What are the sonic benefits of pure class A amps? Are they more "powerful"?
The details are below; the bottom line is that in a class A amp, the output stage is on and passing current for both halves of the signal (+ and -). Consequently, during the time there is no signal, the devices dissipate power as heat. Class AB amps are designed so that there are devices dedicated to each half of the waveform so that less power is dissipated as heat and they're more efficient. The down side is that during the time the signal "crosses over" from + to - there may be a small delay in the device turn on that handles it and this cause a type of distortion known as cross over distortion. Many modern designs today work in class A up to a certain power level then switch to AB. Sorry for the verbose answer, but this isn't a simple topic.

Class A Amplifiers
Class A amplifiers operate over a relatively small portion of a tube’s plate-current or a transistor’s collector-current range and have continuous plate- or collector-current flow throughout each RF cycle. Their efficiency in converting DC-source-power to RF-output-power is poor. DC source power that is not converted to radio frequency output power is dissipated as heat. However, in compensation, Class A amplifiers have greater input-to-output waveform linearity (lower output-signal distortion) than any other amplifier class. They are most commonly used in small-signal applications where linearity is more important than power efficiency, but also are sometimes used in large-signal applications where the need for extraordinarily high linearity outweighs cost and heat disadvantages associated with poor power efficiency.

Class AB Amplifiers
As the designation suggests, Class AB amplifiers are compromises between Class A and Class B operation. They are biased so plate- or collector-current flows less than 360 degrees, but more than 180 degrees, of each RF cycle. Any bias-point between those limits can be used, which provides a continuous selection-range extending from low-distortion, low-efficiency on one end to higher-distortion, higher-efficiency on the other.

Class AB amplifiers are widely used in SSB linear amplifier applications where low-distortion and high power-efficiency tend to both be very important. Push-pull Class AB amplifiers are especially attractive in SSB linear amplifier applications, because the greater linearity resulting from having one amplifier or the other always conducting makes it possible to bias push-pull Class AB amplifiers closer to the Class B end of the AB scale where the power-efficiency is higher. Alternatively, push-pull Class AB amplifiers can be biased far enough toward the highly-linear Class A end of the scale to make broadband operation without resonant tank circuits possible in applications where broadband operation or freedom from tuning is more important than power-efficiency.
No. The sonic benefit is that the entire signal is swung by a transistor (or several in parallel) instead of individual halves of the signal being split up by two different transistors. Since there is no "handing off" of the signal to another transistor, there is potentially less distortion. But then feedback comes into play but that is another story...

Some will say that Class A always sounds better than Class A/B but in fact, it largely depends on the circuit design.
Class A operation is the transistors are fully biased (turned on) meaning there is no crossover distortion (notch??) when the signal swings from + to - and vice versa.

A watt is a watt, so Class A watts are not more powerful than say Class AB watts.

Of course, the drawback of Class A is heat and energy consumption. The amp must dissipate all of it's power whenever it is turned on whether playing music or not. Either the energy goes out the heatsinks as heat, or to the speakers as electrical energy, or a combination of both in most cases.
Much of the newer A/B designs will run up to nearly 25% of the output in pure class A and switch to AB for cooler operation and lower power draw.. and many of the newer A/B designs today rival and beat many of the older Straight Class A in my opinion, newer technology capacitors, transformers, transistors can make this possible. Older Thresholds, CODA, and Levinson's I owned are good, but my newer odysseys and many mcintosh amps sound better sometimes and do not run hot at all... so your taste after audition is going to determine if they are better for you or not, I hated the rediculous Heat and draw of electrical with low efficiency in comparison, so even if not sonically better I find some reliablity, ease of use, and cost benifits to the newer Class A-A/B designs.
Class A amplifiers nearly always exhibit more authority, but this has nothing to do with power.

I should point out that a Class A amplifier can be either push-pull, single-ended, tube or transistor. What is important is that the amplifying device or devices never goes into what is known as 'cutoff'. IOW the device or devices each amplify the entire waveform all the way up to full output regardless if single-ended or push pull.

Distortion is generated when an output device goes into cutoff. In a push-pull circuit, when the device is cutting off while at the same time passing off the signal to its complement that is just barely turning on, the resulting distortion obscures low-level detail (as in the case of Class AB2 and Class B push-pull amplifiers). This can be complicated by the presence of an output transformer in the circuit as magnetics tend to react badly to this sort of thing!

