How to improve this microphone phantom power supply to have better SNR?
I have done many tests during a few weeks, in order to compare the SNR of a few microphones. Here is the SNR measurement protocol I used.

To do this, I bought some components:


1% 680 Ohm resistors 
0,1% more expensive resistors: 536, 665 (680 were out of stock), 715, 1k 
Vishay 100uF capacitors, and other cheap 100uF capacitors
Kemet 10uF capacitors, and other cheap 10uF capacitors
Floureon SPL meter 


Here are my conclusions:


12V battery vs 9V battery powering: doesn't significantly change the SNR
680 vs 715 vs 1000 Ohm: idem (maybe using the 1000 Ohm gave a (very) little bit less high-frequency noise, but the difference is maybe &lt; 1 dB)
680 1% vs 665 0,1%: idem (because the two 680 1% were already close enough! Both were measured at a value of 678.8...)
replace the output 10k resistors by 3.9k ones: idem
Kemet vs noname 10uF capacitors: idem
Vishay vs noname 100uF capacitors: idem
10uF vs 100uF capacitors: it changes a little bit the cutoff of the highpass filter: when using a 100uF capacitor we keep a little bit more low-frequency signal around 10-100 Hz. It is not significant when measuring the "A-weighted SNR" because these frequencies are pre-weighted by -60dB or -70dB before computing the SNR, but it is a (very) little-bit hearable when comparing both.


Conclusion: The good old Schoeps CMT30F microphones seem to have a "Signal-to-noise ratio, A-weighted (re. 94 dB SPL)" of ~ 70 dBA.

As a comparison I measured 76 dBA for a more recent microphone (Rode NT5, the technical specs says: "Signal/Noise: 78dB SPL (@ 1kHz, rel 1Pa per IEC651)").