switches to another mode, but the mode is not Class A, but rather Class AB. As the current source switches into active mode and conducts more, the previously active output device switches off. For example, let us say the idle current is 2 amps through the output stage and that the active device is a MOSFET connected as a source follower. As long as the current demands into the speaker are less than +/-2 amps, the bottom MOSFET continues to draw a fixed 2 amps of current.

Subject: SE / PUSH-PULL
   First of all kudos to you and to your fine magazine. At first, I only understood a third of Issue 1, but now that I just reread it, I understand about 90% of it. I don't know if I will ever hit 100%, but I'll try.
   So here is my question. I love the idea you have of converting a ST-70 into a para feed, SE amplifier using tubes instead of chokes. I have a pair of MK 3's that I would like to convert to style of amplifier. So my question is: Would it be possible to make the amplifier work like the Nelson Pass solid-state SE, the Aleph, which is SE up to a certain amount of wattage and then switches to push-pull Class A. amplification for the rest of output power? I know the solid-stage guys have a huge advantage in that they are willing to use IC's in the signal path, which we tube purist aren't, but at the cost of less design flexibility. So is this worth pursuing? Thanks in advance.
Ian
 
   I like this idea a lot. It's the best of both designs. But is it doable? I have not seen the schematic for the Aleph and I am not even sure it would help with the design of a tube counterpart; as you rightly point out, we have fewer components with which to play. Still, ingenuity often paints the best design with a small palette.
   One point first that needs to be cleared up is that I do not think you have the workings of the Pass amplifier quite right. Yes, it does work in SE Class A up to a certain wattage and

    If the output current exceeds 2 amps, say a peak of 4 amps, the top MOSFET and the bottom MOSFET will be required to conduct the extra 2 amps.  With a large positive swing, the top MOSFET will have to conduct a peak of 6 amps, as the output current is equal to the difference between top MOSFET and bottom MOSFET: 6-2 amps = 4 amps.  On a large negative swing, the top MOSFET can only conduct down from 2 amps to 0 amps, after which it will turn off during the portion of the waveform that the bottom MOSFET increases in conduction to a peak of 4 amps. For this portion, the amplifier will behave as a Class AB amplifier. Notice the purposeful lack of symmetry here. The top MOSFET must conduct up to 6 amps of current, while the bottom MOSFET only conducts up to 4 amps of current. Remember the bottom MOSFET never turns off and the current it passes must be absorbed by either the top MOSFET or the speaker or both.
   How do we mimic this split personality function in a vacuum tube amplifier? The best

Mixed mode amplifier with top MOSFET acting as an SE source follower and the bottom MOSFET acting as a current source up to a few watts and then switching to push-pull Class AB for the rest of the power output .

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