Why do I like Class AB better than Class A?

So I just purchased a Plinius SA 100 MKIII, and have had 2 listening sessions comparing both with the toggle switch. To me clearly the AB is more detailed and focused, the A is warm but a little fuzzy and bloated, although I would give it a slight nod on just vocal songs. Am I just not use to the sound as this is my first A amp or have I not let it properly warm up for class A (1hour or so).
Don't get me wrong, i love the amp, great bottom end slam but I don't hear the holy grail from Class A. Thanks in advance for the input guys.
Interesting, I'm wondering if there may be something wrong with the amp. I've owned 3 Class A amps, but only one of them had the ability to switch between Class A and AB, and even that wasn't a pure Class AB, it was more of a Class A/AB. That amp was a pair of Clayton Audio M-100 monoblocks. Anyway, I love Class A sound, it is amongst the best SS I have heard. On the Clayton amp I had the ability to switch from high bias (Pure Class A) to low bias (Class A/AB) and found that while low bias may have had a slight edge in dynamics, high bias provided a greater sense of liquidity and ease. The sound became more relaxed and natural. I would NOT say it became fuzzy or bloated at all.
What speakers are you using?

Perhaps what you are comparing is the difference in power and not just Class of operation.

Maybe the speakers are just too power hungry for the Class A's limited output to shine.
Doogie - there is no difference in maximum power output in "Class A" or A/B its just a matter of how much bias current there is running in the output stage at idle The term "pure Class A" is a bit of a misnomer too as a speaker load is far form flat, a push pull amplifier biased for 50 Watts class A into a 8 ohm load would only "have" 25 watts of class A into a 4 ohm load although depending on rail voltage it would have a maximum power output of lets say 100 Watts into the 8 ohm load and a whopping 200 Watts into a 4 ohm load.

Nelson Pass wrote a very informative paper on this subject called "Leaving class A " it can be found on his website.

On my own amplifiers I have a built in the option to select High bias or Low bias I prefer the sound in High Bias but if just for background listening I leave them in low bias, no need to use up all that energy.

Good Listening All


"although depending on rail voltage it would have a maximum" should be

"could have"
I am pretty sure the SA-103 (the replacement of the 100) and the SA-Reference that I have has a much lower output in Class A. 100W vs 300W in AB.
Even when in class 'AB' the lower level signal is class A.. up to a point.
And many pure class "A" are also only class A up to a point.
As mentioned it is just a bias voltage, and can be in a wide range..

Pure class "A" are just like small nuclear reactors as far as heat goes....
My experience with Class-A operation (mostly with Parasound JC1's) is very different than yours. My soundstage becomes MORE defined, not less. It also grows in all directions.

If it weren't for the heat I would never use the low-bias setting.
Are you sure you're not responding to one amp being silver and the other black? How you're able to leap from a quick listen sonic preference to a clear cut distinction between amplifier operating states is beyond me.
I have a couple of the same amps. The difference between those modes of operation can be subtle, depending on the recording and the volume. Patience... you'll hear it. After a while, you'll never want to listen in anything other than class A. Probably getting conditioned to the "laid-back" character of the amp as well.
Sounds like God is telling you to upgrade ....
I have owned the Plinius III and always thought it sounded better in class A. How long are you allowing it to warms up? Needs maybe 20 minutes to sound like class A.
My prior amp, a Forte 4a was class "A" and sounded best only if left on 24/7. It took a few hours to sound decent, and to really shine took at least 24 hours..

It was only 50 watts a side, but it was like a little room heater.
My current amp, much bigger Bryston 4B-SST2 produces half the heat in a mass twice as big. (with six times the wattage) and I leave that on 24/7 too.

Your PURE Class A Forte 4A was biased at 800 mA per node - resulting in an astonishing 9 watts in Pure class A into a 8 ohm load - the rest of it in A/B - and yes it ran hot, but it sounded pretty good. I have had several of them through here. My first "real" amp was its predecessor the 1A - in its a day a pretty good sounding amp too.

I owned the SA100MKIII and preferred class A over AB, it was really liquid and had better weight and a deeper soundstage, also more of an intimate and revealing inner glow quality.

not quite tubes but as close as I have heard from SS.

no contest in my book, on Maggie 1.6 btw...
Clarification on my above post. I left my Plinius SA 100 MK III on 24/7 in A/B but when I switched it to class A it took at least 20 minutes to reach the sound I desired.
"I don't hear the holy grail from Class A. Thanks in advance for the input guys."

You should be if all is functioning fine, Class A should have a 3D quality instead of 2d like the A/B would have in comparision.

"the A is warm but a little fuzzy and bloated"

This could happen if the power supply is not up to task, as Class A mode will tax the power supply much more and bring it closer to satuartion point than A/B will, this satuation results in restricted dynamics, soft (bloated) bass loss of transparency. Remeber the power supply is the engine room of the amp.

