What could I expect to hear from a Class A amp?

Thanks for the knowledge guys! Feel free to keep writing your thoughts. I would love to get my hands on ANY class a stuff just to see what observations I could make...

The 'big players' prefer to sell convenience. IMO, their equipment sounds like it is built for that too :)

So there is plenty of room in the market for smaller companies that base their marketing and products on higher performance features like Class A operation.

I did listen to Clayton's at a show. It blends in among the good amplifiers. It's interesting to note that big players have not caught on to Class A in big ways. May be they figure the R&D it takes to compete against Pass take so many years that it's not worth the risk. The little guys are still at Square One and thus don't have much to loose. Just pondering here.

Spatine, As I've said I'm a big fan of Nelson Pass but in my experience I think ( and this is just my opinion) Gryphons class A amps are superior for some applications. The Antileon signature is $31,000 the Colosseum is $50.000 and the Colosseum monos are ( I think) $75,000. What makes them more universally useful is that they continue to increase class a power down to 1/2 ohm impedance. ie. 150w/8 ohms, 300w/4ohms, 600w/2ohms, 1200w/1ohm, 2400w/.5ohms. They have 2 power cords and I plug each into a dedicated 20amp outlet. Now in practice they would rarely exceed 600w output but they are impervious to load. This won't matter to people with efficient speakers with benign loads but it mattered to me in my case. That said Pass represents better value and if you don't need the extra oomph then I would go with them. If however you really dig class A but have challenging speakers or money is no object then Gryphon is a better way to go IMHO. This is not a put down of Pass. I really like their electronics and I especially like their people and business philosophy. - Jim

"...nobody remotely comes close to Pass Lab on Class A amplifiers. Otherwise there would have been well-informed and fairly objective challenges left and right."

I'll submit Clayton Audio as a strong competitor to Pass Labs performance-wise, but they're obviously a much smaller company that sells far fewer amps so they're not close in that regard.

Coda makes some very good Class A amps in pure Class A and biased Class A. Recently heard the Coda S5.5 a real stunner. Also if you can find the Coda 12.0 it is a heavy contender in Class A amps.

Most of the Coda staff is former Threshold staffers, that formed their own company. So the Pass legacy is clearly evident in their product line.

It would be nice to know of non-Pass Class A amplifiers that perform well, and why so.

ATC make amplifiers with what is probably a sliding bias - you get 2/3 power Class A before it reaches Class AB operating point for the final 1/3 power. I have not compared them to Pass but ATC are relatively unknown as an amplifier manufacturer whilst Pass is legendary. You are safe sticking with Pass, IMHO.

It would be nice to know of non-Pass Class A amplifiers that perform well, and why so. Bearotti asks about Krell. At this point I am curious in just about anything Class A. The whole discussion thus far is very helpful to me as a Pass XA60.5 owner, but thus far continues to confirm what's heard through the months, in that nobody remotely comes close to Pass Lab on Class A amplifiers. Otherwise there would have been well-informed and fairly objective challenges left and right.

Shadorne: Correct as to the unamplified nature of crossover distortion. It is only the ouput, or final, stage at which the bias point is placed near cutoff and saturation, respectively rather than centered - in a class B amp. How hard that stage is driven makes no difference as far as the amplitude of the crossover distortion since there is no amplification following that stage. But since the excursions will be greater away from the crossover point, when driven harder, any change at the crossover point (which will be insignificant anyway in an AB design) will be a smaller percentage of the overall signal. Hence, at low values of output signal any such distortion would be a larger percentage of the output signal. As to your comment regarding active speakers - absolutely true - it always makes sense to design the electronics for the load and the more known about that load allows for an easier design with less compromises.

The usual total harmonic and intermodulation
distortion figures do not reveal the abrupt output stage distortions accurately
because of the averaging factor involved in such measurements. A spike of
crossover distortion may reach 2 per cent, but if it occurs only over 5 per cent
of the waveform, it averages out to a respectable 0.1 per cent distortion
figure.