So ultimately, Class A is the lowest distortion form of amplification, regardless of the circuit topology or amplifying device.

The price paid is Class A amplifiers are less efficient as they are 'on' all the time and therefore they make a lot more heat. They also require fairly beefier power supplies for the same reason. Such is the price of increased performance!
Generally, they are not more powerful. Class A designs are extemely inefficient because it requires that a lot of heat be dissipated to maintain stability, robbing power to drive speakers. For technical as well as practical reasons, this limits their power output stages to around 100 watts and their ideal design power rating is usually 80 wpc or less.

This web site goes into a bit more detail concerning the differenct classes of amplifier circuit designs

In general class A is much less effecient than either class B or A/B(most common design) but procduces less distortion (e.g.cross over distortion in class AB) than either class B or A/B. With the inefficency comes heat,caused by bias current, which must be dealt with by utilizing large heat sinks.This generally makes pure clas A unsuitable for high powered amplifiers. An example that comes to mind is Nelson Pass his X seriers is class A/B and ranges in power from 150 (X150) watts to 1000 (X1000)watts per channel. The XA amps are pure class A and range in power from 150 (XA150) watts to 200 (XA200) watts per channel.


There are very high powered pure class A amps. Needless to say they are typicaly very expenisve, very big, very heavy, run very hot and are costly to operate.
Thank you very much to all of you for those very clear and competent explanations. I feel much more smarter now!
Those are all excellent answers but if you want to know how they sound different, visit a Plinius dealer as some of them are switchable between class A and A/B.
I am still at a loss for comprehending the general sentiment expressed in this thread that class A amps are only beneficial due to lower distortion. Heat dissipation and the efficiency of their use of power are well known, but they are also given credit for having a typically fuller sound and are known for their ability to control bass well.
I own very high biased (reportedly) class A amps but can't run them as pure class A. If and when they would switch to A/B is mysterious to me. I don't hear any noticible change in the sound. It is entirely possible they never reach that point, in fact they shouldn't because they supposedly run up to 125 watts class A which would blow my head off except for perhaps dynamic bursts.
Does anyone know if the claim that Aragon Palladiums are really class A in the first place. They don't distort much if at all, but they don't run really hot, temperature wise, either. Any one who knows something about these amps and can tell me something other than the canned corporate stuff would be appreciated. Sorry if that is a thread jack.
Also why all the advertising about class A capabilities if it doesn't matter?
they supposedly run up to 125 watts class A
Wow! They must be HUGE (even with a switching PS)... We're talking about a +500 VA PS per channel for class A... Now that's a nice monster!

BTW, if they claim 125/channel class A, they must be able to back it up; so, you probably DO have 125W one way or another (but it does sound like overkill as you note...)
Even 25 watts of class A will run quite warm. If you want to check if your amp is class A, rig up an ammeter onto the power. While playing music at increasing volumes, when the dial/readout starts to oscillate, that's when it kicks into A/B. For that 25 watts, that would be 0.25 amps and that's loud.
The total output is given as 400 watts into 8 ohms and 1000 watts into 2 ohm, I guess that why the full name is Aragon Palladium 1K. They can't really be running that high in class A they just aren't that big and never run really hot just warm. Look at Avalon's (an agon dealer) listing for the claim. They do have enormous power though, huge headroom, never clip to my ears. I use 936 JM/ Focal Electras with them. They have double 7 or 8 inch woofers and they probably drop their resistance to 4-6 ohm at times. I am suspicious, believe me, but I like them anyway.
I could be very wrong about this, but, I thought Aragon's trick was that they offered their rated class A opperation only so long as they were driven into an 8 Ohm load, below which they reverted into class AB. From what I understand the best ss class A/AB amps half their class A output as their class AB output doubles into progresively halved impdedances.
If its not real heavy and real hot its not running an appreciable amount of class A power. I'm not kidding about the hot. My amp ( gryphon antileon signature ) gets so hot you can not keep a hand on it for more than a few seconds when its really cranking. It runs 150W pure class A into 8 ohms 300w into 4 ohms 600w into 2 ohms and 1200w into 1 ohm and 2400w pure class A into 1\2 ohm ! To do this it requires 200 lbs of amp/heat sinks. Any amp that isn't very heavy can not dissipate the heat produced from this output. This is an inherant drawback of inefficient class A operation. Beware of "class A" amps with A/B output stages. These are not class A amps. Reguarding the sound , it is up to you to decide. I love it. One thing is for sure. It is uniformly stable no matter what load it sees. Good class A amps have ENORMOUS power supplies. I'd be surprised to hear of a class A\B that doubles down to 1\2 ohm. So are they better or more powerful ? It depends. If you have speakers whose impedence dips appreciably then they are probably functionaly more powerful. If you have lots of room, dont mind the heat or the aesthetics of a really big amp with questionable waf then yes I think they also produce better sound. This is just my opinion. - Jim
Another aspect to consider is that Single Ended class A amps do not increase their power output in lower impedence loads e.g Pass XA series amps. Which is why you need to consider the speakers efficency in most cases when using pure classs A amplifiers.

