Cap polarity and powering the opamps after the diode were indeed mistakes, which are fixable. Thanks.
I think somethings come down to design decisions. The question then becomes one of requirements. Remember the primary purpose of this project is not to make a USB sound card. Rather it is to make a prototype module that will be part of a larger project. Yes, the one that has the pre-amps, mixer and headphone amp.
I will likely make several stereo pre-amps PCBs next and inter-connect things on the bench. Then I'll make a multi-input mixer and headphone amp board. At some point a BT module. From each I expect to learn things, as I am now.
Thinking about it, I could save myself some money by replacing the 3.5mm Jacks with headers and interconnect with 3 conductor shielded sound card leads. For initial tests I can always solder a jacks onto those. Depends on what's cheaper.
The output levels might be wrong though. I wanted to aim for 0.615V
rms internal "nominal" level for the project. With the configuration shown in the TI schematic it's 0.6V
pp. For 0.615Vrms I need 1.74Vpp.
Alternatively I could just leave the output levels as they are and rely on the pre-amp to lift it to 0.615Vrms, which is kind of their purpose, to normalise multiple input levels before mixing.
The input levels are a bit of a weirdness. Feeding high line level into an amp with a gain of 1 and then a ADC with a input level of 0.6Vpp will clip it.
The confusing thing is... if you look at the user guide:
http://www.ti.com/lit/ug/sbau195/sbau195.pdfIt states the input AND output levels are 1.86Vpp full scale LINE level. However if I run a spice simulation on those amps they appear to have a gain off 1. The 2906 datasheet says it's analogue output is only 0.6Vpp. So something is a miss somewhere.
The capacitors are now bugging me, for some reason I didn't think 0805/1206 multi-layer ceramics went up to 10uF. I must have been looking for higher voltages or something, but even then 0805 10uF 16V MLC caps are only 4p each.
That will make it much easier to place the caps where they will do better good and save space.
So most of it needs redone. All good practice though.