Diagram

• Dual-gang 50k audio taper potentiometer:
  • PDB182-K430K-503A (Bourns, 50k log, dual gang). Note: verify/choose the variant with integrated SPST “click” switch for the remote control requirement; if not available, pick the switch-equipped version in the PDB18 family.
• RCA jacks (4x, female, board mount):
  • RCJ-041

• Power headers:
  • B2B-XH-A(LF)(SN) (JST-XH 2-pin, +12V/GND)
  • B3B-XH-A(LF)(SN) (JST-XH 3-pin, REM_IN/REM_OUT/GND)
• TVS diode (58 V class for automotive transients; pick one of):
  • SMBJ58A-13-F or
  • SMBJ58A-QH or
  • SMAJ58A-TR
• Reverse-polarity protection:
  • Series Schottky SS34, or ideal-diode PFET topology
• Input EMI filtering:
  • Ferrite bead BLM21PG121SN1D (place on +12V after fuse)
• 5 V regulator (for MCU/display):
  • LM2940S-5.0 (automotive-friendly LDO)
• Input protection fuse:
  • 0805L110WR (PTC resettable)

• High-side PFET (12 V signal line):
  • AO3407A (SOT-23 P-MOSFET)

• MCU (compact, with ADC):
  • ATTINY402-SSN or ATTINY404-SSN
• 4-digit LED display (simplest BOM as a module):
  • TM1637 4-digit module (horizontal)
  • If you prefer discrete IC + LED, use driver TM1637 plus a suitable 4-digit 7-seg display.

• ADC divider (typical values; confirm for your ADC range and impedance):
  • Upper: RMCF0603FG100K
  • Lower: CRCW060333K0JNEAC
• Bypass caps:
  • 100nF0603 (place at MCU, TM1637/module, and LDO pins)
  • 10uF 25V (bulk at 12 V input and 5 V rail)

• Grounding for low noise: prioritize ground separation (audio return vs. digital/display return) because return paths dominate noise coupling. Tie at a single star near the power entry or RCA ground reference.
• ADC divider drive: check divider output impedance using (R1*R2)/(R1+R2). If it exceeds the MCU ADC input spec, buffer or lower the divider values.
• Regulator sizing: ensure the 5 V regulator supports total load current with a 25% buffer, and operate power ICs at no more than 75% of their rated maximum power.
• Supply bypassing: switching/display noise issues are often fixed by proper bypassing first; place 100 nF at each IC VCC pin and keep traces short.
• Audio THD: keep the audio path passive (dual 50k log pot between RCA IN/OUT), route audio away from the digital display and 12 V rails, and use a star ground at the RCA shield.

• Potentiometer with switch: OK to use the Bourns PDB18 family and select the variant with integrated SPST “click” switch? If you prefer Alpha/WH148 style, I can swap to that family.
• Display choice: keep the TM1637 module for speed, or switch to a discrete LED + driver for a slimmer board?
• Headers: confirm pin order for JST-XH 3-pin remote header (e.g., 1=REM_IN, 2=GND, 3=REM_OUT).