Hi Al. The Gryphon Antileon Signature Stereo puts out 5000 watts at 1/2 ohm. The mono's put out 7500 watts at 1/4 ohm.
P.S. I have the Gryphon Encore. 500 watts at 8 ohms and the first 30 is class A.
Hey frankg how do you like the encore ? I've never compared it directly to the signature but I bet it holds up pretty well.- Jim
As "Aldavis" points out, size and heat indeed. At 145 lbs and 16 power tubes each, with a conservatively rated 150w Class A, the CAT JL-3 monos do a wonderful job doubling as floor heaters.
Chuck, I was under the impression that some particular examples of the Aleph amps with minimal gain stages were unable to double down, but, all the other Pass amps were able to double down. I would guess that those which could/can doube down would halve their class A out-put as they did so.

As I understand it (from information gathered from the pass web site) the X series (class A/B) does double down but the XA (pure class A) does not. And as someone mentioned class A amps tend to be sweeter more detailed in the midrange.


Hi Chuck, thanks for the update. I went to the Pass site. I didn't see anything about power into lower impedances, but, you may very well be correct. There was mention of the XA series not having the current delivery of the X series and that the XA's using only two gain stages like the afore mentioned Alephs'.
Unsound, I remember having a conversation with Jon Soderberg, formerly with Threshold and now a top repairman for the older amps. He told me that on the SA-1 that I was actively pursuing that it would double in power into 4 ohms but that the Class A amount of it would be cut in half. Whatever the Class A power was into my Apogees, it was really sweet.

Here is the exact information fronm the pas web site I quoted.

"The XA-200 will deliver 200 watts rms into 8
ohms of impedance. This is the amplifiers class
A limit, driving the XA-200 harder or reducing the
impedance of the load will not convey any additional
power. Driving into a short will not convey any
additional power. These are not design oversights or
flaws in the XA-200, but rather a condition of the
very highly biased Class A operation. However, as
an interesting point of reference, 200 watts driving
a 87dB/1W/1m speaker will deliver a 112 dB (very
loud) average acoustic signal in a 100 cubic meter
room. Transient peaks will be somewhat higher."

Chuck, thanks. It's interesting that Pass suggests that "These are not design oversights or flaws in the XA-200, but rather a condition of the very highly biased Class A operation." I don't recall this "reasoning" from some other Class A amplifier manufacturers. It may even contradict what is commonly percieved as techincally available from Pass' previous designs.
Onemug, Jon recently told me the same thing re: a different Class A/AB Threshold amp.
highly biased Class A operation
This is probably Mr Pass' way of saying the device is operating in "deep class A". Just the PS is well over the expected 4x the energy output rating fro class A.
I think I read somewhere that this is acharacteristic of pure class A amps in general and that it also applies to tube amps. If I remember correctly, and I am getting old, I believe CJ told me that it was a characteristic of my premier 11a, but I am not sure. The topic came up with CJ when I was asking them questions concerning tube amps and driving thiel speakers. I maybe speaking to much to generalities and not enough to specific circuit topologies.

Czbbcl , that is not a characteristic of class A amps although this may be a limitation of Pass designs. As stated above Gryphon class A amps double output from 150W @ 8ohms all the way to 2400W @ 1/2 ohm (5000W peak). In the real world your home electrical circuit will be the limiting factor with well built true class A designs with HUGE power supplies. - Jim
Real Krells can double down and down again from their rated power. I'm not sure if they maintain full Class A operation as they do. I sure would like to find out though.
I have a question whether or not the amp I own differs from those statements made above. Can someone tell me whether my belief that my amp runs Class A all of the time is correct or not. I have had one or the other of these units for years and always thought such was the case.

