What are the advantages to a Class A amp & what are the trade offs?

I've never had a class a amp but am considering one now. So what am I getting myself into?
Ag insider logo xs@2xartemus_5
Typically more expensive per watt of power, higher energy consumption, hot running and requires adequate ventilation, can require more maintenance due to the internal heat build up.  Class A amps tend to be big and bulky.

As for the difference in sound when compared to other typologies, that is subjective. 
+1 testpilot

While Class A has it's fans, there is a lot more that goes into the design of an amplifier besides the heat and power wrecking that it does!

The truth is you need to hear for yourself on your own speakers to tell if it is worth the heat, weight and operating costs.

Modern solid state designs run Class A for most of the time you are listening anyway, so, arguably, the tech no longer matters like it used to.

Other factors, like having no feedback, are often bigger sound drivers. 

The old school answer of course, was that Class A has lower distortion through the zero volt crossing region.

How audible that is anymore though.... I don't know.
Never buy based on design, etc. Buy based on how it sounds. All this other stuff, heat output, etc, may or may not matter depending on your situation. ie small room, poor ventilation, hot weather, probably not the best place to put a 500 watt heater. Unless maybe it puts out music that is to die for. Which you will only know one way, by listening.
Thanks guys. I knew about the heat and power demands. Wasn't sure what sonic advantages it might have. There is a Canary M70 which has drawn my attention. Its in the power range which I have found to work well with my speakers. Its close to my budget. But i can't find any reviews or much info about it. If i can recoup my $$$ I've been known to take a chance. Sometimes you lose. But sometimes you hit it outta the park. Then too, its may be a  sideways move. I'm kinda itching to pair another amp with my new Preamp (CJ CT5). System sounds Great but I can't seem to leaver it alone.
This question seems to imply that the biggest sonic differences between amps is related to whether it's Class A vs A/B etc.  I don't agree if that's the case.  Maybe I'm reading the OP wrong.
Needless to point out, perhaps, but here goes:

Maybe Class A as a topology term and how it’s actually and truly implemented has become somewhat muddied. We may think to have been confronted with "Class A" sound in some iteration listening to various amps, but it mayn’t have been the case in the true sense of the word.
I’m only trying to suggest that the real benefits of pure Class A could’ve escaped a listener or two when in actuality what has been listened to was a watered down version of the concept as a "high bias" or "Class A up until a few watts" amp, trickery that would seem to sonically fall short of a true, pure Class A implementation.
Come to think of it many may feel the honest power output of a pure Class A amp, certainly as a numeric value, is a limiting factor when most audiophiles use low to moderately sensitive speakers, not that it’s necessarily a giveaway into the real-world capabilities of such a (potentially successful) pairing.

That’s also saying: be realistic with regard to the output power you’ll get from a true Class A amplifier - most likely getting anywhere near 100 watts will mean massive consumption, heat and size, so instead expect going for <50 watts if you intend to stay true to concept, so to speak.
And of course, price. Handling these amounts of heat requires very high quality components, large heatsinks and proper casing/ventilation if many years of trouble free service is to be expected. Then again, being topologically more simple than Class A/B amps and possibly even dispensing with a balanced configuration will save number of components used, with more money that can be allotted to the componentry that are - where it matters most.

... Our industry’s marketing people love to find ways to make products more appealing to consumers, which is why we have seen a never-ending race for higher power. High power is better!! But Class A is a great buzz word too and so you also see this creep into the marketing material. High power and Class A, what could be better? The truth is that high power and Class A are pretty much mutually exclusive. It is reasonably easy to achieve Class A in single chassis designs up to about 100 watts but beyond that it becomes extremely difficult because of the heat.

