Why are most High End Amps class A

In transformer-coupled tube amps, the physical mechanisms that cause crossover distortion are very differnt from a direct-coupled bipolar transistor amplifier. In a push-pull amplifier (like both the Marantz and Mac), the concern is hysteris distortion in the output transformer as the current is transferred from one side of the primary to the other. The specific design subtleties in making a quality output transformer are way over my head . . . but suffice it to say that there's a great number of details that can be tweaked to make it work it's best in the particular circuit.

There's also the differnce that the final stage of most solid-state amplifiers is a current amplifier, and the output stage of a tube amp is a power amplifier (both a current and a voltage amp) . . . and tetrodes and pentodes have extra grids . . . so the number of various feedback configurations available increases quite a bit, all of which affect how the amplifier behaves around the crossover point.

True Class B tube amps do seem to be, as a group, less linear than their counterparts running class AB, the McIntosh amps being a bit of an exception to that (they're allmost class B), but the McIntosh "Unithy Coupled" circuit has much more local feedback than most designs, and very high-quality transformers. The Marantz transformers are also very high quality, but designed very differently for the "Ultralinear" circuit. The Model 2 is pretty straightforward Class AB1.

I'm a huge fan of the MC275 in any vintage . . . and the re-issues are excellent, probably better than the original . . . and they're reliable, and available new with a warranty. Also IMO one that's very fairly priced - these were all reasons that I chose it to pair with the existing Marantzes in the system you mention. I just feel that the Model 2 is a big step better than even the MC275, and would have preferred to four Model 2s . . . or even eight of them with a fully active crossover.

But that would be like opening a whole case of 1940s-era Inglenook cabernet . . . which would be awesome, and I tip my hat to those who can afford to do such things. But we didn't have that kind of budget, so I put a 1999 Caymus next to the vintage stuff we already had.

Kijanki, we're definately running into some terminological inconsistencies in this field - but if we're talking about very low signal levels, I'd say that the amp in question must certainly be a Class B amp that for marketing reasons was being labelled as AB. If it occurred at higher power levels (say above a few watts), then I could see how a Class AB amp could exhibit this behavior.

100mA is probably about right for a single pair of bipolar transistors biased Class B across some pretty low emitter resistors, maybe 0.15 ohm? But I think of 0.22, 0.33, and 0.47 ohm as being the common values, so I'd say that most amps are biased proportonately lower for Class B.

It does seem quaint to use an all-NPN output stage these days . . . maybe for an inexpensive paging-system amplifier it might make sense, but for hi-fi, your're right. Crazy.

Kirkus - I don't do much with discrete stuff so I remember wrong definition of class B (as a one without bias current). Now I realized that it has to have bias current in order to break Vbe voltage of output transistors.

Plinius considers all-NPN to be a good way to go for their high-end models, even today. I personally find it a bit clumsy, and that alone might be worse off than slightly different transistor characteristics. Nature seems to value simplicity more highly than complexity.

I agree that many amps marketed as "class AB" are really Class B. The threshold between the two is whatever one wants it to be these days.

I disagree with the "amplifier phase delay" problems with horizontal biamping. There are plenty more phase-inducing circumstances in the system that are worse than the phase of a linear amp, like the speaker drivers' phases. Whether or not a particular horizontal biamp will work well is more dependent on luck than technicalities. However, in vertical biamping, you need the same amps because they are each reproducing the same frequencies and, as most of us know, not all amps sound the same.

Gp phan - Don't worry about what Class the amp is. Just listen and enjoy. If you are really curious about a Class A, just get one and try it for yourself. Besides, there is a lot of sonic performance overlap between Classes. Potential merits can be discussed all day but in the end, they really don't matter. Afterall, it is the sound that should count most.

Arthur

And you're correct in the assertion that increasing bias doesn't improve the problem, it just increases the signal level at which it occurs.

But it does reduce the audibility of the problem. You now have distortion only when the output signal is very high and you have no crossover distortion (GM doubling or whatever you like to call it - lets say transition distortion) when output signal is very low and it runs in Class A (both sides conducting).

A small absolute amount of distortion on a large signal is better than the same absolute distortion on a small signal.

In one case the listener may notice the transition distortion (large part of the overall signal) while in the other case it will be much less audible due to it being a smaller proportion of a much larger signal.