In this drawing the circle on the far left labeled VS3 is the incoming audio signal, the triangle labeled X0P1 is the voltage amplifier and the two transistors, Q21 and Q22 are the push pull output stage. Q21 pushes up and Q22 pulls down.
If this were used in a class B mode, both transistors would be off when there's no signal. In class AB mode, both would be always on, even when there's no signal, perhaps 10% of their total capability. In class A mode both would be on 100% of the time, either creating heat or sending power to the loudspeaker.
Here is a picture of a single ended output stage:
Sorry it isn't more complete, I had to grab and go this morning. Just mentally add the signal source and voltage amplifier of the first picture to the left of this drawing to complete the picture. Notice there is but one device and, where it says Vout is where you would connect your speaker.
Notice it is the same as Q21 in our first picture? It's missing Q22. What that means is it can push but it cannot pull - so any music falling below the center line is asymmetrically feeding the lower half of the signal to the speaker.
To compensate for this designers run a lot of current through this device constantly and, like Nelson Pass's Aleph amplifier of yesterday, that can mean either the ability to fry an egg on the amp or very few watts available to send to the speaker to keep the heat in check.
One last thing: many of you have asked what a SET amplifier is. SET stands for Single Ended Triode and it is exactly what we've been talking about. Replace the single transistor with a type of vacuum tube called a triode and you have it.
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