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"I am not a transistor guy"

I caught up with the rest of this thread and think I understand where you are coming from. Single-ended circuits have to be class A in order to swing load current symmetrically. Push-pull circuits with transformer coupling can use two single-ended stages in anti-phase to swing load current symmetrically. This arrangement can be class-AB and will not be as linear as a single-ended class-A stage with the same output specification for the reasons you have stated.

In solid state power amplifiers the voltage gain is created using class-A circuits. The output stage is most often a pair of followers connected together to source and sink load current to and from the load. This stage can be biased for class-B, class-AB or class-A operation. Let's have a look at an example of a class-A amplifier designed for 50Wrms into 8 ohms. The peak voltage swing of the output stage must be +/-28.3V and the peak load current will be +/-3.54A. So, for class-A the output stage must be biased at 3.6A with +/- 30V supply voltage rails (rounding up). The output stage will be able to source or sink 3.54A without either follower device shutting off (i.e. with no current flowing through it). The idle dissipation of that output stage will be 216W. In operation as the output voltage rises the pull-up follower will supply current to the load and the difference between the bias current and the load current flows through the pull-down follower e.g. at 25Wrms the peak load current is 2.5A so 3.6A flows through the pull-up follower, 2.5A flows out to the load and 1.1A is left to flow through the pull-down follower. When the peak load current reaches 3.54A no current flows in the pull-down follower. For an 8ohm load that happens at our maximum output voltage so the load current cannot be higher. If the load resistance was really 6 ohms then with a 28.3V peak output voltage the peak current into the load will be 4.72A. The pull-up follower is biased for 3.6A so it will turn-on more to conduct an extra 1.12A and all of that current will be delivered to the load. No current will flow in the pull-down follower and we have left class-A operation.

Now let us make another amplifier with double the output voltage swing but leave the bias current unchanged. The supply voltage rails will now be +/- 60V and with the 3.6A bias current the idle dissipation will be 432W. This amplifier is now a 400Wrms into 8ohms class-AB amplifier that will operate in class-A for the first 50W (in 8ohms). To the first order the behavior of these two amplifiers up to 3.54A peak load current is identical. This is the point that I, and I think others in this thread, have been making.

Hope that helps
13DoW

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