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

Threshold 400A class A

I believe it's a push pull 100w Class-A/B amp. And with those heat sinks, about a max of 15w of Class-A,  with rail volts for 100w Class-B.

Cheers George 

Depending on your equiptment and you ears you will appreciate Class A more or less.  For me there is a relaxing and smoothness character to Class A.
I moved from a 540 watt Class D to 60 watt Class A to drive my Maggie 3.7i's.  I liked the Class D over the Class A/B Levinson. The 60 watt Class A has the current to drive the Maggies as loud as my wife will allow but not to the volume of the Class D.  I dont miss the Class D.  The Class A has much more umph and detail to the low end.  The Class A sounds better at all volumes.
I was worried about the heat but in my 20 x 25 room in Southern Ca. it has not been an issue.  I leave them on 24/7. The room is a little warm on warm days but it gets that way in the summer anyway.  I also have 2 tube preamps with a combined 10 tubes that are both on when I listen.  In a small room it may be an issue.
If you can afford Class A you wont likly go back to A/B or D.

@sfischer1 

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

I have one, but it is only my office.  Actually I have an office cat and a Hi-Fi in my office.  Also a Barista coffee machine close at hand.
The Hi-Fi is single source - a Technics SL1210 turntable.  Other components are Ayon Pre, ATC power and Spendor D1 standmounts.  To be honest there is not great synergy but the sound is passable.  It sure looks cool though, and clients love the LPs dotting the office.  In order to up to cool factor I need to have some tubes showing.  The Ayon Pre is a tube based pre-amp but the tubes are all internal.  I want glowing bottles!

MPG on home audio?   Best music?

I listen to nothing but Class A amps now. I have three single ended troide amps using the 45 tube for 2 watts per channel powering a home made set of speakers using a 15" Altec woofer, a two cell Altec compression horn and a RAAL ribbon tweeter. The least efficient driver is the RAAL at 93db/watt/meter. The horn is 114 and the woofer is 102. On the other end of my room, sharing the source, is a pair of Sound Lab A-2 electrostatics. When I built the Altecs, I was using a Musical Fidelity Class AB amp, 300 watts into 8 ohms, 600 watts into 4, on the Sound Labs. I was very happy with that setup but the Altec speakers turned out so well I could no longer listen to the Sound Labs. I've owned the Sound Labs since 1985. I like them. I didn't want to abandon them.

In the early '80's I had the chance to hear the then new Mark Levinson system with the ML-2 amps. That session so impressed me that I remember it to this day. For the longest time I thought I could not use the ML-2s with my speakers since they were only 25 watts pure push-pull Class A. As it turns out, the ML-2 was designed on and for electrostatic speakers. I bought a pair and life is wonderful again. They power my speakers easily. They sound every bit as magical as I remembered from 38 years ago.

I have two other Class A amps; a British (now Musical) Fidelity A-1 and an Amp Camp Amp designed by Nelson Pass and built as a kit. Here is the main limitation, they are low power. They are inefficient. My ML-2s draw 400 watts of power to produce their 25 watts. They do not get particularly hot since they are half heat sink. The tube amps draw about 75 watts to output 2. They are no more of a space heater than any other tube amp. The ACA, I have never measured the current draw, make 6 single ended watts of transistor power, it gets warm but not too hot to touch. The real hot one is the A-1, people claim it is Class AB but it draws 80 watts from the wall at both idle and it's full power of 20 watts. Tim de Paravicini, the designer, says it is Class A, push pull. It has the sorriest heat sink, the top of the amp is ribbed and is the heat sink. It does get too hot to touch for very long.

In case you are worried about the power draw, I calculate my big amps cost one penny per hour to operate--for both of them. And in case you didn't catch my opinion, Class A is the only way to go. I can't guarantee all Class A amps are great, but it is a good place to start.

jspohn - I own an EAR 890 70w Class A zero feedback amp.  It does not have sufficient power for my Legacy Focus speakers which has a tough impedance and 6-12" woofers.  My highly modified Dynaco ST 70 with 30w Class A/B kills it on the focuses (tremendous bass control and dynamics).   When playing both amps on my Legacy Signature IIIs with an easier impedance and 6-10" woofers, the two amps sound very, very similar.  From this experience, I found that a Class A, very hot amp does not deliver into low impedance and/or big driver cones despite a smaller amps Class A/B design (not ultralinear but voltage regulated transformer tap design).  

I have several Yamaha CR620 35 watt Class A/B 1978 receivers that sound excellent and can drive the Focuses just fine (Tom Port of Better Records recommends this pairing).  I use them on smaller smaller speakers for my two video set-ups.

That's where you are 100% incorrect.  That Dynaco is NOT a typical unit. It has been modified to have superior control of deep bass.  My friend Robert Pincus and Audiogon poster Oregonpapa know how powerful my little Dynaco amp is.  It is not as refined as my 130 watt monoblock Class A/B tube amps are, but the voltage regulation, non-ultralinear type is based on the same design concept.  You have never heard a Dynaco ST70 like mine.  The EAR Class A amp has a very poor damping factor and cannot tolerate my Legacy Focus speakers.  Both my Dynaco and mono-blocks have feedback, variable feedback for the mono-blocks although the designer said it was about 6 to 8 db global type at its minimum which we like best.