I own a vintage (1979) Yamaha MX1000U amp and had the MX10000U prior to that. Yamaha states that their Hyperbolic Conversion Amplification Circuit (HCAC) "eliminates the only serious drawback of previous Class A operation amplifiers. These had to switch to non-linear Class AB operation above a certain load current, causing a slight deterioration in the otherwise excellent sound quality. The HCAC does away with this problem, allowing the Yamaha MX1000U to deliver superior performance constantly, without switching or cut-off, over its entire power range.

Advanced Power Supply Circuitry (APS) power supply is also incorporated in the amp. The problem with conventional power sources was their tendency to produce pronounced voltage ripples during medium to large amplitude music signals, particularly under low impedance loads, together with voltage fluctuations in the power transistors. APS solves this problem by providing an active power source that ensures stable voltage in the final transistor stage under all operating conditions, thus greatly improving power supply to the amplifier."

Minimum RMS Output Power Per Channel:
0.003% THD, 8ohms - 260w
0.007% THD, 6ohms - 295w
0.02% THD - 330w

Dynamic Power Per Channel (IHF):
8ohms -- 450w
6ohms -- 560w
4ohms -- 740w
2ohms -- 960w
1ohm -- 1000w

Dynamic Headroom:
8ohms -- 2.38dB
6ohms -- 2.78dB
4ohms -- 3.51dB

Power Band Width: 0.03% THD 130w, 8ohms -- 10Hz-60kHz

THD Main In to Sp Out, 130w/8ohm = 0.003%

Intermodulation Distortion (rated output/8ohms): 0.003%

Signal to Noise Ratio (IHF-A-Network)
Main In 9 shorted) -- 126dB
Main In (5.1 k terminated) -- 122dB

Residual Noise (IHF-A-Network): 21uV


Wow, I wonder if Ken Stevens would upgrade my CAT amps with that APS technology! Oh yes, the 1970s and their super huge spec sheets. Those were the days when all amps measured very differently but still sounded the same. And now today everything still measures very differently but sounds wildly different. Hmmm, what happened? Man oh man I miss Julian Hirsch.

The attached website ( has a pretty good explaination of class A amplification form a lay persons perspective. I believe the Gryphon is designed as a push-pull topology (unsure) versus the Pass which is a single ended topology and depending on ones definition of pure class A the Gryphon may or may not fit the bill. But again I am not an expert on the subject.

Unsound I believe krell is a class a/b design

Chuck, all early Krell's were Class A. Some newer ones (IMHO the only real Krells) are as well, but, use their proprietery sliding bias scheme. I still think these sliding bias amps can make good on their Class A claim. The very early Krells used fans, the later ones used massive heat sinks and the newer ones sliding bias, (still pretty hefty heat sinks as well).

I went to krells web site and he states class A topology. It would be interesting to know if it is push-pull, single ended or what specific design they are.

I'm pretty sure its push pull. Other than Pass and Monarchy I don't know of any other single ended Class A solid state amplifier manufacturers.

That would make sense given the power output. I am sure the Gryphon is as well. It is interesting stuff as I have always found circuit design especially for audio interesting. I majored in Mechanical Engineering as I never had a mind for electronics even though I do enjoy it as a hobby.

Chuck, even single ended Class A amps are capable of respectable power out-put. The solid state single ended Class A Pass Aleph 1's power ratings: 150 Watts > 8 Ohms, 300 watts > 4 Ohms, 600 Watts > 2 Ohms. Of course these amps were expensive exceptions to the norm. The less expensive Monarchy solid state SE 100 single ended Class A power amps power rating: 100 Watts > 8 Ohms, 200 Watts > 4 Ohms. The Monarchy hybrid tube/solid state single ended Class A SE 160's power rating: 160 Watts > 8 Ohms, 320 Watts > 4 Ohms. Needles to say I can't personaly verify these quoted ratings. There does seem to be some confusion about Monarchy's most recent 250 Watt Class A/AB hybrid's power ratings.
I shouldn't have singled out Monarchy for "confusion" with re: to rated power out-put. One of Pass' amps failed to achieve rated power during a test by Stereophile.

