Can someone assist me in understanding QSC's grounding scheme in their RMX amps?

[Fusion916]

http://www.qscaudio.com/support/library/schems/Current/RMX Series/rmx850.pdf

I see they have 3 grounds. Earth, A1, and P1 (speaker ground).

I see Earth is directly connected to A1, but I don't see where P1 is connected.

 

[Fusion916]

Anybody?

 

[Fast Eddie D]

It would seem logical to connect the speaker ground directly to the power supply board; or close to the center tap of the power transformer if there is no separate power supply board.

To me, calling it "P1" implies a power ground. This is consistent with good design practice.

 

[Art M]

This circuit is like a bridge tied load with the speaker placed between P1 (the output signal) and the C120/ C121 junction the speaker return path.
Usually a BTL has both sides driven but in this case only one side of the bridge is driven (P1). The junction of C120/ C121 supplies a passive ( 0VDC) between the supply rails.

P1 is connected to A1 and GND through R20/ 6R8. This is more of a Low level path rather then any high current related to driving the speaker.