Very interesting thread. All I can say is I recently bought a pair of Class A 45w SET monoblocks that greatly improved my system. Built by Vlad of Audio Mirror. I never had monoblock amps or an SET amp so I took a chance. I did not get them because they were Class A design, but because they were SET and I have always been very curious about SET amps. They have a very uncolored, neutral but natural sound. Cymbals sound like..well, real cymbals. They are open, clear space between instruments and voices, yet very organic, dynamic and sound very effortless. When I listen to music I hardly adjust the volume. You hear detail, background at whatever volume is present. They run very hot but my room is very cool in summer so it’s not an issue. I could care less about the power bill...you only live once and can’t take it with you! They seem a perfect match for my Egglesston  Works Fontaine Signatures. Thx again for this thread. Always something to learn here on A’gon!

jdl57 those amps are the nuts!  An other classic John Curl design. Back in the late 70s we had 8 of them driving the ML HQD system, one on each Quad, Harltey sub and Decca ribbon tweeter. 200 very expensive watts and boy was it impressive back them both sonically and visually. The Hartleys were in infinite baffle enclosures! 

Excuse me, but what is an F5 amp?  I can't find it in this thread.          I found excessive global negative feedback is also not my preference.  I like the VAC Class A/B amps with 6 db of same.  

Wasn't the XA25 the amp with superlative technical reviews?  Thanks for the info.

@koestner I would say that this is a better metaphor:
class AB is like having a car with 2 engines and 2 gas pedals. Every few seconds you have to switch pedal and try to hit the same rpm again (we can pretend that the engines would overheat otherwise).

You hope that the engines are not different in strength or something but it is still hard to do the continuous switching without having a little bumpy ride.

Not to be argumentative, but don’t you think that the interface between class AB amps running in class A going into class B is not rocket science and has been perfected years ago? 

Grandstanding a bit, I would guess the majority of great SS amps are class AB, based on sales.  Just an assumption on my part.

Well, I have had some quite powerful small Class A/B amps which deliver 2-3X their rated power. Not only that, but they are about 3X to 5X as powerful on a wattage rated basis as many solid state amps.
The earliest amps to sound super dynamic is the McIntosh MC30s. Next are my highly modified Dynaco’s which can deliver huge and clean bass dynamics into low impedance loads. Lastly, my custom built Altec transformer based 130 watt amps which could drive anything a 500 watt solid state amp can. Maybe my Class A/B amps aren’t as refined in the mids as the best Class A tube amps I’ve heard, but I can put my hand on my big amps transformers hours after being played hard while I could burn my hand in 15 minutes anywhere on the chassis of an EAR 890 70 watt Class A amp which can’t handle low impedances.

Kosst, I mentioned that "the interface between class AB amps running in class A going into class B is not rocket science and was perfected years ago" and  also that while I was "Grandstanding a bit, I would guess the majority of great SS amps are class AB, based on sales. Just an assumption on my part."

You responded that class AB amps are akin to Kia's.  Youe clear meaning being they are cheaply build and inexpensive.  You chose to not acknowledge that I specifically mentioned that sonic problems due to the amps going from class A to class B are non existent and that I referred to great SS amps, not Kias level amps.

I'm just saying

kosst, I don't pretend to understand what would make you say that class AB amps are made with that topology only to be less expensive.  The D'Agostino Momentum monoblock power amplifiers and many other fine AB amps are very expensive in my view of expensive vs. inexpensive.  Lets let this go and I will say ok to whatever.

If what you are saying is that the least expensive class A amp cost more than the least expensive class AB amp I would tend to agree.   

My 70 watt Class A EAR 890 has 8 itty bitty storage capacitors while my 130 watt Class A/B monoblocks have huge 3" X 7" pair of storage capacitors each. The McIntosh MC30 sounds like it is 3X or 4X more powerful than it’s rated. My voltage regulated Class A/B amps can drive low impedance speakers with ease, stay cool and sound great. Maybe they are inferior in some sonic ways, but they are superior in most others. I will stick with my amps and you can have your F5 which I am not sure is a single ended or push-pull design. The great sounding, huge and hot Class A tube amps I’ve heard are usually single ended.

@viridian The Krell you refer to (KSA 250) is an older model. The KSA 300S was the next series that were designed precisely to run completely Class A unless the amp gets very hot. Here's the article that explains the difference:

https://www.stereophile.com/audaciousaudio/krell_ksa-300s_power_amplifier/index.html

BTW, with my low sensitivity speakers (Mirage M3si) I can drive the amp into the highest bias level - and it gets really, really hot. What I don't know is whether or not the top two bias lights still turn on when the thermal protection kicks in. It may be that for some of my extremely loud listening the amp goes into Class A/B. For the great majority of my listening the amp never goes past the third bias level and according to the article, and everything else I've read, the amp operates in pure Class A. I think it's worth noting that this amp weighs 185 lbs. It was obviously built to an extremely robust standard.

