LM1875 as transconductance amp for Fostex

[Antoinel]

The output transistors are operating in the common emitter instead of the customary common collector configuration. The center junction of the 30 Ohm resistors is grounded so as to make their operation common emitter. The collectors are opposed. Since the dedicated +/-15 V supply is a dead short for AC, then the center point of the +/-15 V supply is the output node. Put 2 batteries instead of the +/- 15 V power supply and trace the closed circuit for each of the output transistors. The ground node is the center point of the +/-6 V supply. It is the reference for the input signal and the output signal emanating from the emitters of the output stage

 

[KSTR]

Nice circuit, and yes, that's a "rail shifter" type of amp. The outputs are outside the feedback loop, though, but since it's class-A and Darlington there is no big penalty in distortion from base currents.

 

[Antoinel]

Thank you KSTR. By convention, the base currents; though trivial do flow in the resistors (30 Ohm). Thus they are subject to an (irrelevant) correction by loop feedback. Class A is circumstantial in this design case. The parts at hand dictated this outcome. Still, this transconductance amp was able to step out into Class AB and drive a small loudspeaker (4 Ohm)!