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>
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