To clarify further, operating in Class A was the whole point of this amp's design and explains the design differences between it and the KSA 250.

“The Krell KSA amps are pure Class A.”

The only Krells that were full Class-A biased into 8ohms were the KSA50 and the KSA100 and KSA100 MkII monoblock, all these were internal chimney heat sinking and fan forced.
https://youtu.be/kqSkh08AmKM?t=1

Cheers George 

George, Krell never made a mono amp with the designation KSA.
they were KMA 100s and there were also KMA 50s and 200s. They all used fan powered chimney heat sinks and they were all class A to rated power. I owned a pair of KMA 100s for 20 years and they were great amps.

In simple word Fantastic listening pleasure just like going to an Opera house or Jazz bakery club in LA.  During winter its an alternative heater 

The Jazz Bakery isn't an ideal sounding venue.  I've frequented it half a dozen times.  Bob Wilber played there a decade ago and his sax and clarinet sounded peaky and bright, actually aggressive sounding, so unlike his playing style.  Locally, many churches and concert halls have great acoustics.  The absolute worst with a 5 second reverb time is the Queen of Angels Cathedral.  I've sung there four times and I couldn't hear myself in the choir with all that reverb.  My wife said it sounded like mush.

Class A amps are the least efficient kind. They are bigger heavier and draw more power (per watt output) than the rest so total cost of ownership is high. To make up for that you tend to want speakers that are more efficient. Quality full range hifi speakers that are also more efficient also tend to be larger, heavier and often more expensive as well.

Class A amps from reputable vendors are usually very highly regarded in terms of inherent sound quality.   

@mapman  I think you have given  a good explanation of what i was trying to find out. I have the efficient speakers which somewhat piqued my interest in class A

Class A is a tool in the designer's box which allows for lower distortion.
With all amplifiers, distortion is essentially what causes amps to sound different from each other, moreso than actual frequency response.

The reason for this is that the ear converts distortion into tonality. For example the 2nd harmonic often associated with tubes adds to richness; the 7th harmonic creates a metallic quality. We've know this fact about harmonic distortion since the 1930s.


Now it is the higher ordered harmonics that contribute to harshness and brightness. This in a nutshell is why solid state amps tend to sound harsher and brighter than tubes (keeping in mind that the tube amp may well have bandwidth equal to that of the solid state amp).

Class A in a push-pull amplifier allows for even ordered harmonic cancellation throughout the entire waveform right up to full power. This means it will be lower distortion- and in part due to the fact that the output devices never turn off (so no crossover artifact either). Now its often said that its that first watt that counts the most in most audio situations and this is true. When you operate class A, your distortion at low power levels is lower, and this translates to greater low level detail; distortion is not obscuring that information.

Now using loop negative feedback is good for reducing distortion also, but IMO/IME its a poor means of reducing feedback. This is because feedback, while suppressing distortion, actually introduces some of its own and its almost entirely higher ordered harmonics (plus IMD introduced at the feedback node). So any amplifier employing feedback will be brighter and harsher than real life. This is one reason why class A is important, because it is a tool that can allow the designer to make a low distortion amplifier that does not employ feedback.

@atmasphere  Thanks Ralph. Fwiw you have a way of making the complex easy to understand. Are/Were you also a teacher?

George, Krell never made a mono amp with the designation KSA.

Yes KSA50, KSA100 and KMA100 were all Class-A using chimney heatsinks
I got a letter wrong for the mono's, still talking about the same amps with chimney fan cooled heatsinks as I said and provided a video link to.

Thanks Ralph. Fwiw you have a way of making the complex easy to understand. Are/Were you also a teacher?

I have taught a few classes but mainly I think I got that from working in consumer electronics repair while putting myself through engineering school. I found I was explaining how things worked a lot to customers and quickly learned that converting things to layman's terms was an important skill.

I do believe class A 's design & sonic advantage over Class B. BUT don’t understand  say: 30 W class A vs a 250W class A/B. Isn’t the class A/B has the advantage of both ? Example:Pass Int 250 DON'T leave class A till 15 W and has all the extra class A/B reserve up to 500 W into 4 ohms! Why would a 30 W class A be a better choice over the 250 W A/B??? If 99% of the time I am only playing music at about 2 Watt( under 90db)?

Example:Pass Int 250 DON'T leave class A till 15 W and has all the extra class A/B reserve up to 500 W into 4 ohms! Why would a 30 W class A be a better choice over the 250 W A/B??? If 99% of the time I am only playing music at about 2 Watt( under 90db)?

With many push-pull amplifiers the low level power output below a certain point has high distortion than it does at higher power levels. So its on a case by case basis, but generally speaking if you only use 2 watts its likely that a 30 watt amp will be making less distortion (and noise) than the 250 watt amp will. That distortion is likely audible as harshness and brightness so that right there could well be the advantage.

I'm planning a bedroom system with some small speakers that are remarkably free of breakups. I don't need a lot of power and certainly don't want the size of a 250 watt amp in that environment! Small has its advantages...