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

"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.

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

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.