Why are most High End Amps class A

Actually, not a bad analogy, Kijanki. It's just a question of . . . when do you want the bad news? To deliver more than you promise is usually more exciting than the other way 'round.

But with most semiconductor amplifier output stages, the linearity problem around the crossover point in Class B is substantially less severe than the one that occurs at the transition between Class A and Class B operation on an AB amp. If class B operation is thoroughly and rigourously optimized, then its performance will be less dependent on load and output level (which I think is very important), but it doesn't give true Class A performance under any conditions. Again, the particular application makes all the difference.

Kirkus - one of the problems with class AB is required gain to get rid of nonlinearity. Class AB amps have gain (before feedback) of couple thousands while class A couple hundred. Huge gain (before NFB) and delays just invite TIM if input slew rate in not limited. It might be possible to bias amp just a little higher (class A has about 150% of max current) to move the "kink" a little further away and set minimum gain to get minimum spects like 0.1-0.5% THD and IMD and bandwidth of 50kHz. The key, I believe, is compromise without going to over specifying it.

TIM is one of the reason of class AB sound, but it wasn't known until 1972. "experts" in denial claimed then that all parameters of SS amp are as good as tubes and therefore they must sound the same while average person could hear otherwise.

In all my previous discussion about linearity, I have been talking about the performance of JUST the output stage in isolation. And whether or not the output stage is biased as Class A, Class AB, or Class B . . . has NO effect on TIM.

The "TIM" acronym these days seems to be frequently flung about as a method to justify virtually any school of thought in amplifier design. But if we to talk about Transient Intermoduation Distortion as described by Matti Otala in his early-1970s AES paper -- the main thrust of this paper (and concept) revolves around frequency-compensation techniques. IIRC, Otala was basically proposing alternatives to the ubiquitous Miller compensation around the voltage-amplifier stage, under the supposition that lag compensation would reduce the loading of the differential amplifier under hard-slewing (transient) conditions, thus reducing a major source of nonlinearity.

While I have great respect for Otala's work, there are a few reasons that I feel the TIM concept, as he described it, is long overdue to be put to rest:
- The specific techniques he describes were based on observations in an era when power transistors were extremely slow, even compared to the small-signal stages that preceeded it - meaning that correctly-applied Miller compensation can be far less heavy-handed in the context of modern power semiconductors.
- Reducing the open-loop gain has absolutely no effect on the fundamental mechanism that causes the problem, it just forces the amplifier to work under conditions where it's difficult to occur. Kinda like strapping yourself to a sofa to avoid having foot pain that occurs when you stand up.
- A far more useful method of analysis of TIM is as a conditional reduction of large-signal open-loop gain and phase margin. This predicts the increase in distortion, and explains why a blanket reduction in open-loop gain can reduce the effect. It also gives valuable insight into how to solve the fundamental issue.
- This also lets us look at TIM distoriton for what it truly is -- a stability problem.

There are vastly more resources available today to more fully understand and predict the actual open-loop behavior of an amplifier than in Otala's day (i.e. high-bandwidth DSOs, FFT spectral analysis, and SPICE). This means that the rigourous engineer can more thoroughly investigate and anticipate all stability issues (including TIM), if he/she chooses to do so.

And if they don't, then TIM is the least of our worries.

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.