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Looking for tutorial/description on audio amplifier input circuit operation

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Cliff Matthews:
Very nice to watch this guy at work and a lot of folks learn great stuff on the channel  :-+

Im personally intrigued by q106 q109

Since they power up the output transistors,   not sure if they act as anti-pop   or act as thermal protection or do both, since you see thermistors in their circuit drivers

Q106,109 are the rail switching transistors that switch the supply to the output stage to a higher voltage when the output approaches clipping on the lower voltage rail.

For typical audio, most of the time the system runs quite happily off the lower rail, with only relatively occasional need for more voltage, and using a rail switching design like this makes for a substantial reduction in heat produced and a massive improvement in safe operating area.

Rail switching is tricky and a better approach is usually to have a linear arrangement that boosts the rail once the output stage gets within a few volts of saturation,much better behaved that way, and spreads the heat between the output stage and the passbank, but a rail switcher is cheap.

This is class G (Or H, the terminology gets rather confused).


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