Class A amps

<< I've read a few articles on class A amps but still don't have a good understanding of how it affects sound quality. I've even seen amps which can be swithced into class A mode or AB. Does anyone have experience with this type of unit? Does class A have a true audible advantage or can it be detected only by test instruments? If it is audible, in what resepects? Thanks, Rick M. >> Class A amp will usually give you more detail and smoother midrange but less punch and dynamics. It all depends on what speakers you use and what type of music you like. I am so sure about that because I used to have a Gryphon Antileon power amp from Denmark. This monster amp has adjustable 50%, 75% and 100% class A knob which can be set during play.

Generally, the differences between single ended (class A) amps and push-pull units are very audible: the former have a much wider and more true-to-life soundstage. On the other hand, they tend to dissipate more energy as heat and are therefore usually less powerful than the latter units using the same output tubes or transistors. Tom

The sonic difference between Class A and AB is mostly because the Class A amps usually have better power supplies and regulation, and are also better built (they have to, as they run very hot). However, I have heard Class AB amps that sounded better than most of Class A amps. The switching noise is not hearable in any way with a good Class AB amp. Usually it is easier to make good sound with Class A, but again it only depends upon how well is the AB one designed. In my experience good class AB push-pull transistor amps usually sound better than SE units. The same is true with tube amplifiers: Class A is not necessarily better than class AB. Even when using the same topology or the same tubes, one amp can be world-class and can be mid-fi. As a conclusion: usually class A amps sound good, but not necessarily _very_ good. Speaker-cable-amp interaction is more important than the class of the amp.

In class a amp's amplification is don by just one transistor in the output stage, where class b amp's have two transistors sharing the job. One transistors handles the plus, the other the negative part of the sinus. There is, however, a small area where they both work. This can cause cross-over distortion. A class a amp will be cleaner, without this distortion. Dis-advantages are that they run very hot, do not have a high output (generally speaking) and are expensive. The difference between a good class a and a good class b amp should be quit audible though.

Rick: The sonic difference between Class A and Class AB amplifiers happens because transistors are not "perfect." It turns out that if you have an input voltage of less than about 0.6 volts, a transistor won't work at all; the output is nil even though there is an input. In a Class A amplifier, one ggets around this by "adding" a large constant "bias" to the input signal (about half of the maximum input voltage), so that the transistor will always be in the "active" region, where it amplifies the way one expects it to. In a Class AB amplifier, one uses a pair of transistors (or pairs, or more) to drive the output, one for the plus part of the signal and one for the minus part, as Michiel said. This way, there isn't a need for a huge constant bias, which wastes lots of power as heat when there is no signal. Instead, one just needs to add a little bias to get the transistors above the little part near zero when the transistors don't amplify. Unfortunately, it's near impossible to eliminate the little distortion just by biasing, and it will manifest especially in quiet, delicate passages. With the little Class B amplifier I built, this makes percussive elements sound like crumpling wax paper and things like flutes sound like kazoos. Certainly not acceptable, but fortunately it isn't anywhere near that bad in commercial amplifiers (or so I hope).

I have an Accuphase power ampliefier. If I play in class A. the music will be more clean and soft, but I can not play so laud as in class b

it amuses me when a class "a" amplifier does not generally have a high output. Tell that to krell .

All this discussion about operating class is really moot if it involves ASSUPTIONS. Since I have been designing amps for over 40 years I am somewhat of an expert on the subject. The most important thing is the OVERALL RANGE OF LINEARITY. This property is obviously much easier to achieve with a class A design albeit at great expense. In the last 25 years or so, this whole situation has become rather moot as the better designs by most competent engineers have long overcome the biasing problem. The sad truth of the matter is that the average audiophile layman has absolutely no way of verifying that any given unit has been correctly designed. One must un-fortunately trust the reputation of the designer and today even that is no guaratee as there are a lot of what I call dilletantes touting themselves as great engineers. The real truth is that everyone can't be "King of the mountain". James Bongiorno formerly of Dynaco, SAE, GAS, & Sumo.

listening to all of the above, all i can say is with all the amps i had a/b amps never came close. But theres one company that mad my jaw drop was sim audio, great product, i dont own any of sim,but i gotta say,it sounds good for a class a/b amp, and there intergraded amps too.like i say I may not be someone building electronics but I can say I have the ears of years hearing all kinds of components. so is class a amps better,to me yes,if you dont think so i guess your still in maze.there are a lot of a/b amps out there,very few sound good and are some that do a good job, class a is better.im just dieing to hear a digital amp make its way. well bye everyone enjoy listening.