A number of “tricks” have been used to claim both high power and Pure Class A. While it is perfectly OK to bias an amplifier into partial Class A while allowing additional power for short term transient headroom often this is done without declaring the true operating parameters. Commonly the amplifier is claimed to be “Pure” Class A, and yet only delivers Class A performance up to a percentage of its output power. Other schemes have been invented that cause the amplifier to vary its Class A operation depending on how loud the music is. The idea being that it only runs in Class A to as high a power as needed moment to moment, and thence avoids the heat. This causes the music to modulate the amplifier’s design parameters, and in turn introduces other forms of distortion.

How do you tell determine the truth? Its easy really, just ask what the amplifier’s Class A power consumption is when there is no music playing. Take the number you are given (Watts) and divide by two for a mono amplifier, or four for a stereo amplifier. The number you get is the maximum power that the amplifier could possibly deliver in Pure Class A without some trickery.


Never buy based on design, etc. Buy based on how it sounds.
+1 to that by @millercarbon. I see many audiophiles who pay a lot of attention to the particular technology used in a product. To a large degree, the sound of a product comes from general implementation and engineering, not the class of operation, the chip used, and so on.

Also +1 to those who warned about heat. It is a real factor in choosing power amps, and Class A typically runs hottest.
To a large degree, the sound of a product comes from general implementation and engineering, not the class of operation, the chip used, and so on.

It’s a design choice. Ralph Karsten designs his Atma-sphere amps to run in Class A.
So does Tim de Paravicini. I believe it’s a design to lower distortion and colouration.
Nelson Pass designs some of the Pass amps and First Watt to operate exclusively in Class A.

I own an EAR Push-Pull Class A amp and the sonics are a  clean reproduction of the recorded material. The downside is the heat generated from the amp. But this was not a consideration when choosing which amp to buy, I chose the component based on performance and  
sound quality. 

People never complain of heat with tube amps only with SS class A amps. Yes, unless you get some trick Boulder or Krell design, class A amps run hot. The biggest problem with this is durability. Nothing benefits from recurrent heat cycling. A well built class AB amp will out live a class A amp built to the same spec. So, if you can find an AB amp that you really like, go for it. The problem is I can't. Not only that but power is everything. It is not unusual to have 12 db transients in music. That would be 16 times the power. So if you were cruising along at 8 watts and said transient comes along that will cost you 128 watts. If your amp can not support that you clip. Unfortunately for Boston Edison I am in love with big, heavy, powerful class A amplifiers. The best compromise may be the one that John Curl landed on which is build a big, heavy 400 watt class AB amp and bias it up to run 25 watts into 8 ohms class A. 
I wish I could tell you why class A amps sound better but I can not. Is it psychological? I do not think so. It is very hard to find a luke warm review of a class A amp. It might be because of the over built power supplies these amps usually come with that makes them sound more powerful than they actually are. There is also a certain airy effortlessness these amps seem to have that I have not heard in either AB or D amps. I favor the warmer tube like sound of Pass and Curl amplifiers. Some prefer the drier low distortion Boulder version. You either have to have a bunch of consistently good reviews,  try the amp in your system or a system like it before you pull the trigger. Of all the electronics in your system the amplifiers will make the biggest difference in sound quality. 
Always buy on design elements. Sound is over rated.

Class A amps have no zero crossing distortion. An amusical distortion as it is unrelated to the fundamental. In push pull amps one transistor, or tube, or bank of transistors, or tubes, handles the positive going waveform and the other bank the negative waveform. Notch distortion is created, to varying degrees, as one bank of amplification devices hands off to the other.

In class B amplification the output devices power down while not handling any signal, increasing efficiency and reducing heat.

In Class A devices, the output devices are not switched at all and are biased so that they are on all of the time, making the handoff virtually flawless. Single ended amps are completely flawless in this one parameter as a single output device handles the entire swing of the waveform, making it even less efficient than class A.

