Q: Do I have to modify my NwAvGuy O2 Headphone Amplifier PC board any to use the O2 Boster Board?
A: No you don't! The O2 Booster Board plugs into the U3 and U4 IC sockets on the O2 PCB. You just remove those two NJM4556A chips in U3 & U4 and plug in the O2 Booster Board. You also have to solder 3 wires from the O2 Booster Board onto the O2 PCB. Then you slide both PC boards, O2 and O2 Booster Board, into the O2 case at the same time. The O2 PCB goes in the bottom case slot while the Booster Board goes into the case top slot. To see the two new green LEDs on the Booster Board you would have to drill 2 new holes in your O2 front panel. Measurements for doing that arein the build instructions. ACAD file for a new O2 front panel with the holes included is also published in the materials. But a lot of people simply don't drill the holes and leave the LEDs covered up, or just don't solder them on in the first place if you are building your own O2 Booster Board.
Q: You list both the OPA827 and OPA140 as optional choices for the two op-amps. What is the difference?
A: The OPA827 is the better chip, but only by a tiny amount. The OPA140 has lower idle (quiescent) current so it is the best choice if you run your O2 Headphone Amp on batteries for a long time. If you primarily use your O2 on AC power I would recommend the OPA827. The "tiny" differences are a slight improvement in THD+N in the 827, and slightly lower input resistance induced THD with the 827. But both are outstanding chips with extremely low THD+N, low noise, and extremely low DC output offset.
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