I am thinking about upgrading my ARCAM AVR 550 to a new ARCAM AVR 30.  The ARCAM AVR 30 has G Class amplification.  I believe the first 50 watts are pure Class A.  This will cost me an extra $3,000 ($6,000) total.  Will I hear the difference in sound quality and is it worth the cost difference.  The reason why I use an AVR receiver it to watch TV through two front towers, a center channel and two REL S2’s.  I use this to listen to 2 channel.  I need the flexibility.  
I would be curious what some of the experts in this group would say?

Kurumuka gave a great explanation.  Class A and AB can be both balanced and single ended.  Both are push pull and can be tube or ss.  There are some Class A SETs - single ended triode or single ended transistors where the +/- is handled by one tube or transistor.  These are always single ended (RCA) and are almost always very low power.  

Class A amps, because of the constant current, tend to be very fast and responsive.  They also waste 75% of the power going through them and run very hot.  Many at 130 degrees and in a small space, this is a huge issue. 

It also matters little how hard your drive these with the exception of low wattage, Class A tube SETs.  Because current running through the amp is constant, it doesn't make a difference if the amp is at 10 or 100, the amp is being "driven" the same. 

One issue is that they tend not to sound right until the get scorching hot and this can take some time.  Tube amps often have direct heated triodes which get them to temperature faster.  SETs are an exception and unless you are in the market for a low wattage SET, I will not share further.  

Class ABs are much more efficient, run in Class A mode to a point and then switch to class B.  This can vary wildly from amp to amp.  When driven hard, these can get very hot and you do not want to be pushing these to high volumes.  They can also seem a little slower than a Class A because of the non-constant current.  In practice, this is not a huge issue unless the amp is poorly made.   

Class B tends to be highly distorted and there are very, very few pure Class B amps on the market.  AB amps switch from Class A to Class B so that it can deal with increases in power needs with changes in volume. 

If you have an amp that has a switch from pure class A to AB (borg dune and warp), your speakers and power needs drive what mode you should use.  

Lets be perfectly honest here folks, who actually listens to music on output levels of 50 watt and above, certainly not many of us with an interest in sound quality IMO. At this level you probably listen to more room distortion than audio reproduction distortion.
 
Most modern design transistor Class-A amps run at this class at lower output levels thus providing good source reproduction without any crossover distortion. Once you ramp up the volume the circuits are design to operate in AB mode.

For sure your input source and speakers can make a huge difference in sound quality, as can the positioning of your equipment.

I guess the thing with sound quality (audible not oscilloscope) is its very subjective, your friends and neighbours will always think there sound system sounds better than yours. My wife claims she makes the best lasagne ever, I've tasted better !!!

Lets be perfectly honest here folks, who actually listens to music on output levels of 50 watt and above


I use way more than 50 watts, my apogee duetta 2 speakers are not very sensitive, but they sound phenomenal. 

@invalid. I am curious how you are making that determination?

I've owned many excellent class A amps.  I've settled on a class AB and a class D integrated for my systems and they produce the best sound I've experienced in my home.  Topology certainly doesn't tell the whole story of what an amp sounds like.

We no understand black hole or why woman trying to make conversation with me at the gym but we know everyone brain wired to like something best. All blonde and brunette like me equal but me thinks ear and speaker prefer certain topology best.

Class A amps

I’ve read a few articles on class A amps but still don’t have a good understanding of how it affects sound quality.

One simple thing, zero cross over distortion, changing from class-a to class-b .
A great designed class-A solid state cannot be bettered

If you patient read this
https://www.firstwatt.com/pdf/art_a40.pdf

Cheers George

Folks, just so you know, one reason there is class AB is so that there will be no crossover distortion, as opposed to class B where this is a problem (and usually class B amps are not intended for hifi operation on this account). The amp is biased just enough so that it has an operating point in the A region (at idle) so crossover distortion due to cutoff of the output devices is simply not possible.  Whether you're class A to 1 watt or full power isn't going to make any difference insofar as crossover distortion is concerned.


However you do get better distortion cancellation when the amp is class A to full power and hopefully the output devices are being operated in their most linear region.


The reason to do this is that most solid state amps made in the last 70 years don't run enough feedback to really do the job that needs to be done, so to help the amp make less distortion to begin with its set up to run class A. Put another way this reduces distortion when the amp is running open loop; thus the feedback has less work to do cleaning things up.

There are two types of solid state class-A
Complimentary push/pull solid state amps that can be class-a/b or class-a if biased to the max so neither of the push pull transistor "ever" switch off and give crossover distortion. If they did you’d have crossover distortion
Or Class-A that uses a constant current source and the bottom instead of complimentary push pull arrangement, and never gives cross over distortion

When a class-a/b amp leaves it’s biased level of class-a and enters class-B then there is cross over distortion.
Only an amp that is biased hard all the way in class-a avoids this.