Another benefit of class A design is that you can often turn your heater down in Winter, the benefits being offset somewhat by having a heavier load on the air conditioner in Summer.
Post removed 
It gets hot, eats electricity,less then 40% efficient 
pure class A rarely gets to 100 WPC.
Nobody needs a pure class A amp with 100wpc unless you use it to weld as well. I’ve got a class A 80wpc and it’ll drive any speaker made. It has such a full, lively 3D sound that even non-audiophiles that hear it are amazed. I’m a big fan of class A topology. Don’t think I’ll ever go back to A/B. 
Mickeyb, what is the brand of amp you are using, if you don't mind me asking?
Once I listened to a Class A amp, many years ago, I never wanted to listen to anything else. Some of the older Class A amps used fans which increases your noise floor. The Sumo Gold I ran for a long time used to hit its thermal cutoffs in the summer so you’d lose a channel every once in a while and would have to shut it off until it cooled.

As I mentioned on other threads, I now own a Gryphon Colosseum stereo amp. I think it is one of the most beautiful amps I’ve ever seen and it doesn’t take up a lot of floor space because it is laid out vertically. It has two massive aluminum heat sinks.

What Class A amp are you considering?

There is one maker of a well-regarded tube power amp (I’ll allow he and it to remain anonymous, so as to not appear to be "pushing" them) who offers the amp in both Class-A/B form, and in pure Class-A. Both versions share the same circuit architecture, power supply, tube compliment, etc, differing only in the biasing required to create each version.

The A/B version produces 100w into 4 and 8 ohms, the A version 40w. So why would anyone go with the 40w version? The designer claims (and there’s no reason to doubt him, at least in my mind) the Class-A version produces 1/10th the distortion of the A/B version. How audible is a tenfold decrease in measured distortion? I can’t answer that question.

I learned long ago that the low powered amps sound better to me. I've had amps for 35wpc to 250 wpc. The 35wpc sounds best in my system. Unless one is pushing an 84db speaker load, high wattage is not needed. Even then, the low power may work. The amp I am considering is 40wpc and has pushed the 84db speakers

I recently built a Class A 35wpc power amp based on the First Watt design.  The Class A was much better but I also use the best parts available and point-to-point wiring.  I also recently upgraded a few Counterpoint hybrid power amps with bipolar transistors with Class A output and the difference was much better. It added more clarity, depth, dimension and faster dynamics. You have to try them out for yourself as always.  I agree with the posts above mine.

Happy Listening.

Class A amps are like driving your car around with the gas pedal floored, and controlling your speed with the brakes. It is a rather inefficient method.
The class of the amp is only one criteria on which to make a buying decision. I have a mid 90's Krell KSA 300S that I dearly love but the Class A feature has its downsides. It weighs 185 lbs. and it runs so hot when I play it loud that I can't touch it. But it puts out 1200 watts at 2 ohms and it sounds gorgeous. It's a perfect match for my speakers (Mirage M3si) which are very low sensitivity (I routinely drive the amp into the highest bias level). This amp is a technology statement and there are very few new amps which can match it's performance, at least for under $50k. I am willing to live with the downsides of this amp because I admire the design philosophy and it sounds very good to me. It's kind of like owning a vintage Porsche that isn't very practical but it's just so fun to drive.

The Canary is a very different animal than the Krell but If the M70 flicks your Bic go for it but understand that the Class A feature is only one aspect of the amp.

@artemus_5, I would have my 100w version modified to run in Class A, but it is powering a loudspeaker which contains magnetic-planar drivers, known for their very low sensitivity. Not as much so as Maggies, but close.

I for years used a 25w/ch Class A ss amp with my old QUADS, and that was a magical combination. If I set them up again, the amp designer alluded to in my previous post offers a real nice little tube amp, available in both Class A/B (35w/ch) and Class A (25w/ch) versions, I will definitely get the 25w version for the QUADS. The QUAD Mk.2 amp was only 15w, and that was plenty for the ESL.

8th-Note, very interesting, but a true class A amp will run hottest at idle, not while playing music.

