Where you biased the Q2 would mean that the base of q3 is at 4.8v for the reference which would give me a dropout voltage of around the reference if i bias it from C3 through 5k6 resistor i could have a lower dropout voltage. What is the purpose of the D5? my guess was decoupling.
I think we have a rather different concept for the meaning of 'dropout voltage'. Dropout voltage is
*usually* understood to refer to the minimum input voltage required by the regulator to maintain regulation, and is usually expressed as a voltage differential across the regulator, so adjustable regulators and regulators of different output voltages can be directly compared.
However you appear to be concerned about the minimum output voltage,which obviously in this configuration cannot be less than the reference voltage. As your stated application was to power a microphone preamp, I didn't consider it to be important as long as it allows a suitable range of output voltages. If you are building a bench supply so need an output that can go right down to zero, you'd be foolish to start with this circuit. Its hard enough to get right with RRIO OPAMPs and a well stabilised negative bias supply.
Regarding D5:
Normally a capacitance multiplier follows the bottom of the ripple, as when there is insufficient collector voltage, the base capacitor discharges via the base-emitter junction to supply Iload, instead of the normal Ib of Iload/h
FE, and only recovers slowly via its feed resistor. The Schottky diode D5 lets the supplemental reservoir capacitor C5 charge up to the ripple peaks of the Unreg18 rail, and as the load on the capacitance multiplier is small, the ripple at its input is significantly reduced, resulting in a lot less ripple on its output. It also lifts the capacitance multiplier output voltage, resulting in a lower dropout voltage for the regulator as a whole. It becomes increasingly important as you increase the load current, as that linearly increases the ripple on any unregulated supply consisting of a rectifier directly feeding a reservoir capacitor.