bcfa5dd7f0
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>
47 lines
1.6 KiB
Text
47 lines
1.6 KiB
Text
* PM_K-1 : input loading -- line receiver (25k) vs Hi-Z instrument buffer (1M)
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*
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* This is the circuit-level proof of a decision we made by hand: why a guitar/bass
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* needs a HIGH-impedance input. A passive pickup is inductive, so the load impedance
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* it sees shapes its tone. Plug it into a low impedance and you damp its resonance
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* and lose treble + level; a high-Z buffer preserves it.
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*
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* Pickup model: open-circuit source + winding resistance + inductance + self-capacitance.
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* Plus a typical ~300 pF guitar cable. We drive two identical pickup+cable networks
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* from one ideal source and load them differently, then compare.
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*
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* Run: ngspice -b sim/input_loading.cir (from inside the EDA container)
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.title PM_K-1 input loading: line 25k vs instrument 1M
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Vpu src 0 AC 1
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* --- Branch A: LINE input, ~25k receiver impedance ---
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RdcA src a1 6k
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LpuA a1 a2 3
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CpuA a2 0 150p
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CcabA a2 0 300p
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RinA a2 0 25k
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* --- Branch B: INSTRUMENT input, 1M Hi-Z buffer ---
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RdcB src b1 6k
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LpuB b1 b2 3
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CpuB b2 0 150p
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CcabB b2 0 300p
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RinB b2 0 1meg
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.ac dec 100 10 100k
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.control
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run
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meas ac a_1k find vdb(a2) at=1000
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meas ac b_1k find vdb(b2) at=1000
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meas ac a_peak max vdb(a2) from=1000 to=20000
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meas ac b_peak max vdb(b2) from=1000 to=20000
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echo
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echo " Level @1kHz : line(25k)= $&a_1k dB inst(1M)= $&b_1k dB"
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echo " Resonant peak : line(25k)= $&a_peak dB inst(1M)= $&b_peak dB"
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echo " -> the 25k load drags the signal down and flattens the pickup's natural peak;"
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echo " the 1M input preserves level and tone. Hence the switchable Hi-Z front end."
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wrdata ../eda/sim/input_loading.csv vdb(a2) vdb(b2)
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.endc
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.end
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