Inverting summing amp (OPA1612 section) mixes STAGE1_OUT (line/instrument) and
CLICK_OUT (filtered DAC) at unity into MIX_OUT. Each source enters its own 10k into
the op-amp virtual ground, so they sum with no interaction.
stage3_sum.cir confirms: each input alone = 0 dB, both together = +6.02 dB, and each
input's gain is unchanged by the other (virtual-ground isolation). ERC/netlist 0 errors.
Note: inverting summer flips phase -> corrected at the Stage 4 balanced driver via
hot/cold assignment. At integration, this summer can use the parked 2nd half of the
Stage 2 filter OPA1612 (U4) instead of a separate package.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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>
Audio chain, stage 1 (balanced input receiver + protection) validated in ngspice:
- stage1_cmrr.cir: CMRR vs resistor matching -> 1% = 46dB, 0.1% = 66dB, perfect
= amp-limited; justifies the laser-trimmed THAT1240 over discrete resistors.
- stage1_phantom.cir: +48V phantom step -> clamped to ~16V blip, steady-state
~0.12V; the DC-block cap + clamp + series R make a miswire a non-event.
Container: add kicad-symbols + kicad-footprints (for symbol placement) and skidl.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Pinned toolchain under hardware/eda/ so the design can be checked/simulated
identically in the future (system KiCad is 7.0, which has no CLI ERC):
- Containerfile: Ubuntu 24.04 + KiCad 9 (PPA) + ngspice + python3.
- run.sh: build-if-needed + run with the repo mounted; lands in hardware/kicad.
- sim/input_loading.cir: ngspice deck proving the line(25k) vs instrument(1M)
input-loading decision — Hi-Z preserves a +16dB pickup resonance the 25k load
flattens to -3dB.
Verified: KiCad 9.0.9, ngspice-42, ERC runs clean (0 violations) on
pm_k1_core.kicad_sch.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