What he doesn't say which is important is that the crossover distortion is
fixed (it is not amplified as you crank up the volume, at least that is my
understanding) - therefore Class A becomes critically important at low power
output => this is what makes designs with sliding bias or several stages that
include a Class A stage attractive. (it allows distortion to remain low at low
output levels or quiet passages but then still gives you the ooomph needed
for loud passages....the crossover distortion is less critical during a loud
passage as it is a relatively small amount on a good Class AB design
compared to a big signal output)

This also makes an active speaker topology attractive. Since the distortion is
high order then some of the worst stuff is going to come out the tweeter. In a
biamplified active speaker design you simply feed clean Class A for the feeble
power requirements of the tweeter whilst you can power the mid/woofer with
a beefy Class AB design without much worries over high order harmonics or
intermodulation distortion being all that audible

Al - read also this http://stereophile.com/news/10065/

Class A is probably the best SS. The real question is what is the best amp for the money one has assigned to spend. For me it was class D - maybe not at the level of top class A amps but way better than my previous class AB amp.

Thanks, Shadorne. I subsequently found this article by Nelson Pass, which appeared in "Audio" magazine in 1977. Figures 1 and 2 enabled me to visualize what you were saying:

http://www.passdiy.com/pdf/classa_amp.pdf

The article also contains the following interesting statements, relating to the original poster's questions:

The usual total harmonic and intermodulation distortion figures do not reveal the abrupt output stage distortions accurately because of the averaging factor involved in such measurements. A spike of crossover distortion may reach 2 per cent, but if it occurs only over 5 per cent of the waveform, it averages out to a respectable 0.1 per cent distortion figure. Considering this error factor, it is easy to see why two amplifiers with the same specifications can sound so different. To properly evaluate the distortion, peak distortion and harmonic distribution must be considered. Typical class A amplifiers will exhibit low order harmonics, and their peak distortion is less than twice the average distortion. In class AB amplifiers, very high orders of harmonics occur, and the peak distortion can be as much as thirty times the average distortion.

Another problem common to class B and AB output stages is due to the unequal turn-on/turn-off times of the transistors. Because the turn-off time is greater, both transistor sets can conduct uncontrollably under high slew conditions, making it dangerous to operate the amplifier at high frequencies, a particularly bad problem with some quasi-complementary designs. In a class A output stage, however, there are no abrupt nonlinearities and no turn-on/turn-off delays. The smooth transfer characteristic yields low order harmonic distortions, and these harmonics can easily become unmeasurable at low power levels.

Regards,
-- Al

I'm not sure I understand, though, why Class A would run cooler when outputting signal into a load. It's tempting to think that could result from a portion of the current that would otherwise flow through some of the output transistors (at zero signal conditions), being diverted through the speaker instead.

Yes that is the reason. At no load (or I meant to say no input signal) you are actually running at 50% current output and all that current has to be dissipated internally in the amp somewhere, across the bias circuit if you will (it is the high bias that makes the current flow at 50% when at zero input signal). When you play music the +ve signal requires more current (but it all dissipates in the speaker) and on the -ve cycle it actually diverts current from the bias circuit to the speaker (meaning the amp generates less heat).

The amp may use more power from the AC but it will likely run a bit cooler - no enormous difference and I gues it may vary a bit on the exact topology and type of output devices.

Hey, I already said, the XA-100.5 monos did 115 dB without ever choking up on
any dynamic swing, and the X-600.5 never gassed out on heavy bass runs. The
needle never moved, staying class A. It blows your hair back, and to think they
are working a 1 ohm load throughout. The long ribbons never get warm.

I use a class a amp with 88db 4ohm speakers and it is definately worth it imho. My experience is that it takes 45 minutes to warm up with improvements up to 4 hours. I leave the amp in "standby" mode and never turn it off. Standby does not generate heat but keeps enough current that it doesn't experience a cold start. It generates 150W into 8 ohms 300W into 4 ohms and 600W into 2ohms. It will play ear crushingly loud. Compared to ab amps I've had it has more low level detail and never shows a hint of strain even with the minimum impedance of 2.5 ohms on my speakers. With any given setup any given individual may prefer a different set up and few generalizations can be made. My personal belief is that properly executed ss amps with large power supplies capable of doubling down far more than their ab counterparts with much smaller power supplies are ideal for high impedance loads with reasonable sensitivity. - jim

Kijanki, this waiting does happen during the day, on the weekend as well. You are correct about me being picky. Yet it seems that if I want to get the typical benefit of class A, I'll have to wait. Otherwise it sounds closer to my former class AB amplifier which is not bad really.

