i didn't want to hijack a thread to post this, but there was a discussion in another thread about grounds and power supplies and power amp circuits. i stated that there were some very strange designs that looking at the schematic , don't look like they would work at all, but they do, and are very successful designs. here's one of them. the amp is made by QSC, which uses this design in almost ALL of their PA amps, and it is so successful that other companies have copied it shamelessly. in this design, the power supplies float, and the supply common is NOT ground, but is the speaker output. the output transistors are grounded, and the collectors attached to the rails. the amp works by using the output devices to "walk" the power supply rails in reference to ground in unison with the audio input, and so the power supply common also changes voltage with the supplies. the negative feedback is applied to the noninverting input of the input stage (which in this case CAN BE an op amp running from +/-15V rails driving the driver transistors directly since drive current with hardly any voltage swing is required to drive them)
notice the transposing of the ground, and that the power supplies are floating in the QSC amp.
notice the transposing of the ground, and that the power supplies are floating in the QSC amp.