Why are most High End Amps class A

High slew rate input signals come back to summing junction thru negative feedback delayed because of signal path delays. For a moment amplifier has no feedback and overshoot appears at the output (or earlier dependent on design). This will happen to any amplifier if slew rate is not properly limited at the input.

The only part of this I disagree with is the phrase "at the input". And yes, these are the very fundamentals of proper frequency compensation. The only nit-pick I would add is if this is indeed done "at the input", that implies a passive network before amplification . . . and while slew-rate and bandwidth are two different things, you can't limit one without the other in a passive network. Which makes it a bandwidth discussion, and again takes us right back to the fundamentals of stability in feedback amplifiers, and phase margin.

Also, a Class AB amp cannot "exhibit higher order of mostly odd harmonics at very low signal levels" as compared to a Class A design . . . very simply because at "very low signal levels", it's a Class A amp. That's its raison d'etre. I could maybe see how other non-crossover nonlinearities (this "kink" you describe) may have been a slight contributor at very specific power levels in days of output devices like 2N3055/2955, but is virtually nonexistant in properly loaded constant-beta modern bipolar power transistors.

But to answer Oem's question . . . I don't think that Class B is "the answer", it's one of many valid options. Why is it not used more? I don't know for sure, but I'd speculate that the main reasons are because it requires extremely accurate thermal bias compensation, and the nonlinear base currents demanded by the drivers from the voltage-amp . . . both present significant (but not insurmountable) design challanges for good results. Class AB is significantly less critical in these regards, but the trade-off is a greater variation in performance with load and signal level, both of which are dynamically working to pull the amp from Class A operation towards Class B. And this transition isn't a particularly graceful one.

Kirkus - class A amp at very low levels has both output devices conducting simultaneously doubling their voltage gains. It creates wobble in output linearity - no escape from that. It is known as "gm doubling". Increasing bias won't help since overbiasing creates higher order of odd harmonics (because of gm doubling)as well as underbiasing.

The term "gm doubling" as I understand it refers to the increase in gain that occurs from when one half of the output stage is conducting, to when both halves are conducting. The idea is that there's "double the transistors", so there's "double the transconductance". The static gain is of course no where near double because of the local feedback intrinsic in the output stage.

"Gm doubling" is the main problem with a Class AB amplifier, and manifests itself as a sharp nonlinearity at the point of transition between Class A and Class B operation. This can easily be seen when looking at the distortion output on a 'scope - as the signal is increased into a resistive load, the nonlinearity on the distortion residual slides, in alignment, from the tips of the waveform toward the center, as the amplifier transitions from Class A to Class B. 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.

Class A amplifiers don't have this problem, because "gm doubling" occurs all the time - since both halves are always conducting, there's no change in gain to the point where they're not. Class B amplifiers must walk a thin line between cutoff (underbiased) and gm-doubling (overbiased) - this is why bias tracking is so critical on class B designs.

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.