Are There Any Inherent Advantages to Class A amps?

All things being equal, power supply size, wpc into 2 ohms and everything else, Is a class A design inherently better than an A/B design? Disregarding the heat issue with class A amps, what makes them so desirable?

I'm saving my money for a pair of used serious monoblocs. I'm thinking a pair of used Pass X-600 monos or a pair of used Krell 750 monos. On the used market, the Krells are approx twice the cost of the Pass monos.

The Krells are pure class A, the Pass are class A for the first 160 watts, then they go to A/B.
The Krell is probobly pure class a for the first few watts or so. If your looking for true class a look into set amplification.
Thanks Tripper. Those links are extremeley informative and they were very well written.

I believe Audiogon should have a glossary page. The links you provided are candidates for such a page.

The Krells are Class A to their rated output.
"The Krell is probobly [SIC] pure class a for the first few watts or so."

That's class AB, Radiob.

Mitch, I believe class-A-biased amps have an inherent sound-quality advantage over class-AB-biased amps, but in well-designed amps, this advantage is other differences. Buy according to the usual criteria...

Class-A amps do make great room heaters! :-)
Thanks for the references Tripper!

I have an amp that can be switched from pure A to A/B with a toggle switch in the back. I move it to A/B sometimes instead of turning it off (it runs much cooler in A/B). At other times, when the music is strictly for background listening I'll also run it A/B. I can always tell the difference! Class A sounds more "right" and much smoother overall.
I don't think the class of operation will (or should) make one amp be more advantageous over another. To answer the question directly: yes, there is an inherent advantage. It's the elimination of cross over distortion which means one less level of distortion to worry about during design. If one were to design a cost-not-matters sota amp, then class A will probably be ONE design criterion.

But it doesn't mean that the advantage results in a better amplifier - too many other factors are involved. I personally prefer the class A/B Classe amps over the class A Krells. Nothing against Krell, but to me their amps are just not to my taste. So to toss the coin with an eye towards operating class will not, in my view, bring about the end result of a good sounding system.
Dear Mitch4t: Switching-cross distortion: as a fact almost every amplifier works in class AB, what this means?: that it works in class A, for a few or severals watts, and in some time switch to works in class B. So, there are amplifiers that works the first 5, 10, 50, 100 watts in class A and when you are asking for more ( watts ) switch to class B.
This switching-cross distortion ( up and down ) it can be audible. So, a pure class A amplifier does not have this switching distortion because always is working in class A and this is the inherent advantage of pure class A. Theoricaletly the class A has less distortion than class B ( it can be measured ) but it is almost inaudible ( in good designed amplifiers ) , example: if you have two amplifiers ( that have same specifications, design and designer ), one pure class A and one pure class B, working between their best linear power ( not near clipping point ) it is almost immposible to detect any differences in the quality of the sound reproduction .
BTW, I don't know why you need the Krells 750 or the Pass XA 600 in an audio system where your front loudspeakers are biamplified. Can you explain this?
Regards and always enjoy the music.
Gs, I generally agree. I stopped short of recommending that Mitch should go for the Class A because there are simply too many design characteristics that influence overall sound. There are clearly A/B designs that beat A designs -- that was also made fairly clear in the great references shared by Tripper. On a straight design to design comparison (i.e., same amp), Class A rules. As always, judge with your ears. Good point Gs.
Raul, I'm considering the Krell and Pass monoblocs to upgrade from my current amplifiers. My current amps are the Carver Silver 9t monoblocs, they are/were the price point that I was able to attempt to enter into the high end audio arena. I started with one pair and I liked the sound, I got another pair and bi-amped, then a whole new world opened up. Then I got the AR LS-10 preamp and the system took another quantum leap. I recognize that my current amps do not rank high on the heap of hifi, some even consider them mid-fi. So, the reason that I'm considering the Krell and Pass monoblocs is to see if they take my system to another dimension. My Infinity Kappa 9 speakers dip to really low impedances and I want unquestioned authority on the bottom end....I keep hearing Krell is really strong there. I tried using a Sunfire Signature on the bottom while bi-amping, and while good, it did not deliver the slam and control that the Carver monoblocs delivered.