Cheers George

Hey, thanks for all the feedback. I have Snell type AIII's, now I know there not very efficient but this amp certainly has brought more bottom end slam than my BAT. The rest, AIFF,Pure music, MacMini, Wyred4sound DAC 2, Aesthetix Calypso Pre with Nos Tubes, oh and dedicated 20 amp outlets plugged straight in. I was hoping someone would tell me I would need to get used to the class A sound. I will let her warm up more than the hour I usually give it and see what happens.
Most transistor gear I have seen really have long warm-up times- 24 hours or more.

Theoretically class A gives you lower distortion. When you remove distortion, the amp should sound smoother and more detailed as distortion can result in harshness and obscures detail due to the ear's masking rule.

Now when you get more detail, one of the things that happens is that there is more room ambiance that shows up. There is more of the acoustic signature around each instrument. This can make the amp seem to have less image specificity, until you realize that the image is being presented with more of the ambiance around it intact.
I'm sorry, maybe I'm slow or missing something. but your initial post said that you compared both with a toggle switch. Compared both what? I only saw one amp. So, my initial reaction without benefit of knowing which two amps you are talking about is that you compared apples to oranges. The amps were either not rated the same or they were in totally different price points. Like comparing a $10,000 amp with a $400 Yamaha 100wpc amp. I don't know what you compared.


you can flick a switch on the Plinius and change it between class A or class A/B, one amp offers both functions
Minor1, there are some amps out there that have a switch between high bias (Class A) and low bias (Class AB). The OP's Plinius SA-100 mk III is one such of these amps. Clayton Audio also had amps that used to do this. There may also be others that I'm not aware of. So you have the ability to switch between Class A operation and Class AB operation via a switch on the amplifier.

To answer your question: Compared both what?
He compared both forms of operation (A vs AB) that his amp is capable of. Yes, there are other forms of operation such as Class D, also.
marketing. I would bet you a lunch that the Plinius is biased class A up to a particular bias point and even within class A/B and when you flick the switch the bias is reset to a higher bias point.

Got it. Still once you turn a transistor on, it is on. It can be turned on at a really low bias point, but, it is still on. Then you can really turn it on at a much higher bias point where you are cooking at full rated output without an input signal.

This is marketing and semantics. But on is on. Some devices are biased such that they aren't turned on until the AC signal is passed through it. Others are biased such that they are turned on with DC bias with or without an AC signal passing through it. Again, many of the amps I have tested and repaired state they are class A on the specifications and are rated at 50, 100 150,etc. wpc. This does not mean that they are biased at the rated output capacity. They are biased on at 2 watts or 5 watts of 20 watts etc.
also, thanks for the clarification. I didn't know this amp had a switch that altered the internal circuit bias. However, to be clear, all it did was change the bias point from a lower to higher output driver bias. Which is a good circuit design idea as long as you have the power supply capacity and the proper heatsinking for the output drivers. Full output class A is wonderful for those cold days and nights when I want an extra space heater. Tube amps are also good for this.

I don't think Plinius designed the SA10x and SA Reference/250 to be Class AB amps but they wanted the amps to be left on all the time, so providing that AB switch provided owners a way to leave it on 24/7 without busting the electricity bill.
FWIW, when comparing Threshold model amps that had similar chassis, though with differing bias, etc., that were sold as different models with either Class A or Class A/AB (Class AB) output; other than running hotter and with lower total power output, the Class A amps were smoother and more laid back, while the Class A/AB (Class AB) amps ran cooler, had more total power output, were more dynamic and more upfront, though not as suave.
Depending on the speakers, the room, preferred live musical event seating, sonic priorities, and of course budget, I could imagine different people having different preferences.
FWIW, I'd prefer the best of both worlds, cool running, dynamic, neither laid back nor forward, suave sounding, Class W hatever. So far it's typically been high powered Class A amps that have come closet to most of those ideals (except of course for the heat, which I could most easily make an exception for), unfortunately the costs of such amps have just as typically escaped me. High biased Class AB amps have so far become the prudent choice, at least for me. YMMV.
Class D might change all that, we shall see.
it is possible you prefer greater resolution in the ab mode that you feel is lacking in the a mode.

i would imagine that in class a, your amp is an electric bill.

the plinius was recommended to me to use with panel speakers.

i would like to hear what they do with electrostatic speakers, but have not heard them.
Mrtennis, you can expect most transistor amps to be somewhat bright and lacking bass on most full-range ESLs like the Quad ESL 63.

The impedance curve of such speakers and the way they operate simply does not favor most transistor designs. there is more information at this link:
I also almost always prefer AB to pure class A. I do love my my Reimyo PAT 777 but it was purchased specifically to drive the ribbon teeters in my self designed 3 way active speakers. Push pull simply sounds more focused, and real in most cases to my ears. Now I do remember loving a pair of Aleph 1.2' I had in my possession briefly about 17 years ago with a set of Miller modified Infinity RS1-b panels I used to own and that combination was STUNNING (on certain types of music). No saxophone and the Emit drivers were subject to resonances that destroyed the sound :(
That would depend on the amp, preamp and speakers.
I had the Plinius a few years ago and preferred listening in AB mode most of the time. For rock or more dynamic music AB overall sounded better and more accurate. I agree with the original poster it sounded bloated and a bit fuzzy. For softer sounding music like classical or Jazz I could see where Class A would be preferred.