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The strange HK A500 power amp circuit

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I've been studying the Harmon Kardon A500 power amp circuit. It's a strange little beast with both positive and negative feedback applied. Does anyone know why the designer added positive feedback? What problem was he trying to solve, or otherwise why did he feel it was needed? Was it just to boost the gain? If that's the reason, it seems like he would have had plenty of gain without positive feedback because there's another 12AX7 gain stage (not shown in the schematic below) that sits in front of the power amp circuit that acts as a preamp. (The pot shown in the schematic on pin 2 of the 12AX7 is a balance control, not a volume control.)

It's a neat little circuit that I might try to work into one of my own builds in the future--if I knew what the designer really had in mind with this positive feedback thing. BUT I think I'd like to try to avoid the need for that coupling cap between the 12AX7 gain stage and the 12AU7 split load phase inverter, meaning DC couple these two stages--if that won't screw up the positive feedback loop. I'd obviously need to rebias the stages appropriately to get the DC and AC conditions to work out simultaneously.


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