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The constant current source also allows for DC coupling at the output (with a bipolar power supply). The simplest circuit topology would require only two MOSFETs. (Transistors would not work as well because of the transistor's low input impedance, which would drag down the line stage output stage.) Added features would include a DC servo loop.
One feature, which is not optional, is a carefully designed fusing arrangement. The danger of placing the all of the biasing resistors on the amplifier side of the fuses lies with blowing a single power supply rail fuse, which would protect the output devices, but would not protect the loudspeakers, as a steady 2A current draw (from the top MOSFET or the constant current source) into loudspeaker would vaporize all but the most buffed voicecoils. Sown below is a push-pull power buffer with complimentary output MOSFETs. Should any power supply fuse, the output will remain close to ground and not slam to the opposing rail.
The downside to driving the output MOSFETs directly is the MOSFET's high input capacitance.
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