I took the power supply section from Figure 7 of this app note, also copied here, as part of a phantom-powered project, but I can't figure out what the author was thinking for R3 and R14:
I've studied it a while and simulated it in my head, and I think I understand how it works:
- R1,R2 provide the base current for Q1,Q2 to operate. Other than that job, they're supposed to stay out of the way to avoid loading the AC signal.
- R3 completes a voltage divider with R1||R2 to set the base voltage around 10.7V or so, which is stabilized by C1.
- Q1,Q2 are used as emitter-followers, providing a stable 10V or so, which is stabilized/decoupled/bypassed/[your term here] by C2,C5 and used as the supply for the rest of the circuit.
- As seen from the SIGnal lines, Q1,Q2 are high-impedance current sinks and therefore don't affect the differential-mode signal very much while drawing common-mode power from them.
What I can't figure out is the author's comment of:
R14 (marked **) is described as 'S.O.T', or select on test. This resistor needs to be chosen so that the DC is about 10V with a normal phantom supply of 48V. A zener may be used instead, which will give a little more voltage range - depending on the opamp used.
I thought the better place for a zener would be in place of R3, not R14. Then R14 could be removed entirely. Is this also a high-impedance current source into the ~10V point, so as to behave like a classic zener regulator as the author suggested?