XO 1.75 - +3.3V_AUDIO is not driven

[wmb@…]

The voltage on U24 pin 5 should be 3.3V but instead it varies between 0 and 2V depending on the mic bias voltage.

Inspection of both the schematic and the gerber reveals that the +3.3VA_AUDIO net is connected to several loads, but is not connected to any voltage source. Instead, it is getting its voltage from the mic bias supply via forward conduction of the top halves of D16 and D17.

One result is that the AC/DC switches U24 and U25 do not work correctly, thus preventing the use of the loopback cable for audio testing. Another result is mic signal distortion.

The audio subsystem works much better with a blue wire from that net to +3.3V_AVDD at C218. Perhaps +3.3V_AVDD and +3.3VA_AUDIO were supposed to be the same net?

[wad]

Confirming that +3.3V_AVDD and +3.3VA_AUDIO were supposed to be the same net.

To be fixed in B-phase.