The reason is simple, since the output devices are always on, with no signal playing that energy is released as heat. With music playing, a small percentage of that energy is going into the loudspeaker load and the balance is released as heat.
I think people underestimate the amount of power required for the fast transients that typically occur in music, all the time in symphonic music. The Gryphon Colosseum can generate over 4000w instantaneously. It has two huge capacitors that can store 340,00microFarads. 

Low power amps have never done much for me. There is something about having a huge reservoir of power to draw on that makes musical reproduction so effortless. I don’t play at high decibel levels, I just like the clean and effortless sound I get from my 175lb Class A amp, lol.

340,000uF might sound like a lot, but that's about on par with what any class A amp at that power output would call for. My F5 having about 1/3rd the power has about 1/3rd the reserve capacitance. Nothing particularly special there. Plenty of DIY'ers have built low power amps with 180,000uF per channel. 

Concerning your claim of 4000 watts... uh, no, it can't, at least not into a practical speaker load. Most likely that thing is running rails around 55 volts, give or take a little. That would allow it to drive it's full power into a 16 ohm load. That's the limiting factor of your head room, NOT the reserve capacitance. Driving 55 volts into a 3 ohm load would call for 18.3 amp which equates to a little over 1000 watts. Of course, that's just idealistic theory that doesn't take into account topology, transistor limitations, or the ESR of the caps which tends to be pretty high for very large capacitors.

What anyone considering a class A amp needs to understand is that the topology is a huge factor. There is no "class A sound". Class A can be single ended, complimentary, fully balanced, or a combination of those, and these basic topologies have very different audio characteristics. A fully balanced complimentary topology built with decently matched parts can produce VERY low distortion and noise with no feedback at all. A single ended class A topology will produce an abundance of distortion of low, even order, which isn't exactly offensive to listen to. Plenty of people have made good money selling single ended class A amps making 2 or 3 percent distortion and gotten glowing reviews for them. There are hundreds, if not thousands, of choices concerning topology, parts, and adjustments that produce different sonic results. 

Personally, I feel the versatility class A allows in parts, topology, and adjustments is it's biggest advantage. When you accept the amp isn't going to be efficient, that it's going to make gobs of heat, that it's going to require a massive power supply, you open doors. The attraction to class A is in the well founded belief that anything class AB can do, class A can do better. Class A is what every topology strives to measure up to, and class A can do things that are simply impossible with any other class, like giving you a rich, warm, organic, spacious sound; the result of lush, natural, even order distortion. 
“The attraction to class A is in the well founded belief that anything class AB can do, class A can do better. Class A is what every topology strives to measure up to, and class A can do things that are simply impossible with any other class, like giving you a rich, warm, organic, spacious sound; the result of lush, natural, even order distortion. ”

This is all I need to know and it is what my experience has been listening to Class AB amps vs Class A amps. I listen to music every day, I glance at my electricity bill once a month. :-)
I listen to music every day, I glance at my electricity bill once a month. :-)
I like that. Sounds like something I'd say

Class-A has yet to be equaled in my experience.
I can beat that! My wife glances at the electricity bill once a month:)
Class-A has yet to be equaled in my experience.
Correct, if done right.

Why do many of us spend $$$K's on our equipment especially BS cables, for only 10% "if any" in sound quality increase over something that cost a 10th or 100th of the price.
Yet we cry murder for nowhere near the extra cost of using Class-A amps and the power they consume, (Class-D users are especially known for this)

Cheers George 
It’s true, George. I think the extra electricity may come to $50 a month, maybe $70, if I listen a lot. It does annoy my wife sometimes but I try to calm her down by pointing out that we don’t own a car, lol, and save a lot of money that way.

People in New York burn $50 at Starbucks in a couple of weeks. I make my own coffee for fifty five cents a cup, I calculated the figure. So, skip Starbucks and listen to a Class A amp. 

Larrykell, you have to try San Fransisco Bay coffee. French Roast. Great stuff!.