This "one" of the very best amps push pull solid state that was hard biased into pure class-a, it was the massive and maxed out doing only 25watt @ 8ohm, the Classic Mark Levinson ML2 monoblocks, but they could do almost 200w into 1ohm they were so good at giving current
https://img.usaudiomart.com/uploads/large/2229206-5c605986-mark-levinson-ml2.jpg

Nelson Pass on Class-A and crossover distortion:
In class AB, this effect is "alleviated" by a small bias current, and then is eliminated in class A where the bias current is high.
As he says crossover distortion it's alleviated in A/B with some bias but not eliminated, that only happens in Class-A amps



Cheers George

Just YouTube but you can get the idea. 
https://youtu.be/LYn8Fj7ItnY

Very good demo (2nd half ) even via youtube, this poster says it all

"Class AB sounds like a request but class A sounds like a command"

Cheers George

When a class-a/b amp leaves it’s biased level of class-a and enters class-B then there is cross over distortion.

This statement is false. Crossover distortion is a phenomena that occurs at the zero crossing point of the waveform. There is distortion that happens when the output devices go into cutoff, but that is harmonic distortion, not crossover distortion.

There are two types of solid state class-AComplimentary push/pull solid state amps that can be class-a/b or class-a if biased to the max so neither of the push pull transistor "ever" switch off and give crossover distortion. If they did you’d have crossover distortion
Or Class-A that uses a constant current source and the bottom instead of complimentary push pull arrangement, and never gives cross over distortion

This statement is rubbish, on account of its word salad quality, rendering it without meaning. In particular the second paragraph with the words ’and the bottom’. Constant Current Sources are used in many solid state amplifiers (usually at the input where a differential amplifier is used). Sense cannot be made of this paragraph.

There are two kinds of solid state amps: push-pull and single-ended. Nelson Pass has made a number of single-ended solid state amps but for the most part such amps are quite rare. 99 44/100% of all class A amps are push-pull.

Complimentary push/pull solid state amps that can be class-a/b or class-a if biased to the max

If there is any AB operation at all, the amp is an AB amp and not class A. Class A is a pretty strict definition when transistors are involved: the output devices never go into cutoff at any time right up to full power. When AB is happening, one of the two output devices does go into cutoff. If this occurs at the last watt before the amp clips, its still a class AB amp!

Crossover distortion only occurs at the point where the signal is going from positive to negative or vice versa. Even if the amp is being driven to full power, right around the crossing point the power is nearly 0 Watts. As long as the output devices are biased so that they are conducting around the zero crossing, crossover distortion will not occur. It is *eliminated*.


I’m always open to learning new things. If anyone can find an example of where a class AB amp is making crossover distortion, I’d like to know about it! The thing about crossover distortion is that its really audible. Back when I was putting myself through engineering school, I worked at an Allied Radio Shack (and later simply Radio Shack) service department that serviced a 5 state region. I worked on literally 1000s of amps and receivers. I noticed that certain receivers didn’t sound good at low volume levels (they sounded a bit ’hoarse’ for lack of a better word, sort of like when you have a voice coil rubbing in a loudspeaker). So I tested them on the bench and found that from the factory the output section was under biased. So I would set them up according to the specs in the service manual. I found that the point where the crossover distortion was eliminated **on average** was the bias spec in the service manual, but also that some units needed more bias and some didn’t need as much due to variance in the devices. After being set up properly they sounded fine and with no crossover distortion whatsoever- the correct bias **eliminates** crossover distortion. Nelson Pass or no, lacking crossover distortion is not an advantage of class A operation. Lacking some higher ordered harmonic distortion **is** (and higher ordered harmonics are easily heard as brightness, so this is important).


Again, if anyone can provide a bench test example that disproves what I’m saying here I’d love to see it!

This statement is false.
This statement is rubbish

No it’s the opposite


Take it up with Nelson Pass. The Messiah of solid state class-A amplification (not you)
And why his top of the line amps are low power Class-A!! and always have been.

As he says, "crossover distortion it’s "alleviated" in A/B with some bias but NOT ELIMINATED!!, that only happens in Class-A amps"

Nelson Pass on Class-A and crossover distortion:

In class AB, this effect is "alleviated" by a small bias current, and then is eliminated in class A where the bias current is high.
As he says crossover distortion it’s alleviated in A/B with some bias but not eliminated, that only happens in Class-A amps

So no examples then. The simple fact is you can't find any, or I'm sure you would have presented them. Res ipsa loquitur

Your the one that started personal mud slinging.

