PM_K-1 hardware: Stage 1 audio (input receiver) sims + container libs

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>

hardware/eda/Containerfile

@@ -19,10 +19,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     add-apt-repository -y ppa:kicad/kicad-9.0-releases && \
     apt-get update && apt-get install -y --no-install-recommends \
         kicad \
+        kicad-symbols \
+        kicad-footprints \
         ngspice \
         python3 python3-pip python3-venv \
         git make && \
-    apt-get clean && rm -rf /var/lib/apt/lists/*
+    apt-get clean && rm -rf /var/lib/apt/lists/* && \
+    pip3 install --no-cache-dir --break-system-packages skidl
 
 WORKDIR /work
 CMD ["bash"]

hardware/eda/sim/stage1_cmrr.cir

@@ -0,0 +1,56 @@
+* PM_K-1 Stage 1 : balanced line receiver -- why CMRR needs matched resistors
+*
+* A balanced receiver rejects "common-mode" noise -- hum/interference that appears
+* equally on both signal wires -- by SUBTRACTING the two wires. How well it cancels
+* (its Common-Mode Rejection Ratio, CMRR) depends entirely on how well its four
+* resistors match. This is the circuit-level reason we chose the laser-trimmed
+* THAT1240 instead of a discrete op-amp + four 1% resistors.
+*
+* Three identical unity-gain difference amplifiers, driven common-mode (same 1V on
+* both inputs), with 0% / 0.1% / 1% mismatch on one resistor.
+* CMRR(dB) = -20*log10(|Vout|)  since differential gain = 1 and Vcm = 1V.
+*
+* Run:  ngspice -b ../eda/sim/stage1_cmrr.cir
+
+.title Stage1 line receiver CMRR vs resistor match
+
+Vcm cm 0 AC 1            ; common-mode drive: both inputs tied to cm
+
+* ---- A: perfectly matched ----
+EA outa 0 vpa vma 1e6
+R1a cm   vma 10k
+R2a outa vma 10k
+R3a cm   vpa 10k
+R4a vpa  0   10k
+
+* ---- B: 0.1% mismatch on one resistor ----
+EB outb 0 vpb vmb 1e6
+R1b cm   vmb 10k
+R2b outb vmb 10k
+R3b cm   vpb 10k
+R4b vpb  0   10.01k
+
+* ---- C: 1% mismatch on one resistor ----
+EC outc 0 vpc vmc 1e6
+R1c cm   vmc 10k
+R2c outc vmc 10k
+R3c cm   vpc 10k
+R4c vpc  0   10.1k
+
+.ac dec 10 100 10k
+.control
+run
+meas ac acm_a find vm(outa) at=1000
+meas ac acm_b find vm(outb) at=1000
+meas ac acm_c find vm(outc) at=1000
+let cmrr_a = -20*log10(acm_a)
+let cmrr_b = -20*log10(acm_b)
+let cmrr_c = -20*log10(acm_c)
+echo
+echo "  CMRR  perfect match : $&cmrr_a dB   (limited only by the amplifier)"
+echo "  CMRR  0.1% mismatch : $&cmrr_b dB"
+echo "  CMRR  1%   mismatch : $&cmrr_c dB"
+echo "  -> hand-matched 1% resistors give poor rejection; the THAT1240's"
+echo "     laser-trimmed internal resistors are how we get >90 dB for free."
+.endc
+.end

hardware/eda/sim/stage1_phantom.cir

@@ -0,0 +1,39 @@
+* PM_K-1 Stage 1 : input protection -- a +48V phantom-power step at the jack
+*
+* "Phantom power" is +48V DC that mixers put on their MIC inputs. If it ever reaches
+* our input (miswire, wrong cable, our OUT plugged into a phantom'd input), a bare
+* op-amp input would die. Our protection makes it a non-event:
+*   - a series DC-BLOCKING film cap passes audio (AC) but blocks the +48V (DC),
+*   - a series resistor + clamp diodes to the +/-15V rails survive the turn-on EDGE
+*     (the cap passes the fast step before it charges).
+*
+* We hit the input with a +48V step at t=1ms and watch the OP-AMP INPUT node.
+*
+* Run:  ngspice -b ../eda/sim/stage1_phantom.cir
+
+.title Stage1 phantom protection
+
+Vin in 0 PWL(0 0 1m 0 1.001m 48 5 48)   ; +48V appears at t=1ms, stays
+
+C1  in a 1u            ; DC-blocking film cap (audio passes, DC blocked)
+Rs  a  n 1k            ; series current limit
+Rb  n  0 1meg          ; bias / input impedance (audio high-pass corner ~0.16 Hz)
+
+.model Dclamp D(Is=1e-14 N=1 Rs=10)
+Dp n  vp Dclamp        ; clamp to +15
+Dn vn n  Dclamp        ; clamp to -15
+Vp vp 0 15
+Vn vn 0 -15
+
+.tran 2m 5
+.control
+run
+meas tran vn_peak  max v(n) from=1m to=5
+meas tran vn_final find v(n) at=4.9
+echo
+echo "  op-amp input PEAK during the +48V step : $&vn_peak V   (clamped near a +/-15 rail)"
+echo "  op-amp input STEADY-STATE              : $&vn_final V   (DC blocked by the cap)"
+echo "  -> +48V at the jack becomes a clamped blip that decays to ~0. Nothing is harmed;"
+echo "     a wrong patch only ever sounds wrong. (Audio passes -- the high-pass is ~0.16 Hz.)"
+.endc
+.end

hardware/kicad/skidl_REPL.log

@@ -0,0 +1,9 @@
+WARNING: KICAD8_SYMBOL_DIR environment variable is missing, so the default KiCad symbol libraries won't be searched. @ [/work/hardware/kicad/<frozen importlib._bootstrap_external>:995=>/work/hardware/kicad/<frozen importlib._bootstrap>:488]
+WARNING: KICAD6_SYMBOL_DIR environment variable is missing, so the default KiCad symbol libraries won't be searched. @ [/work/hardware/kicad/<frozen importlib._bootstrap_external>:995=>/work/hardware/kicad/<frozen importlib._bootstrap>:488]
+WARNING: KICAD9_SYMBOL_DIR environment variable is missing, so the default KiCad symbol libraries won't be searched. @ [/work/hardware/kicad/<frozen importlib._bootstrap_external>:995=>/work/hardware/kicad/<frozen importlib._bootstrap>:488]
+WARNING: KICAD_SYMBOL_DIR environment variable is missing, so the default KiCad symbol libraries won't be searched. @ [/work/hardware/kicad/<frozen importlib._bootstrap_external>:995=>/work/hardware/kicad/<frozen importlib._bootstrap>:488]
+WARNING: KICAD7_SYMBOL_DIR environment variable is missing, so the default KiCad symbol libraries won't be searched. @ [/work/hardware/kicad/<frozen importlib._bootstrap_external>:995=>/work/hardware/kicad/<frozen importlib._bootstrap>:488]
+WARNING: fp-lib-table file was not found. Component footprints are not available.
+WARNING: fp-lib-table file was not found. Component footprints are not available.
+WARNING: fp-lib-table file was not found. Component footprints are not available.
+WARNING: fp-lib-table file was not found. Component footprints are not available.