My Carver monoblocs cost $1500 per pair used. The Krells will be at least $15k used, whereas the Pass will be about $8k used. I hear that the Pass and Krell monoblocs are light years better my Carver monoblocs. What kind of performance leap can I expect by upgrading to the Pass or Krells? I prefer to bi-amp with matching amps, but at those prices, that will be prohibitive for now.

I liked the sound of my system at every point before I upgraded it, and I liked it better than before every time I upgraded.

I plan to spend no more than $1k for speaker cable and interconnects for the entire system.

Any of you, please weigh in on this for me.

Good thread guys, thanks for your kind words. BTW, nothing against [or for] Google, but I've always used and liked for metasearches [where I searched for class A amp operation]. On systems, Everything is the sum of that which preceded it as Ozfly points out. But Class A does seem to be an excellent point of departure. My single-ended Class A amp has been in my various systems longer than other component. "Only" 50 watts but every one is as sweet and unforced as can be, and has driven a dozen very different speakers with grace and ease. Same for my 35 watt custom 85# brute, truly an iron fist in a velvet glove. Good luck to all, whatever Class you choose! Tripp
The Krell FPB amps are class A to their full rated power. The sustained plateau bias helps to prevent (through magic and witchcraft) the amp from running as hot as Class A amps have in the past.

It is true that Krell amps before the FPB series were only class A for the first 25 or so watts, which is where most of our listening is done anyway. The Krell monoblocks are very expensive, but I think they are worth it. The Pass Labs X 600 are a great pair of monoblocks, and as mentioned previously will only cost about half the price of the Krell.

In my limited experience Class A amps sound better than class AB or those wretched digital amps. They are more 'there.' So yes, along with the drawbacks there are many real advantages.

...and dats da name ah dat tune!
Don't forget the oil savings in the winter, as you can turn down your thermostat a few notches :)
Dear Mitch4t: I think that the JC-1 by Parasound it will be a good alternative to you and for the price you can get four units for bi-amp instead two Krells/Pass
Regards and enjoy the music.
Rauliruegas, can the Parasounds drive low impedances and deliver serious slam on the bass?
Mitch4t, I think there are no worries in that area.

Mitch, if you want an amp / amps for just the bottom end and REALLY want them to thunder, save some money and buy some used Electron Kinetics Eagle 7A's, a Perreaux PMF-5150B, etc... It might take some looking and waiting, but you can typically find these for about $1000 - $1200 if your patient. If the demand goes up all of a sudden* or they are really clean units, figure $1200 - $1500. The big Aragon mono-blocks might also do the trick for you.

As far as your experience with the Sunfire Signature goes, i agree. These amps in stock form DO lack bottom end impact and authority. This was a real sore-spot for me with these amps. The newer models are better in this regard but they can still be improved upon. If you really just want it for bottom end, send it back to the factory and tell them to perform what Robert refers to as their "Apogee mod" to it. This changes the tonal balance quite drastically and will "thump" your woofers for all that they are worth.

If i personally was choosing between a Krell and a Pass JUST for use as a woofer amplifier, i would give the nod to the Krell in a heartbeat. If i was going to listen to the amp full-range, i would choose the Pass in a heartbeat. Obviously, these are my personal preferences, so take them with a grain of salt. Sean

* Has anyone ever noticed that products that get repeated recommendations on the Agon forums tend to creep up in value? I've seen this with several different models, both here and on Ebay.
As others have stated - the Krell FPB "Class A" series are
Classe A up to their rated output.

A Class A amp can't switch to Class A/B - it doesn't have
the second "mirror-image" amplifier chain to handle the
opposite polarity. An amp that is Class A only has a single
chain - and it is biased so that it conducts during 100% of
the cycle - both positive and negative.

The Krell KAV series amps are Class A/B.

The advantage of a Class A amp is that there is no
"crossing distortion"; inherently. The amp does not
shuffle the load back and forth between an amp chain that
handles the positive half of the cycle and an amp chain
that handles the negative half.

In a Class A amp, a single amp chain handles 100% of the
cycle - so there's no distortion due to the "hand-off" to
the other chain - because there is no "hand-off".

