Problem
- You're wiring sub RCA and remote turn-on from the headunit.
Symptoms
- Your sub stays off and the amp sits in protect.
- You hear hum with the engine running.
Cause
- If your grounding layout is poor, alternator charging current modulates the RCA reference.
- The remote turn-on sequence couples with the way your headunit mutes or delays.
Fix Step-by-step
- Establish the amp ground first: run a short lead to bare metal, sand the paint back, then torque to spec. Common mistake: a tidy factory bolt through painted sheet gives you a high resistance ground.
- RCA routing: run them down the rocker opposite the big power, and cross existing bundles at 90°. If your amp sits under the rear seat in a ute, avoid parallel runs with the alternator charge lead through the cab tunnel.
- Remote (+12 V turn-on): meter it at 0 V key off, and around +12 V with ACC on. If your headunit output is weak, drive a fused relay coil from the remote and take the contact from battery+.
- Long canopy or tray runs: use proper shielded RCA or a balanced DSP feed. You'll often hear hum from cheap RCA on a 4 m run on a diesel.
- Smoke-test gains: set the headunit sub level −6 to flat, amp gain at minimum, then check you get no protect LED with the engine off and again at idle.
- Hum only with the engine running: reroute the RCAs first. If you still hear it, treat it as charging or ground layout, see the whine article.
Typical install mistakes
- The RCA shield brushing seat rails or custom plate steel.
- Remote tapped to the power-antenna lead, so your sub wakes with the radio only and pops oddly.