Shadorne -- Excellent thoughts, as usual, which are not commonly realized. Thanks!

I'm not sure I understand, though, why Class A would run cooler when outputting signal into a load. It's tempting to think that could result from a portion of the current that would otherwise flow through some of the output transistors (at zero signal conditions), being diverted through the speaker instead. But I don't think that would happen to any significant degree in the typical situation where the amp's output impedance is much less than the speaker's impedance, so that the amp behaves essentially like a voltage source.

In any event, it seems clear that the presence of signal would not raise the operating temperatures significantly above what they are at idle.

Thanks again!

-- Al

Shadorne - I suspect that ferrofluid used in many tweeters changes viscosity with temperature.

Ahhh the wonders of ribbons. It's like running a space heater in a cold breezy outdoors.

Class A would be most evident at low power operation (less higher harmonic distortion from zero-crossing effects).

If you use highly efficient speakers that barely use one watt then I'd definitely consider Class A. If you use something a little inefficient say only 91 db SPL for 1 watt at 1 meter then you could go for a design that runs Class A up to a certain point (this kind of topology exists and is well documented - sliding bias if you will). If you are driving something very inefficient like 85 db spl then I'd probably say it is not worth it and stick to Class AB or you favortie Class D etc., as running Class A will be like adding a furnace to your house...

On the warmup issues I think one needs to be careful about making too many assumptions. Firstly a Class A amp will actually begin to cool down when you first start running it (yep, it will run cooler when you have a load). This then opens up the question what kind of load or what kind of RMS power is running through your amp during different tracks and passages of music...all to say that temperature equilibrium doesn't actually exist - it will be changing all the time (so good design is what matters most - agood design will also give the product a longer life as heat generally causes components to fail more quickly). A warm up of 5 to 10 minutes with some heavy music will actually get the major components warm pretty quick (even if the cabinet or heat sinks does not yet show it - at the component level things are already well on the way to approaching design operating temperatures).

Finally, is anyone even aware of the 150+ degree temperatures in the voice coils of your speakers? Did you know that this tempertaure is constantly varying depending on the type of music (quiet and loud passages). Ever play a song that sounds real loud at the beginning - great bass slam and then it seems to lose its punch???....well that would be thermal compressiono in your speakers and it happens in mere seconds from hot voice coils...

All to say that there are big differences in sound in the majority of speakers becuase many are not designed specifically to eliminate thermal compression (by using humongous voice coils for example - this is expensive and not immediately obvious so manufacturers do not often focus on this aspect - given that a better cabinet or better price point will be more appreciated by consumers). Not only will this affect dynamics but it will affect how well the crossover behaves, as the impedance of the individual speaker drivers drift with increasing/decreasing temperature.

If you prepared to worry about your CD/amp warm up - then you should be much more worried about your speakers!!! Ask a simple question - do they have pro drivers with large diameter voice coils? (large diameter increases surface area, which increases heat dissipation and reduces thermal compression effects)

Spatine - why do you call it waiting period? Your gear most likely plays at 99% of quality and you can still enjoy music.

On my system it takes short time probably half an hour and it might be related to tweeters warming up.

If I can toss monkey wrench here: Is your time of 4-5 hours same at different hours of the day? I suspect that minuscule improvement might be caused by drop in radio interference at about 6PM since radio stations have to lower their power (FCC rules). I hope that Bearotti doesn't mind that we sidetracked his thread a little.

Kirjanki, during that 4-5 hour warm up, sound quality fluctuation is serious. It can sound pretty good almost at start up, then sound hemped in again a few minutes later. Of course it still sounds better than the previous amp. despite the fact; however, that's not the point here. As to placebo effect, my hearing capability definitely changes from moment to moment. But this 4-5 hour waiting period has been going on for some time now. This definitely isn't so before I got the XA60.5, and behaves as such immediately right after I start using the XA60.5.

"It seems I have to continue putting up with listen to music late at night after giving the system most of the evening to warm up."

Spatine - Can you hear it improving over time. Would you know if amplifier was ON for one or three hours? If you cannot tell then don't listen to somebody's recommendations.

