Version 2 immediately started with bodges & etching defects. Despite this, it ended up surprisingly clean. Got all the electrolytics & through holes on the bottom. Noted that the rainbow wrapping wire has insulation which melts extremely easily.
Unfortunately, the Metcal was unsuitable for drag soldering the chips in. Had to break out the Hakko. The Metcal may require a lower temperature, which requires ordering a new $30 tip & waiting a couple weeks. Its only advantage is the extremely fine point tips which actually heat up, unlike the fine pointed Hakko tips. Not sure inductive heating is necessary for getting heat.
The new board worked flawlessly on powerup & revealed new discoveries. WIFI at 5dbm doesn't work when the PI is upside down. The preamp is actually noisier at low frequencies when right side up. This might be from the table grounding something. The noise was unchanged from the dead bug circuit. With the gain at 100x, it only got 10 bits.
Tried cleaning flux from the preamp under the microscope, to no effect. The preamp would be quieter with metal film resistors. A test with the preamp disabled gave twice the noise of the dead bug circuit. It's now 13 bits & picking up switching noise. Moving the 8V to 5V converter under the PI produced the switching noise.
The AK4524 can actually take differential inputs if both converters are used. This got it up to 15 bits, but the extra space required to hook up another op amp & the noise from the op amp defeated it.
This noise testing is a bit academic, since the headset is going to obliterate the noise floor.