Why are most High End Amps class A

Kirkus,

We touched on Bi-amping a bit and using class "A" on the high frequency, and class A/B on the bass. I noticed on a system you helped create you used a Marantz Model 2 with EL34 tubes for high frequency and then a McIntosh MC275 with KT88 for the low frequency.

Is the Marantz running in class A, or do you like the mix of a nice EL34 amp for the high frequency and then the KT88 for the low frequency.

How can you bi-amp economically to try and get nice high notes, leads, vocals, and then get good bass as well. If you bi-amp should you stick to all tubes, or all SS or can you mix it?

Kirkus - I read article that shows harmonics of output stage (class AB) with underbias, proper bias and overbias. It's a little eye opening since many people believe that increasing bias will make class AB sound like class A. It is not so simple - otherwise everybody would have done that.

What is typical bias these days? I remember vaguely something around 100mA.

I also read on the subject of complementary pair distortions that you mentioned once. It pretty much confirms what you said but I just couldn't believe that, according to it, some companies use all NPN transistors because PNP transistor is a little more expensive. Crazy.

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.