You made excellent point about power outage and surge. I use surge protector but still turn off the switch while not at home and unplug my gear during thunderstorm or long absence. Surge protector works for small spikes but is useless for direct lightning hit. My insurance agent told me that stereo equipment is always insured independently of value (One doesn't need special insurance) but it's a little hard to believe.

My amp is class D and uses about 10W idling but I still turn it OFF - I don't see (should I say "hear") reason not to. In case of class A amps electric bills should be one more argument - unless one can really hear big difference over time and is willing to pay for it.

How about blind test - you come home from work and find your gear ON. You have to guess how many hours it was ON. This would perhaps tell us what is real and what is placebo effect.

Class A is the opposite of what A-B and Class D type equipment needs.. So Class A and Tubes need "LITTLE" time to warm up and sound there best... My Class A gear gets to good hot and excellent sounding idle in less than a Half hour.. Same with most tubes.

Class A-B and Class D designs do benefit from being turned on for hours or even a Day before critical listening, but that process can even be sped up by playing a music signal thru it and leaving the room.

In general I have tested several times and found that Class A gear and most tubes 40 minuets after start are pretty much 99% the same sounding as if you come back leaving them on for a day or three... Thats why many think Tubes and Class A sound better than some standard solid state gear which is the hidden potential, its simply Class A and tubes do in fact warm up and sound musically enhanced faster. This is from my experience, and some A-B stuff does sound nearly as good after a few more hours of music signal and warm up.

Class A turn it on a half hour prior, and turn it off when your done. A-B will take some experimentation to see if you find differences leaving them on all week.

It seems I have to continue putting up with listen to music late at night after giving the system most of the evening to warm up. I didn't have the time yet to read carefully the thread on whether to power on/off (thanks, Almarg) just yet. But from what gathered, it seems like a hot debate. Also water evaporation on the electrolytic capacitor is a time-dependent thing. The longer it stays hot, the more time for water to evaporate. Yet regardless of where the truth lies, leaving expensive equipment on and unattended at the risk of power outage and surge is asking for trouble. Does anybody have positive experience using a constant power source to drive your equipment. I once tried a hefty computer battery. It sure kept the equipment on regardless, but the sound is awful.

Kijanki -- Really excellent post!

As you undoubtedly recall, we discussed (and somewhat disagreed about) these matters a while back in the following thread, which discussed whether or not it is best to leave a system on all the time (although it was not addressing Class A specifically):

http://forum.audiogon.com/cgi-bin/fr.pl?htech&1228170308

For those who may be interested, in the last of my posts dated 12-3-08 in that thread I addressed the statistical aspects of this question.

Re your post above, I agree 100%, with the slight qualification that the phrase "last forever" be taken to mean "very unlikely to fail within a reasonable period of ownership."

Near the end of the other thread, in which a lot of people threw in a lot of conflicting opinions, one person asked if a consensus could be stated. I was the only one who responded, and I said:

I'm hesitant to declare that we have a general consensus. But in terms of reliability I would say the diversity of opinion we have seen is consistent with the opinion I expressed that, considering your frequent but not extreme usage patterns, you won't be going too far wrong either way.

In terms of sound quality, optimal warmup time is going to be system dependent, and probably listener dependent as well, so if you choose to have your system off during the day see if you notice continued improvement in sound quality following whatever warmup period you choose to use initially.

Perhaps this is a bit of an anticlimactic conclusion in view of all that has been said, but I think that any conclusions beyond these simple ones are unprovable, are probably not universal, and in terms of longevity probably don't make a lot of difference anyway.

Musicnoise --

Nor does an amplifier that is on but not driving a load use a great deal of energy so it shouldn't change your utility bill anymore than leaving a porch lite on 24-7.

That is not even close to being true in the case of a Class A amp.

It takes about 1 second of on time for a solid state amp to sound its best.

I don't think too many Audiogoner's would agree with you, based on their own listening experience. Although I am an electrical engineer, and I tend to be among the first here to recognize that perceptions about equipment performance that derive from listening experience can often be distorted by expectations, failure to control extraneous variables, etc., I would agree with them that you are incorrect about this. Notwithstanding the fact that to my knowledge a solid technical explanation of warmup effects has not been offered in the context of audio.

