What is the maximum raw voltage of the power supply? Let's assume 13.2V and 0.6V drop on the emitter follower, that's 12V output, six stages makes 72V. The zener has minimum min. 44V (source: Distrelec Diotec 1.3W Zener SMD diodes). With 47k it makes 600uA current. Let's assume the electrons clump in clumps of 5 in the Zener (to simulate increased noise) - that's shot noise current of 3mA.
Excessive phase shift forced me to extend the bandwidth to about 100kHz. There the noise contribution will be the worst since the 100nF blocking capacitor has higher reactance of 16 Ohm there. Let's assume worst case bandwidth of 30 MHz and parasitic inductance of 10nH, that makes 1.88 Ohm. The capacitor dominates the noise leakage. The leakage has three ways: 390k resistance, say 500fF stray capacitance vs. 100nF and 0.5pF vs. 10nH @30 MHz. The ESR is negligible 0.1 Ohm or less ibid.
The resistive leak gives 5nA, the 0.5pF vs. 100nF gives 75fA, the 0.5pF vs. 10nH @ 30 MHz gives 88pA. All negligible.
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