In late 80s I came across the article about Quad 405 in Wireless World (it was a British electronics magazine). I liked the idea so much that I decided to design and build own version of forward feed design following Quad 405 architecture. I had a crappy AB class amp - totally non-complimentary design with pretty cheap components. I essentially threw away everything except of power supply and output bipolars which looked exactly like on this picture. Then I designed forward feed amp with minimal amount of components going for simplicity. Did not try match beta or anything. If I remember I used 1% resistors and avoided caps. I do not remember all the details but I made driver complementary A-class and kept output stage non-complementary B-class, i.e. crudest output stage you can imagine. For preamp/input I used op-amp as in original design to keep things simple, it was like 250mv.
The problem with this kind of design was that the idea was based on balanced bridge which included inductor and capacitor. If I recall correctly it was necessary for stability and for forward feed. But it made impossible to balance the bridge for all frequencies. I chose to balance at 1Khz using scope and a passive subtraction circuit (input/output) to amplify distortion. Yes, distortions were eliminated completely at 1 Khz. But it was impossible to eliminate them at all frequencies. It sounded much better than original amp but I did not have high end system so cannot judge how good forward feed idea was but I know that it is far from perfect and would rather design perfect amp differently like no caps and DC servo instead, inductors, fully complimetary balanced, cascode JFETS as input stage and MOSFETS as drivers and output current amplifiers, high precision resistors and so on, as much A class as possible basically how modern high end amps are designed.
I not a native English speaker so I am sorry if my language is far from perfect.