Regards,
-- Al

>>03-16-09: Musicnoise
It takes about 1 second of on time for a solid state amp to sound its best<<

If you read Nelson Pass' comments on his First Watt products, he feels it takes about an hour for them to reach optimum performance.

Not one second.

It takes about 1 second of on time for a solid state amp to sound its best. The last piece of electronic equipment I saw that took any longer on time to sound its best was a tube television set from the 60's. (cannot speak for tube amps of today). Even that TV took only a few minutes to settle. Turning the amp on and off between listening sessions will not change the amp's lifespan in any meaningful way. Nor does an amplifier that is on but not driving a load use a great deal of energy so it shouldn't change your utility bill anymore than leaving a porch lite on 24-7. In short, these issues are of no more practical significance than the issue of how many angels can dance on the head of a pin.

"On separate issue, would all that heat for 24/7 at full power take toll on the circuit board? Of course the counter arguments would be the on/off as you said, and the expansion/contraction fatigue cycle due to the hot/cold."

Spatine - the only perishable component inside of SS amp is electrolytic capacitor (other than ON/OFF switch). Rush current does nothing to electrolytic capacitor other than heating it slightly (very short time and very low ESR) in spite of large momentary current. Electrolytic caps die of dehydration. Each increase in temperature of 10 deg Celsius cuts their life in half (water evaporates from electrolyte). Eventually drier capacitor has higher ESR and starts heating up to the point of runaway. In critical application (lots of current) they get to thermal runaway and explode (they have fuse to relieve pressure). The other way to damage them is to leave them without voltage for years. Aluminum oxide layer that serves as isolator is eaten by electrolyte but presence of voltage builds this layer back.

Fairy tales about rush current (turning ON) damaging equipment comes from two sources. First is the bulb or tube that breaks on power ON since cold filament has many times lower resistance. Second is the fact that if you do anything around your amp like for instance moving speakers while amp is OFF and create short it might damage amp when you turn it ON and you will blame turning ON (rush current).
Sometimes also mechanical component tends to break at start-up like for instance computer's hard disk.

Circuit boards, resistors, film and ceramic caps, transformers etc last forever even in extended temperatures. As for semiconductors - as long as they operate at sensible junction temperature (let say under 125 deg C) they also last forever. There are electronic circuits with semiconductors being turn ON/OFF hundreds times a second and they last for over 25 years. Simple example of it is seven segment multiplexed display.

As for sound being optimal after a week of continuous operation - it's possible. I don't have class A amp, don't have experience with them and perhaps less than "golden ears".

People who calculate reliability are probably ready to kill me for saying all this but statistical analysis they use is something else. When I walk with my dog we have statistically 3 legs each. Also statistically tattoos are causing motorcycle accidents.

My class A runs damn hot.
It really needs a week of warm up before sounding it's best?
On the advice I got calling the Sugden factory, I turn mine off between listening sessions.

I should have made it clearer. The 4-5 hours is going from standby to full power on. The standby mode is just not enough. So to rephrase the question, did you find the standby mode insufficient to let 30-60 minutes at full power to finish the job?

On separate issue, would all that heat for 24/7 at full power take toll on the circuit board? Of course the counter arguments would be the on/off as you said, and the expansion/contraction fatigue cycle due to the hot/cold. Do you know what's the typical age of a class A amplifier before the heat kills it. At least the XA60.5s are hot but not that hot.

I'm confused about the comment on Henry Ho. The DAC is either positioned between the transport and the preamp, or is part of the preamp itself. Yet we are talking about the amplifier here. It comes across that you actual benefit from improvement to your source instead.

The dealer I am referring to told me, "I am not an audiophile." He has a huge collection of discs. He sells monitors of his own design to recording studios where he knows recording stars by first name. Of course his amps and preamp are on all the time.

They sound wonderful, and to virtually all ears the music is as good as it gets. His Full Range speakers are spooky good, much better than stock. I have watched his gear and speakers improve steadily over the year.

I have a couple tell tale discs I took with me on one of my visits. One is a cathedral pipe organ recording. I told him you can hear the organist pull and push stops between pipe changes. It is something I take for granted.

