regarding
A mild debate has arisen elsewhere as to what function(s) the OUTPUT CAPS serve (2200uF, C9-12 in schema above).
Are they to BLOCK DC and/or (as contended by another) "all push pull stages that operate only on a single rail power supply utilize a SERIES capacitor in series with the speaker, which acts a a voltage source during the negative half cycle." ?
"
This is really arguing about the veracity of "lies to children" , after all an easter bunny is a sufficiently good explanation of the source of easter eggs.
If you want to think of the speaker coupling capacitor as a 6.0v DC source when connected to a single rail 12.0v amplifier , then that is sufficiently good enough to explain the operation in broad terms. (After all if you run a circuit simulator that is exactly what it does with capacitors for a small signal analysis).
C9-C12 are essential in the schematic if you intend running single rail amplifiers with speakers grounded , it is only a "debate" in the same way that King Canut "debated" with his advisors. It is obviously not the best solution, but adequate. A better solution would be to connect two of the 2200uF caps in series from +12v to gnd , and tie all four speaker common terminals to this point, A bit of clever phasing of the input signals would see low frequency AC current flowing in one speaker and out another. So, as drawn, 2200uF and 8ohms is a 10Hz rolloff , using my scheme each channel individually sees 4400uf in parallel with 2.66ohms from the other speakers, that would push the rolloff down to 5Hz. With normal musical content however , the low frequency component of all channels would nominally be in phase below 50Hz, so with "clever phasing" , the LF rolloff would be pushed down below 1Hz (determined by input coupling capacitors) . It's a bit of a moot point however , as human hearing drops off at ~ 20Hz , as does speaker efficiency.