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

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.

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.

"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.

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

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.