Hi George, I have to agree even if you like funny looking class A amps.
but I do make my own cables:)
Use of class A allows achieve low distortion with less feedback applied. That is the main advantage. Main problem is the heat: at full power amplifier will convert into heat 3 times more energy than it sends to speakers. At zero output power energy that is not sent to speaker is converted into heat too.
From 1980 - 2019, Classe' Audio's DR-3 is still the sweetest amp I've heard. It just didn't work in my 25x15 room.
So what does that say for class A amps?
That said, I have not heard Threshold's SA12, or Pass Labs xs300.
A nice thread to read this evening. Well done all.

I completely agree. There are adjustments you can make to an amp for FREE that do more than most of the insanely priced cables out there. Change the bias. Change the feedback. Maybe adjust some resistor values to load the circuit differently for very little money. The question of how much power an amp uses is irrelevant to me. It's such a small price in the big scheme of things. What other hobby can you get into where once you've laid out the money for the gear, it requires nothing more than the power to operate it? It's not like this stuff becomes obsolete like computers do. It's not like it needs tires and oil like a car or a bike does. It's not like there's any actual consumables involved. You flip a switch and it makes you happy! This is actually really cheap fun once you put together something you're happy with. 
Class A may well have been superceded by other topologies. Even some chip gaincones sound very good. 
Sugden have made and continue to make, Class A amps. They're handmade in the UK. 
My introduction, as a musician, to class A was an epiphany. 
I built and listened to commercial class B amps at university.
I performed in a concert in a church with a large organ in 1974.
The concert was recorded. 
On playback the revelation was how every upper rank of the organ was crystal clear. There was no blurring I'd been accustomed to with other amplifiers.
My main instruments of study were piano and flute but I played a lot on pipe organ.
That was my epiphany. 
I've listened to a DIY Sugden 10w class A driving Lowther speakers. 
My degrees and diplomas are in classical music. 
I spent 30 yrs producing recordings of concerts for broadcast on national radio. 
We used proper commercial equipment with standard Neumann, b&k and akg microphones. 
My class A + Lowther combo was used as a reference. If the recording sounded good on them, then it was fine on anything. 
Our studios used B&W and Tannoy monitors, but most recordings were made in our mobile van. 

It depends on the Class A and whether it is tube based or transistor based but couple things are universally true.  

They are horribly inefficient and run very hot.  Due to that heat, they tend to require large heat sinks.  They tend to be very heavy and power consumption is very high.  Because they are so inefficient, they tend not to be that high power.  

Sound profile is a varied as there are amp companies.  I have heard Class A amps that are dark and warm and others that are bright and shrill.  Tube amps vary as much as transistor amps and careful pairing with your speakers is still required along with plenty of ventilation.  

@verdantaudio I’m glad Gryphon integrated two chimneys in the Colosseum, haha. The amp actually runs pretty cool on low bias, 30w Class A, because of the huge heatsinks, but it runs very hot at 165w Class A. Yes, I can hear a difference between the two.

 @mijostyn I will try that coffee. How much is it a pound? For those who wonder about my math to get to fifty five cents a cup it was $25/454g*10. I only use ten grams a day, that’s my dose.
@larrykell41, we used to have a nice Pasquini espresso machine and grinder in the office which was used on a $1 suggested contribution model. It worked well, providing artisan roasted beans, organic whole milk, and the occasional necessary parts. We were using double shots of 18-20 g per drink. The $1 didn’t account for energy use.
The espresso bar definitely made the office a better place and improved the barista skills, and coffee appreciation of many.

Perhaps a good stereo in an office space, for break time, would revitalize the love of good music reproduction?

A big advantage of Class A amps is that by the standards of any AB amp they have a hugely overbuilt power supply and they can drive low impedance loads without faltering.

One of my systems has a pair of Class A monoblocs that put out 45 watts a channel in stereo configuration, but 500 watts bridged into a 1 ohm load.  Hard for AB amps to match that.  They are also large and heavy, if that matters to you - mine are over 100 lbs. each.

