For today's game, reduced the home made inductor from 10uH to 5uH & added an LM317, so it now boosted 7V to 55V.  Changed the feedback to frequency instead of duty cycle.  With no load, it hit 100khz.  With 2 microphones, it hit 40khz, well below the nyquist but hopefully cancelled out by the differential pair.  Suspect a flyback transformer is in the future.

This dropped the common mode noise by another 10db, but the differential noise was still there at -78db.  Depending on where in the room it was, it had more 60hz noise or more wifi.

So the next step was measuring the differential noise while playing with the preamp inputs.  With them disconnected from phantom power, the differential noise completely went away or -89db.  With them shorted, the differential noise hit a whopping -60db.  With an 8k pot between them, it was -72db.  With a 100k pot between them, it was -86.

It became clear that when connected to phantom power, the preamp was like an instrumentation amplifier with Rgain being the 2 * 6.8k + 2 * 1k & R1 being 100k.  The 2nd stage is the ADC with gain 1.  The input was going into the negative inputs instead of the positive inputs.

Though each op-amp has a 100:1 gain, the maximum difference the preamp can generate is 1 + 200k/(2*6.8k + 2k) = 12x but much noisier than each op-amp alone.

The voltage follower from a few blog posts ago was only a single ended test, so tried it again in differential mode.

The gain was so much higher, it had to be viewed in 1/16384 per y.  This revealed the noise was a function of the op-amp simply not doing anything when no current flows through its input.  If the input is an open circuit, the gain is 0 & there's no noise.  If the input has current flowing through it, the gain is nonzero & there's noise.  There's nothing the phantom power can do to reduce the noise.

While the matcher achieves 100x gain, there's little point in leaving it in because even 12x gain gets the noise floor well above the ADC noise.  The microphone is on top of that.

Lions are fans of using the fewest components possible.  It's all a game to do everything in a single layer & a single op-amp.  If every circuit had to be horrendously complicated, they would give up & focus on web apps.