Remember, he is using Nelson Pass's best 2 part preamp, XA 100 monos, and 600.5 monos. Even on listening for those wooden pegs, I could not hear them. On another disc, a brass quintet. The tuba and trumpet were recognizable, but the other three horns just became a background blend.

Where the XA amps excelled was on simple fair, such as voice and guitar.

You know these arguments can go on and on. SET/single driver adherents think we are both idiots.

Class A amps should run 24/7 for maximum thermal efficiency. By turning them on and off shortens the operational life considerably due to convection. In a way its like a light bulb turn on and leave it on it will damn near last forever as opposed to the constant on/off cycle.

One of the reasons for listener fatique is due in part to the on/off cycle. Run it for a solid week of 24 hour operation and you will hear the difference and the difference you paid for a Class A amp.

In fact I run my all my amps regardless of Class of operation 24/7. They just sound better by being continually on. Many of the newer amps have a standby feature which allows to the unit to stay on, but at a less power consumption.

Obviously say a Class A amp running at 60 WRMS per channel, if its pure Class A it is drawing 300 Watts at the wall. Something to consider in regards to your monthly power bill.

There are trade offs in using Pure Class A amps, but over the years I have found, for me it is well worth the penalties that come with Class A operation.

I talked with the manager at Sugden.
His opinion is the amp needs 10 minutes warm up time.

Ferrari, that would be calling me a fibber. Like I wrote, I used other conventional amps, including Pass Labs fair. Nothing ever came close, including the latest class A ware that approaches the fierce speed of resolution to the faintest nuances offered by my amps and preamp by the same maker.

I hope you don't mind me offering just few pointers when auditioning class D amps. First of all, don't be sure your system is conducive to class D amps. Secondly, there is a quality hierarchy applicable to class D amps as you found out.

On the first point, I have had the same monos and preamp for years of listening. During that time, the resultant sound moved through a series of personalities. At one time, it was airy and whimsical. Another time it sounded warm but focused.

After eschewing commercial speaker cables, and employed the short ribbons I use now, the sound took a revolutionized step. I upgraded my non oversampling DAC which I sent to Henry Ho of H2O to work his genius on. The stage stretched fore and aft, bass grew in heft and musicality, highs perfection, mids as natural as it gets. The last amazing surge in fidelity occurred when

I could go on and on. If I thought any of the class A amps could do me more, I would be happy to buy them. The knowledge Henry makes class D amps and class A preamps to be as squeaky clean as can be, I knew there was a way to harness the best possibilities.

Could somebody please comment about the 24/7 operation needed for Class A. Typically I heard that you want to warm it up for 30-60 minutes. Yet it seems to take about 4-5 hours before my ears could not distinguish a difference.

I definitely agree about the non-fatigue factor since your brain does not have to fill in. Before getting the XA60.5, it's like a love-hate thing I have with the stereo. I must listen to it because I love music. But about half an hour later, I got tired, somewhat disappointed, and turn on the TV instead.

Detail without fatigue.
Smoothness without the mush.

Class D amp have miles to go, to even be remotley considered in the realm of pure Class A amps. Even with the attendent penalities of Class A operation of high cost, high heat, heavy chassis, the need to be powered 24/7, additional usage of power to your monthly bill. Even with that all against Class A amplifiers, once you hear and become accustom to the signature of Class A, there is no turning back.

I myself at the moment using a Class D amp in the PS Audio HCA 2 which I would never compare to a properly made Class A amp. While close, it is no cigar. No doubt that Class D will evolve in years to come, but not just yet. I have listen to the Rowland, Channel Island, Nu Force as well as the PS Audio I have here. The only Class D amp that comes close enough to warrant a close examination is the Rowland.

The rest are nothing more or less than contenders at this point in time.

Muralman -- Thanks for the good clarifications. I must say that is probably the most compelling testimonial, among a great many that I've seen, to class D excelling in high current applications.

Enjoy!

Regards,
-- Al

Hi Almarg, I anticipated the questions you forwarded. The story of my visits to this dealer, to be honest, encompasses several visits. On one visit, I heard the 600.5 amps driving Apogee Scintillas, actually an even more ravenous ampere pig.

