I have been repeatedly told by a lot of people that an input capacitorless phantom-capable "active" mic preamp cannot be done.
I'm here to tell all of them that it has been done. Twice already.
Input-Capacitorless DC-coupled Preamp 2012 Protoboard
I promised to re-build the prototype and make a few improvements.
Here it is.
It works, I've used it with real microphones, and it doesn't blow up.
I decided to take a pic of it while I clean up the bench.
I don't like having a messy bench with almost 100V floating around. (The supplies are +72, +48 and -20.)
One of the first comments I always get is "Well, some microphones pull unequal current out of each leg. You'll be amplifying all that offset. XYZ tried that and gave up."
Excuse me, but isn't that what servos are for?
The differential servo is capable of correcting - as it's currently built - about 100 uA/leg or 200 uA of total error. It can easily be made to provide more correction current.
The common mode servo keeps things from blowing up by flying the supply rails based on the microphone's DC quiescent point.
It's particularly fun to switch phantom on and off: The recovery time with an actual microphone is about 1 sec from off to useable, monitorable, audio.
Shorting legs to ground to create
phantom faults is also a thrill.
Microphones with transformer outputs will typically equalize the offset due to the low DC source resistance.
Microphones with direct-coupled outputs e.g. emitter followers will also have fairly low voltage offset errors.
Microphones with capacitively-coupled outputs current errors will usually be based on resistor tolerance.
The only microphone I've found that pulls all it's current out of one leg - and doesn't meet any standard - is the unbalanced Behringer ECM8000 measurement microphone. This preamp is not for it.
Pardon my temporary use of carbon film resistors while I tweak values.
From left to right are the passive common mode integrator (orange cap), 1510 preamp (buried under the gain switch leads), OPA2272 "Deboo" non-inverting integrator (orange cap), LME49720 low-gain offset trim stage and common mode servo and finally the flying rail power stage. The heatsink is way oversized. The big blue cap is the output coupling cap.
I'll draw up schematics after I clean up the bench and play some more.