What is the simple -* 15 volt supply
a center tapped winding
a full wave bridge
two caps
think more on the cap's a second. two conductive cylinder with an insulator, A cap to what is around it!!
all of those parts have some Capacitance to what is around them.
So if you change the world around them then you start charging or discharging that Capacitance.
Now think on that Op-amp that gets it's power from that floating supply.
The Op-amp inputs functions best when the inputs are in the center between the supply voltages and stays good until the signals get close to supply rails.
The Op-amp outputs will pull on the supply rails to produce their outputs. If the Op-amp output is at what should be the floating supply center then the Op-amp will pull the floating supply so that it is centered.
So with careful design that floating supply will float right where is it suppose to be. But remember that Capacitance, for a fast change, it will be slow to charge/discharge and may need help for the very fast changes to where it should be floating.
CR40 takes care of the +Out increasing faster then the normal float adjust that the +-15 volt supply can handle.
I would guess that Q7 handles the problem that the +-15 volt supply wants to float up higher than wanted for static operation. And also handles the -Out dropping very fast.
You have here a +-15 volt supply. What is the dynamic range that the floating supply can be off center and the control system can still regulate? Add that the greater the change from the static float voltage center, the greater the pull on the supply rails to get back to that static float center.
So why float the +-15 Volt supply instead of just connecting the center of this supply to +Out?
There are probably many reasons.
One could be to keep some of the control noise from being transferred to +Out.
The big problem with floating electronics is not that it is floating, it is that some repair tech will damage the circuit by thinking it is not floating.
C