I use to own the Pass X 600 monos which I used driving my Scintillas. The ribbon array of the Full Range is very similar to the Scintillas. I was comparing the sound from the Scintilla powered by the 600.5 and the sound of the Full Range powered by the XA 100.

These Full Range speakers are not stock. They have been structurally reinforced, and the transformer loaded crossover has been chucked for a much simpler one. Ridding the speakers of it's transformer makes the speaker an even more torturous load for any amp. It was simply awe inspiring how a 100 watt amp can push the FRs to shocking loud levels, without any whimper.

Saying all that, I have to confess, the Pass sound from either amp does not approach the performance I enjoy with my class D system. Whereas the Pass 600 bloats the Scintilla's bass, my amps fully rule the Scintilla.

Next time I am there, we will replace the XA with my amps, the Pass XP -20 preanp for my Fire, the SCs for my ribbons, and even swap the sources. This should be fun, and instructive.

I am familiar with Nelson Pass's very best current products. I have the pleasure of knowing a dealer who employs Pass components powering the towering Apogee Full Range speakers.

There are 600.5 amps handling the huge bass panels. The XA.5 100 finesses the mid and tweeter ribbons.

Through all types of music, and loudness, I never saw the power needles ever move on any of the amps. That would mean they are all running class A.

So, do both the 600.5 and the XA amps sound the same? The answer is no. I don't know why they sound different, but they do. The class A amps are clear, and easy listening amps. The 600.5 is forceful, and less open.

The question here is what is the difference between a wholly class A amp, and a powerful Class AB biased towards running in class A for around a fourth it's power rating?

Muralman1 -- Some questions and comments, which are sincere and not meant to cast doubt on your statements.

The Apogee Full Range speakers, as I recall, had one of the most difficult-to-drive impedance curves of any speaker. If I'm not mistaken they went below one ohm at some frequencies.

Therefore isn't it conceivable that the differences you perceived between the two amps were simply the result of their driving differing (and difficult) load impedances?

Second, how were you able to distinguish the sonic character of the two amps considering that they and the speaker elements they were connected to were reproducing different parts of the spectrum?

Third, although I have no significant experience listening to Class A amps, intuitively I'll say in relation to your closing question that I wouldn't be surprised if the perceived differences between a given pair of Class A and Class AB amps differed significantly as a function of the degree of difficulty of the loads.

Regards,
-- Al

"Bearotti, class AB amps usually operate in class A up to some power level"

Yes, but won't give class A performance. Much higher signal gain and deeper feedback were already applied in order to remove THD and IMD and you already paid the price. At this point it doesn't really matter if you listen loud or soft.

Class AB is a compromise for another reason. In order to linearize transistor that is completely OFF (class B) it would take much higher signal gain and deeper feedback than class AB, creating even more TIM problems and odd harmonics.

Over the course of 50 years in this hobby have heard or owned or near owned damn near all the high end amplifiers produced. Such as Levinson, Threshold, Krell, Classe, Coda and many others to numerous to menetion here. But when it comes to Class A ampd I have always gravitated back to the Nelson Pass Threshold and Pass Labs units. At present I do not have a Pass unit in the system. Moving shortly to the X350.5 in a few months. Now running a PS Audio HCA 2 Class D amp. Very happy with it, but no where near the level of the X350.5 While it is not a pure class A amp such as the Aleph series, it delivers the signature I want, with less heat and drain on the power bill.

As far as Levinson and Krell go I have never kept any of them longer than 3 months. Impressive at first, but in the long run just have not pleased my ear.

As far as tubes go, when I got into this hobby that was all there was and mono to boot. So from 1957 to 1977 was in the tube camp until the Threshold gear came out. Dumped the tube gear in 1977 and haven't looked back. Twenty years of fiddling with tube gear was entirely enough for me. And that was during a time when you could buy new good tubes made in U.S. or great tubes from Europe. I have heard the Russian and Chinese tubes, to me pure junk as opposed to what we had in the tube era.

One of the great Class A/AB amplifiers I have heard recently was the Musical Design D150B signature unit. Now thats one Class A/AB amps I could live with nicely and totally recommend.

