原创 QUAD405,how it works?

     This KIT is designed 30 years ago,it amuses a lot of people.

     I think in the original Quad405,LM301 is not a fast OP.
So,the C8 120pf capacitor  used here as the feedforward network.
the high frequency part of signal went stright into the current dumper(push/pull pair transisters).bypass the Tr3-Tr5,Tr7 transisters.

47ohm and L2 3uH，together with R5, as feedback（current parallel negative feedback ）.

so,the total gain is Io="A"*Ii,current dump to the speaker.