Where to connect that black loudspeaker wire

I was asked where to connect the black (negative, cold) loudspeaker wire. Red wire clearly goes to the power amplifier's output pad, but where should the return current flow?

One wrong way is to make it flow directly to the power supply (e.g. to the midpoint of the capacitor bank). The reason is that the amplifier has little control over the voltage between its output terminal and the midpoint of the capacitor bank.

The control in an amplifier is performed with negative feedback, whether global or local. That includes no-global-negative-feedback amplifiers, too. Feedback compares a portion of output signal with the input signal and steers the output in the correct direction. The output signal of the amplifier - the input to the feedback network - should be the voltage fed to the loudspeaker. It you take feedback from between two points on the amplifier PCB but connect your loudspeaker somewhere else, feedback cannot do it work properly.

The correct way is to connect the loudspeaker where the feedback is taken from. Or to take feedback from where the loudspeaker is connected.

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