OPA1641 non-inverting DI buffer (1Mohm in, +12dB) + TQ2SA DPDT relay that both
routes the jack tip (line receiver vs DI buffer) and selects the output. Default
de-energized = LINE (common case, fail-safe). Driven by the shared ULN2003 via
net K1_DRV from GPIO SEL_LINST.
Pinouts verified from datasheets before capture (per the no-guessing rule):
- OPA1641 (TI SBOS484D): 1=NC 2=-IN 3=+IN 4=V- 5=NC 6=OUT 7=V+ 8=NC.
- ULN2003A: GND=8, COM=9, in 1-7 / out 16-10.
- TQ2SA (Panasonic TQ-SMD): pole1 COM=3 throws 2/4, pole2 COM=8 throws 7/9
(from contact-resistance terminal pairs). NC/NO orientation + coil pins (1/10)
follow the standard single-side-stable diagram -- flagged in-file for a final
connection-diagram cross-check (not over-claimed).
ngspice stage1b_di.cir confirms +12.04dB gain, flat across the audio band.
ERC 0 errors; netlist 0 errors.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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