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

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

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

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

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

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