I agree and I didn't mean to suggest that single ended amps couldn't produce high power what I mean't, and I may be wrong here, is that they don't produce the power output of a push-pull design e.g. Pass xa versus x series amps. And yes I remember reading the Stereophile article concerning the power output of the XA200 at least I think it was the XA200 falling short of rated output.

In general, and my logic may be off here as well, I think of class A amps (SET vs. Push-Pull) like I think of tube amps (SET vs. Push-Pull). SET tube amps typically have power outputs in the range of 3-50 wpc while push-pull varieties have power outputs in the range of 50-500 wpc. Anyway sure is fun to have these discussions as I usually learn something in the process.

Chuck, I think your right about single ended amps being less likely to produce as much power as push/pull, but, when it comes to solid state the differences aren't as great as in tube amps.......
I'd still like to know if those ss amps that are touted as being pure Class A and being capable of doubling down, maintain pure Class A operation as they double down.

The Krell FPB "Class A" series maintain Class A operation
from 8 ohms to 2 ohms.

They have well regulated power supplies that have the current
capacity to deliver the current that low load impedances

Given the well regulated nature of the power supply [i.e.
the ability to put out the rated voltage independent of load ]
and the current capacity to back it up - that's WHY a
Class A Krell will double down.

Dr. Gregory Greenman

You can also check out the reviews by Stereophile, such as
Martin Collums April 1997 review of the then current FPB-600.

In the Measurements segment, Martin addresses the power that
the amp can deliver into low impedances:

"I wasn't able to run my long-term continuous testing at
2 ohms, but compromised with five-second bursts-long by
peak-measurement practice (eg, 20ms). The FPB 600 could
sustain a 29.3dBW level into this load, corresponding to
3.4kWpc-an extraordinary figure."

Dr. Gregory Greenman
Unless I missed something, everything in these aritcles suggest that the Krell's are capable of maintaining Class A operation as they double down, but, they don't actually make that claim. As for me, I believe they do. Wretched excess? I love it! I may be part of the minority here on Audiogon, but I really like the "real" Krells and thats based on what I've heard not on what I've read. Though it is pretty impressive reading.

The circuit topology of the Krell FPB series is Class A.

Class A amps don't automatically resort to A/B when they
are asked to output large amounts of power.

No - that is a characteristic of Class A/B amps that are
designed to maintain Class A at low levels - and then they
revert to what they really are Class A/B.

However, a true Class A like the Krells doesn't automatically
revert to A/B at high levels.

Dr. Gregory Greenman
Moribus, thank you. That really clarifies it. Am I correct in assuming that power increases by ss amps into lower impedances is not determined by Class of operation?
What caught my eye in the Stereophile measurements was paragraph 3 where they state it's a 935 watt amp continuous, almost 30db. It doesn't put out 1870 wpc into 4 ohm or 3740 into 2 ohms continuous. "However at it's rated power of 600 wpc, which is 28 db, it would hold that rating for all loads from 8 to 2 ohms". I read about many an amp that put out 100 into 8 and 150 into 4. What if that manufacturer decided to rate it at 75 into 8, even though it put out 100? Then he could say it "doubled down" to 150 into 4. Impressive little amp then. Nobody is going to sue anybody if they buy a 75 or 600 watt amp and it puts out 100 or 935 respectively. Gee, I got more than I paid for. But it's the wordage I look at. Doubles its "rated" power is honest but to me, it is what it is, and if its "really" a say 250 watt guy, I'd be curious how close it came to 500 into 4 but most important is how it sounds into 4 or 2 ohms. Some amps start to get a little unhappy down there. If it makes someone happy to believe their amp doubles down that's fine but I don't listen to specs so my criteria is just "does it draw me in".
Onemug, may have a point. Bear with me for a moment and forget the "most speakers are damaged due to being under powered rather than to being over powered". What if a speaker manufacturer refused to honor a waranty because one used amplifiaction that was rated within the speaker manufacturers standard 8 Ohm limits but actually exceeded its 8 Ohm rating?
Interesting reading, and discussion men. Thank you. Now I know why I love my FPB400cx! The "real" Krells do sound wonderful on every type of music in my system. :-)