Yet again "product protection mode", as with preamps and the imaginary Class-D that’s yet to materialize.

If I did some mudslinging please provide an example. You might review your last sentence and consider what is really happening.


Now regarding that crossover distortion thing, most solid state amps of traditional design are class AB. By 'most of them', more than 98% (assuming that we are leaving class D out of this...). That's a lot of amps going 'way back to the 1960s...


Out of all those, you mean to tell me or anyone else that they have crossover distortion that class A amps do not, yet to back up your assertion, you can't find even **one** example?? Stereophile alone by your words would have to be full of them!


Occam's Razor suggests a simpler explanation, that in fact crossover distortion isn't a thing- unless the AB amp is under-biased/malfunctioning.


You might consider picking up a class AB amp and putting it on the bench and testing it, and setting up its bias and DC Offset. Its patently obvious that's a thing you've not done.

It’s like flogging a dead horse.
Whatever Ralph, you only post when there’s a potential sale, or are in product protection mode, we can all see this it’s that obvious.

Take it up with Nelson Pass. The Messiah of solid state class-A amplification (not you)
And why his top of the line amps are low power Class-A!! and always have been.

Aren't his top of the line amps the XS series, which are not low power.

@invalid, correct. Pass’ high end amps, (Pass Labs brand) whether class A or AB are the high powered amps. The low wattage amps (either type) are considered the entry level. 

A great class A SS amp cannot be bettered, paraphrasing.

That’s true, when you discount the class D amps that best Class A.

Want an example? How about several? Read my review of the Legacy Audio i.V4 Ultra Amplifier, which has bested ALL other classes of amps I have reviewed at Dagogo.com

invalid
Aren’t his top of the line amps the XS series

Yes, but not as pure Class-A as the XA series, but they the XS (a/b amp) have large amount class-B after leaving Class-A with more crossover distortion, where the XA don’t
As he says

"crossover distortion it’s "alleviated" in A/B with some bias but NOT ELIMINATED!!, that only happens in Class-A amps"

If you were using 4-8ohm speakers that were efficient, the the XA’s would be the choice.
But if you had say the Wilson Alexia’s that are 1ohm EPDR in the bass, then the XS would be the better choice, for better current into that 1ohm epdr bass load, and class-B in the bass is fine

We listened to the Gryphon Antillion Evo, a 150w amp but it can give 5000w at 1/2ohm, and on the Alexia’s and it can do the 1 ohm no problem
It’s able to be switched from 20w Class-A bias to 40w or 100w on the fly, and all 8 of us could believe just how much better it was at 100w class-A bias.
But the room did get very hot after 1 hour of listening.
https://gryphon-audio.dk/shop/power-amplifiers/antileon-evo-stereo/

Cheers George

A great class A SS amp cannot be bettered, paraphrasing.
That’s true, when you discount the class D amps that best Class A.

I like to see/hear any class-d better the Gryphon Antillion (in 100w class-A) in the above post on the Alexia, I’d put my house on it.

Cheers George

@georgehifi… every single thread around this topic you bring up Wilson Alexias. Every, single, one. You describe the load of the Alexia as a 1 ohm load etc etc, and basically anything but a bi-polar amp, namely a Gryphon, has little to no chance of driving them. Your assertion is based on a discussion you had with John Atkinson on the original Alexia tests where he showed the load just under 2 ohms at 80hz with a -45° phase angle. Challenging load, but not crazy, and it is the load as specified by Wilson. However, you mentioned another test (that you pull your figures from) as the “evil” load you ALWAYS mention. Perhaps you should go back and read that original article where John challenges and disproves this test and states why. You failed to answer that post in the article, yet you continue to drive the narrative. Overall, I could really careless, but others might look to this, not read the article and thus not buy or perhaps overlook the Alexia. Time to pick another whipping post speaker. BTW, my Alexias are driven with Pass XA60.5’s currently, and not until about 90db does the needle even begin to fluctuate showing the amp reaching into the AB realm. The amps cruise along running about 50°c and don’t even blink. The Alexia, at 90db efficiency do require a bit of grunt, but are not the near impossible load you assert.

I think that would be a fun comparison. It would be a shame if you lost your house. Good thing I'm not a betting man.  ;)

geof3
every single thread around this topic you bring up Wilson Alexias

Because they are a classic for a heavy load on the amp even though efficient, and have been documented and measured in many article’s with graphs just for you. And if your driving them with a Pass XA60.5 sure they’ll work, but I can guarantee you, that you have not heard them at their best. Because the owner of the Alexia’s had the XA160 (mosfet) and it sounded good but they were quickly replaced by the Gryphon Antillion (bi-polar) once he heard the difference.

douglas_schroeder
It would be a shame if you lost your house. Good thing I’m not a betting man. ;)

Don’t think there’s a chance that would happen, care to name the Class-D amp you think would better the Gryphon Antillion?

