Why are most High End Amps class A

Kirkus - The magnitude of TIM is highly dependent on the open loop gain (everything else being equal) up to point where output transistors go to momentary saturation and stay there for a moment (having charge trapped at the junction). We cannot hear it (brain fill the gaps) but it make us tired.

TIM can be easily shown with just sum of two signals and the scope but it doesn't show in normal measurement of THD IMD etc. That was the problem in 1970 and is still now.

In an article in Stereophile "A future without a feedback"

http://stereophile.com/reference/70/index.html

Maritn Colloms claims that sound of 700 amps he reviewed was inversely proportional to amount of global negative feedback. One amp he mentions is a CARY monoblock with a strange feature of negative feedback adjustment. It sounded best at the lowest feedback.

In order to guarantee that amp would be free from TIM designer has to limit input slew rate (or frequency) to levels that output has (slew rate or frequency) before feedback is applied.

The issue here, I believe, is not a lack of resources but lack of discipline. I wouldn't buy class AB amp that has 0.0001% THD - that would be insane. At certain point of open loop gain very low level THD distortions (mostly odd harmonics) will be traded by for higher level TIM artifacts (also mostly odd harmonics). In both cases there will be also more (than in class A) harmonics of the higher order.

Yes TIM is a stability issue - when somebody decides to put gain of 10000 into audio amp and publish perfect spects.

Todays output stages are much faster than at the time Otala published his paper but desire to make class AB amp that is as good as class A amp - still exist.

The magnitude of TIM is highly dependent on the open loop gain (everything else being equal) up to point where output transistors go to momentary saturation and stay there for a moment (having charge trapped at the junction).

No, I think you're getting a couple of concepts confused.

Saturation of the output transistors happens at clipping or at reverse bias, the latter of which being a point where the charge carriers are accelerated maximally away from the transistor junction. Whether or not this happens is indeed a function of output stage slewing, but is completely an open-loop phenomonon and occurs independently of loop gain. The amplifier need not have feedback (actually it doesn't need small-signals stages at all!) for it to occur. If for some reason the designer wishes to never reverse-bias the output transistors, this is easily acheived by making minor changes in the driver connections - and the result is a slightly slower output stage.

The concept of slew rate limiting that Otala discusses in his seminal paper on TIM is related to the charging and discharging of the capacitor(s) used to set the small-signal bandwidth of the input and voltage-amp stages, and thus the open-loop response corner frequency. Since these small-signal stages are always biased Class A, their slewing performance (and ultimately that of the whole amplifier) is dependent on the quescient current flowing through them (as used to charge/discharge the capacitors), not the open-loop gain (which BTW I'm assuming means the o/l gain below the corner frequency). Otala advocated the use of capacitors in different places (lag compensation), which basically simply changes which stage in the amplifier is responsible for their charge/discharge current. Both Otala's method and the conventional approach have their pluses and minuses . . . and both approaches can be much less drastic with modern semiconductors than the ones available when he wrote the paper.

My point is that TIM can be understood, analyzed, and avoided - and we don't need to go down the "is feedback good or bad?" road to do it. The latter is of course an unsolveable debate at this time (so let's not go there). The biggest point to me about THD, IMD, and TIM is not so much is not what the numbers themselves are -- but what's causing it, and what the best ways are to fix the problems.

Hi My question is:If Class B is the answer why hasn't it been done.And why does it seem that Higher End designers try to stay away from Class B with the use of Class A/AB

Kirkus - Yes, saturation can appear without any gain in the circuit but it has nothing to do with the issue we're discussing.

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.

Amount of this overshoot is a function of amps open loop gain and in really bad case will take output stage to momentary saturation.

Let forget what is causing it, I agree, and look what to do to fix it. Class AB amp exhibits higher order of mostly odd harmonics at very low signal levels while THD and IMD is measured at substantially higher levels and doesn't show it. In order to lower it - either components have to be very linear or feedback has to be deeper. Careful selection of transistors and better circuit will help to a degree but will never eliminate big "kink". Local feedback will help as well but most of the linearizing will be done in the global NFB (global vs. local is a separate discussion). Bandwidth has to be limited at the input to bandwidth of the amp without the feedback (open loop). That's all. It is tradoff between low level THD and bandwidth on one side and TIM on the other.

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.