Additionally, the latest offerings from Krell - the
"x-series" [ because the model number ends in "x" -
e.g. FPB-400cx] are somewhat of a departure from the
earlier Krell amps. They have gone away from the typical
"Krell sound" and lean toward a sweeter, more musical

Dr. Gregory Greenman
Dear Mitch: It can drive low impedances with out any problem and their bass reproduction is stuning.
Like I told you, you can get four of these exellent amplifiers for bi-amp. I agree with you that the best way to do the bi-amp is with the same model of amplifiers.
Regards and enjoy the music.
bryston 6b and 7b appear to be decent values used for 500 watt monoblocks and most with years of warranty left....they can be wired up in parallel for different speaker types, too...2000 or a bit more for a pr
"A Class A amp can't switch to Class A/B--it doesn't have
the second "mirror-image" amplifier chain to handle the
opposite polarity. An amp that is Class A only has a single
chain--and it is biased so that it conducts during 100% of
the cycle--both positive and negative."

Morbius, that's not correct. You're confusing 'single-ended' with 'class a'. The first is a circuit-design principle wherein one output device does indeed handle both swings of the alternating-current signal. The 2nd deals with how much bias current is pushed continously thru the output device(s). Every solidstate class-A amp I know of (and that's not many) is push/pull and NOT single-ended.

These are 2 different design considerations.

You are incorrect my friend, a single ended amp is just a Class A amp with a single (as opposed to multiple) devices in the output stage per channel. Class A amps BY DEFINITION amplify through 360 of the duty cycle. Class A amps ARE NOT push pull!!!


Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1
Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB)
Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1.
Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! ** This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts (single-ended operation is assumed, and yes this does work for RF)
Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - because of frequency limitations (and the fact that they nearly all seem to sound disgusting), many are only suitable for industrial control of motors and loud but crappy sub-woofers (this may change if transistors with an infinite bandwidth become available soon - yeah, right!) All Class-D amps have a major limitation in the output filter, whose response is highly dependent on the load impedance.
Well, we seem to be splitting hairs here.

I was specifically addressing Morbius's comment, which is still incorrect in that class-A amps can indeed be push/pull as are all the solidstate class-A-biased amps I've ever seen (and, as I said, that's not many, as I do not study everything this crazy industry markets).

So I'll agree that the output devices of push/pull class-A-biased amps do indeed conduct the entire waveform if you'll agree that Morbius's statement is incorrect.

We are NOT splitting hairs - go back and read Steve's post -
where he states, "Class A amps BY DEFINITION amplify
through 360 of the duty cycle. Class A amps ARE NOT
push pull!!!"

A true Class A amp does not have two "mirror-image"
amplifier chains - one for the positive half of the wave,
and one for the negative half of the wave - as is found in

In a true Class A amp, a single amplifier chain is biased
to conduct through all 360 degrees of the waveform.

Dr. Gregory Greenman
Please help me out here. There seem to be lots of conflicting definitions of class A. The references Tripper cites clearly indicate that class A can be a push pull design. Virtually everything I've read in the past here on the Gon also support that. I'm wondering if the definition that Stevechan cites is also consistent with that: The definition for class A reads that "a single output device is possible" -- I would interpret that as "possible", not "required". If someone elects to provide the bias for 360 operation but also elects push-pull, then it may still be class A. The definition for class B indicates that "push pull must be used". That is true. It must be used in class B designs for the full wave form -- and depending on bias, the full wave form may not be reproduced at some volumes even with push pull. I don't believe the definitions are at odds. The definitions per Steve allow for push-pull in A but insist on push-pull for B. That is consistent with the definitions supplied by Tripper. I may be missing something -- besides my mind, which I know is gone ;-)
My personal experience says yes.I switched to a class A gryphon antileon signature from a very good chrord AB amp which had a much higher output rating.The gryphon has much better low level detail and much more powerful dynamics.It is a true class A amp throughout the signal path.With huge power supplies 150w/8ohm becomes 5000w/.5ohms of pure class A.I believe the pass amps are merely Class A biased and therefore become AB after a certain output is reached.Also be careful of most classA claims.They typically denote bias or dont include output section.In general if its not big and hot its not class A.