Since certain power amp designs do split the signal at various points, such as the dividing point between the upper and lower part of a sound wave, ie., positive and negative, these designs do exclude, or remove, some or all even order harmonics, especially the all-important "2cd" harmonic. Class AB is one of these designs.
Class A (especially, or only, single ended?? I'm not sure!!) on the other hand, is a Linear power amp design and amplifies the entire signal. It does not divide the signal, or remove harmonics including the important "2cd" harmonic. The reason the 2cd harmonic is important is that it is almost always reproduced an octave above the fundamental tone or pitch. This is one of the few harmonics that is consonant. In other words it adds to the pleasing quality of a particular sound, pitch, or tone, rather than detracting from it the way odd order harmonics do. Odd order harmonics sound "harsh" or are displeasing, because they tend to be dissonant. they do not form what most humans would consider pleasing intervals with the fundamental tone or pitch.
This is why, or most of why, Class "A" power amp sections using valves, or tubes, are considered to sound more pleasing, all other things being equal, than class "AB" power amps. This is especially (always?) true in "single ended" Valve power amps, or sections.
It is a subjective phenomenon, and one of many parts of a signal chain, that make it sound pleasing, or just plain good, or great!!
Others would include using valves, what type of preamp, tone stack, or rectification. The type and brand of speaker, and cabinet, the quality of valves and other electric parts, etc., etc.
I happen to believe that it is the artist himself, or herself, that plays the biggest part in shaping the quality of his or her sound, or tone. However don't discount the value to your own tone of the operating class of the power section in a particular amp. It can make a significant difference. This is where I disagree with the gentleman I've quoted, but disagree in degree only!!!
This is a rather subjective question, that will have to be decided by the individual. No consistent "rule of thumb" can be developed for this question because of it's subjectivity. It is much like asking which piece of art is better; Vincent Van Gogh's classic paintings or Monet's impressionistic ones.
Good single ended class A amps sound great!! Poorly designed and cheap single ended class A amps don't necessarily sound any better than any other cheap, poorly designed amp.
Just my VERY subjective opinion.
Try out an amp if you can before you buy it. Or better yet, buy it from Sweetwater or a place like it where you can return it if you aren't satisfied with it. Relying on a spec, however true it is, that says an amp is single ended class A, or any other impressive sounding spec, is always a gamble and seldom will you get what you want, or need.
You are much better off relying on your own good judgement and ears!!
November 20, 2013 @01:17am