This is the Input Capacitor-less DC-coupled Mic Preamp's Common Mode Servo and Flying Rail Supply. I'm using this one for testing.
Direct Coupled Input Capacitor-less Mic Preamp Common Mode Servo and Flying Rail Generator
The input has a passive pole with a 3.4 Hz corner frequency; I may need to tune this. This is for a relatively fast response with a tau of ~ 50 ms.
Op Amp "A" is connected as a voltage follower.
The MJE243 and MJE253 bootstrap the supply voltages.
I'm looking at some higher-voltage TO-126 parts (these are 100V) to provide a little more margin. The pickings are somewhat slim: MJE15034/35 and 2SC3503/2SA1381 are candidates.
The 16V Zeners and 10K resistors set the flying rail voltages at ~ Vcm +/-15V to provide 30V supplies for the common mode servo itself, the THAT1510 preamp IC and the differential servo an OPA2277.
The positive flying rail, with 0 to +48V common mode voltage, ranges from ~ +15 to +63V. The negative flying rail goes from -15V to +33V.
This keeps the THAT1510's power supplies and it's input common mode range centered about the microphone's quiescent "Q-point" voltage.
The base-collector capacitors, 100 nF/100V, provide stabilization.
Various 100 nF capacitors provide bypass.
My current op amp choice is an LME49720 dual to provide a low output impedance vs. frequency.
Though bipolar, the LME49720 has a significantly lower bias current than a 5532. The 1 nA bias current does develop an error across the 100K but it is not significant.
I've also used 5532s and the rather large ~1V error from Ib*Rin does not have significant impact on circuit operation.
My concern was that DC-precise op amps, such as the OPA2277, or BIFETS with low bias current, might not have low output impedance across that audio band.
No build-out resistance is required since the op amp is not driving any significant capacitance.
An optional differential bypass capacitor can be used.
I have not experimented with using electrolytics in that location.
Both ends of that bypass float together - but 30V apart - and essentially bootstrap it.
I've raised the Zeners in the past to permit the 1510 to run on 36V but the LME49720 has a 34V maximum.
I will be trying the 44V part, the LME49860.
Total current draw, not including phantom, is around 22 mA for the entire floating section comprising the THAT1510, LME49720 and OPA2277.
This works out to be about 2W most of it appearing as heat in the MJE243/253 and 10K resistors.