Ferrari, I must agree with you about the exemplified work done by Nelson Pass on Class A design. I own the XA60.5. Based on his paper, it appears that primary difference between the X.5 and XA.5 is less on circuitry, and more about higher current bias setting for the XA.5. Secondly, people have contrasted the 2 series just like Muralman1 did. Thirdly, nobody receives such level of praise when it comes to bridging the gap between SS and tube amplifiers the way Pass amplifiers do. Yet with good end results we suspect Class A design is responsible for giving SS that wonderful tube sound without the overkill. Thus for these observations I tend to think Class A design is the way to go on amplifiers, as it provides the superior foundation. However there are enough unknowns that it's up to the astute designer to finish the job. There is only so much mathematics, theories, and the oscilloscope can explain.

Well it sure sounds like I am advertising Pass amplifiers, am I? It would be nice to hear comments from people who own non-Pass Class A amplifiers, especially when the author of this thread was wondering about Krell. It would be nice to balance personal bias when hearing contrasting opinion.

I am familiar with Nelson Pass's very best current products. I have the pleasure of knowing a dealer who employs Pass components powering the towering Apogee Full Range speakers.

There are 600.5 amps handling the huge bass panels. The XA.5 100 finesses the mid and tweeter ribbons.

Through all types of music, and loudness, I never saw the power needles ever move on any of the amps. That would mean they are all running class A.

So, do both the 600.5 and the XA amps sound the same? The answer is no. I don't know why they sound different, but they do. The class A amps are clear, and easy listening amps. The 600.5 is forceful, and less open.

The question here is what is the difference between a wholly class A amp, and a powerful Class AB biased towards running in class A for around a fourth it's power rating?

Bearotti: You may want to further your knowledge by looking at texts and articles from other than manufacturers of audio equipment. Those sources are generally biased to promote their product and do not provide a complete picture. Keep in mind that there is nothing particularly difficult in processing audio signals. Techniques that work in one area of signal processing work in other areas. For eg. in the mid 1990's I spent considerable time on a design project which applied signal processing ideas initially developed in radar target identification to extracting late potentials in ECG signals as an early predictor of cardiac problems.

Also keep in mind that in some 'perfect' sense there are tradeoffs in such things as class a vs class ab amps. However, those tradeoffs are meaningless because - as I indicate above - properly designed AB amps will not result in signal degradation that is in any way noticeable (unless of course one uses the magic 'audiophile ear').

A good beginning point for gaining more knowledge are the books by Douglas Self. These have very little mathematics in the explanation and are readily understood by anyone with a high school math background. The same goes for amateur radio publications. (Engineering texts and publications require a minimum of a formal course, or the equivalent, in differential equations and some background in Fourier analysis - which takes a considerable time investment - generally not a good investment unless one expects to make a living as an engineer)

Tubes have their problems too! It's very hard to find good "tubes" that will last like they should "these days". New TUBES can go BAD "at any time"...at any price! That's a BIG Problem.....!

For further and deeper understanding of Class A design and operation visit http://passlabs.com/articles.htm There you can find several articles dealing with Class A and other amplification devices. Well worth the time to visit and read the articles from the man that for all practical purposes responsible for and defined high end in the mid 70s to today.

Nelson Pass when he was at Threshold clearly defined and manufactured some of the worlds best Class A amps and preamps. In 1991 he left Threshold and started Pass Labs and the in the years since these products have further pushed the envelope of Class A design. In my opinion no one does it better than Nelson Pass.

I do appreciate the technical explanation. On one hand it's about education. On the other hand it builds credibility to one's belief. From what's expressed thus far, can I take it that Class AB has more issues to overcome? The combination of designer skill and his stand on debatable subjects makes it difficult to come up with a comparable sound delivery system. Whereas Class A design is more simple, but with the problem of heat and power consumption.

My second question if I may would be what enables Class A solid state design to incorporate the strengths of tube amplifier concept with less of the problems. Is it because of shared design? Somebody mentioned about 2 tubes and a resistor.

Fabulous descriptions. I have visited this site regularly for a few years now. Had some basic input of my own.

But I'd like to add this.

The people on this site continually help build my audio knowledge with their unreserved pouring out of knowledge. It makes my keen interest in music and my secondary hobby in audio more interesting by the day.

An ongoing thank you to all the folks keeping Audiogon so alive and vibrant. There really is nothing else like Audiogon. Bravisimo!