Cheers George

I submit the Legacy i.V4 Ultra for comparison to the Gryphon Antillion. The i.V4 Ultra is the best class D I have used, so that would be my choice.

I know you consider Pass Labs products as well designed, as do I. Yet, 3 Pass Labs amps, including the $42K Pass XA200.8 Monos, and the First Watt J2, all were bested by the i.V4 Ultra. I discuss this in my review at Dagogo.com. All of those falling to the Legacy i.V4 Ultra would sufficiently falsify your statement, "A great designed class-A solid state cannot be bettered."

Has anyone done this comparison (Legacy Audio i.V4 Ultra vs. Gryphon Antillion) in multiple systems? I would build approximately 8-12 systems to see how the amps would react across a spectrum of components and speaker genres. It’s not uncommon for an amp or other components such as pre, DAC, etc. to swap places to a degree, depending upon the ancillary components and cables used. It is quite possible that the comparison would end in a draw, where there were roughly equal appealing performance characteristics to each amp. No one knows apart form a comparison.


"Don’t think there’s a chance..." Thank you for your opinion. Opinions apart from actual comparisons are not too important to me. :)

I know you consider Pass Labs products as well designed

Yes they are, and that’s where it ends for me, much prefer the sound of Nelson’s Threshold amps he designed using bi-polar outputs. I don’t like the sound of Mosfet output amps, and that they don’t drive very low impedances as like the great bi-polars can, like Class-D also has diminishing performance well below 4ohms as well.

I submit the Legacy i.V4 Ultra for comparison to the Gryphon Antillion.

They’re just IcePower 1200 modules https://icepower.dk/buy/as-series/1200as1/ maybe with a couple of mods.
Your a very game man putting up an IcePower 4ch Class-D ( 610 watts per channel into 8 ohms, 1,000 watts into 4 ohms peak current 38A😴) They never seem to show 2ohm specs for Class-D as it usually ***** itself

A Gryphon Antillion https://gryphon-audio.dk/wp-content/uploads/2018/10/ANTILEON_1.jpg even though lower in 8ohm wattage nails the Icepower, (150 watts per channel into 8 ohms, 300w into 4ohm, 600w into 2ohm, and 1200 watts per channel into 1 ohm, 5000w!!!! peak per channel into 1/2ohm ohm peak current over 100amps)😵
a very game man indeed.

"A "great designed" class-A solid state cannot be bettered."

Legacy Audio i.V4 Ultra Amplifier vs CODA #16 will be done by me when I go to upgrade my CODA #8 to the #16. The dealer has the  Legacy Audio i.V4 Ultra Amplifier amp sitting right beside the #16. He is a huge Legacy Audio dealer. I was thinking of listening to it last time but I was having too much fun with the #16.

Easy to wire up

Just get 2 of these https://icepower.dk/buy/as-series/1200as1/ $600pair

And one of these https://icepower.dk/buy/power-supplies/1500s/ $400

Throw them in a nice box https://www.ebay.com.au/itm/162028925013?hash=item25b9ad3055:g:K~kAAOSw5VZXA2Ad $100

And for $1100 you’ll have a stereo version that’s just as good. You'll save $5k!!!!!!!!

Cheers George

I still have a pair of T400s an a S500 it's loaned out..

My T400 were set up by a serious Threshold NUT. They will push anything. He worked with James B for 2 years.. I had a pair of 2000 Ampzillas too. They will push a load too..

I haven't heard EVERY class D but I've heard a few and own both ICE and Hypex Ncore.

For high fidelity I'll take the Threshold stone stock.. Tuned correctly.. 
Nothing even close for the money...

I'm a hard core Krell guy too.. Say what you will. They can cost you a fortune in every way, but 1-400 class A watts is pretty mind blowing. Sound lab, MLs, Custom planars.. Krell active crossovers..  130 lb MBs
I've placed and wired a few pairs at 220/240. THEN they sound right..

I'll take them over big Macs any day. For HIGH fidelity. Same with Threshold over any SS Mac.. I like Mac SS (kinda :-))

Pass cost WAY to much, and has for a while, for me.

I'm looking at 20 acres of land 200k. An amp that cost 1/4 of 20 acres. 

Pretty simple choice for me.. The Threshold will outlast ME...

Regards