If you don't run difficult loads, then the choice comes down to sound vs. power usage and heat production.  I really like the sound and still own 4 Class A amps, but I use an AB tube amp to run my main system which has quite efficient speakers.(92dB)
@viridian4, The Krell KSA amps are true Class A. They have a feature called "Sustained Plateau Biasing." where they have a sliding bias feature that anticipates the loudest signal and then biases the amp into more Class A wattage before the signal is amplified. There are 4 bias levels and when a higher bias level is triggered it stays at that bias level as long as the signal strength justifies it. All the signal going through the unit is amplified in true Class A but if you are playing quiet music it only uses the lowest bias level. This is why the amp runs hotter when it's played loud. But it's important to note that at no time does the amp run in Class A/B.

Some amps run in pure Class a for a few watts then switch to Class B. The Krell does not do this - it only runs in Class A.

There are a couple other companies that use a similar Class A biasing feature in their modern Class A amps. They, like the Krell series, only run in Class A but don't consume their full power unless demanded by the input signal.
@sfischer1 I own a Baratza Forté coffee grinder, a smart scale, and a programmable kettle. My coffee is pretty consistent from day to day. I watch my coffee to water ratio, you know. I use the Kalita pour over system, it’s simple and it works well. An app on my iPhone that pairs with the scale leads me through the steps and provides a timer.

Every office I’ve ever worked in has had atrocious coffee. I used to bring in my own beans and a little French press to make a fresh cup so I didn’t have to drink burned coffee or that awful Keurig stuff.

I don’t think a shared stereo works in the work environment. The most obnoxious people will blast crap all the time. Music is a personal choice, not a group one. I worked in one place where they blasted horrible dance music or Stevie Wonder. Stevie is cool, but every day, all day? I used to unplug that stupid speaker whenever I had the chance. We were all trying to listen to our own headphones anyway, nobody cared to listen to the house music. 

The Gryphon Colosseum allows me to select from three bias levels but it does not slide between one or the other automatically. Medium bias is 70w Class A. I think vocals sound absolutely amazing on this amp. 

We live in good times. We’ve got great gear options and an easy way to stream a huge amount of music. 

Coffee is another area that the masses could care less about. Give em Dunkin Donuts and they think they have the best. I used to take my own coffee on vacation & when we went to see family. I'm sure They thought I was a coffee snob. I'd offer them some but they just didn't get it since it didn't taste like theirs. I never did get into the expresso though. I don't really know enough about it

A big advantage of Class A amps is that by the standards of any AB amp they have a hugely overbuilt power supply and they can drive low impedance loads without faltering.
Yes they should have, but not all have, I’ve seen them with supplies that are close to saturation in the Class-A idle state, which means there’s nothing left for any Class-B transients, so they sound like a limp ****.

This amp I have here can do 3-100w user adjustable of pure Class-A, then to 180w Class-B, and it has the power supply to back it up, 5kva transformer with 300,000uf of supply caps. But it also weighs close to 60kg

Cheers George
I have a Nelson Pass designed Threshold 400A class A driving a pair of Kef LS 50's in one of my systems.  I've owned this amp since new. Recently the capacitors and power switch were replaced and an Anticables level 3. something power cord were added. (The power cord made more of a difference than I expected.) The little Kef's sound  effortlessly clean with this amp. I  listen to this set up for hours without fatique. Is it because the amp is class A or because the tremendous power reserve - probably some of both. 
At the end of the day, it is your ears that decide.  Class A has certain very distinct attributes that factor into its choice.

I have reasonably inefficient speakers (planars) that require a bit more power / current than is typically availabke from Class A amps.   

If I were running efficient speakers I would most certainly audition both Class A SS as well as tube amps and lwt my ears and wallet make the choice.

The beauty of this hobby is the myriad of choices we have on the journey.  Let no one criticize what your ears tell you as it is your money and your preferences based upon informed (read as having auditioned the choice) decision.