12 Volt Encyclopedia Volume XIII
Quick answer: when it goes into protect tells you what to check first
Protect at key-on usually means a short. Protect after a few seconds of bass usually means the load is too low or the voltage is sagging. Protect after several minutes with a hot amp is thermal. Work through five checks in order: power, ground, remote, impedance and heat.
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When does it go into protect?
| When it protects | Most likely cause | Start with |
|---|---|---|
| Right at key-on | A shorted speaker wire or speaker, a short to the chassis, or an internal amplifier fault. | Check 4 (impedance and shorts), then check 6 (bench test) |
| After a few seconds, or on bass hits | Impedance too low for the amp, voltage dropping under load, or a weak remote signal. | Check 4 (impedance), check 1 (power) and check 3 (remote) |
| After several minutes, amp is hot | Thermal protection: low impedance, high gain or clipping, poor airflow. | Check 5 (heat), then check 4 (impedance) |
| Only at high volume | Voltage sag from wiring or the battery, or clipping into a low load. | Check 1 (power) and check 2 (ground) |
| Even with speakers and RCAs unplugged | Power, ground or remote problem, or a failed amplifier. | Checks 1–3, then check 6 (bench test) |
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The 5 checks
Turn the volume down and keep a multimeter handy. Measure with the system on and, where noted, while music plays.
| Check | How to test | Good result | If it fails |
|---|---|---|---|
| 1. Power | DC volts from the amp's +12 V terminal to its ground terminal, at rest and while playing. Check the fuse near the battery and at the amp. | Within about 0.5 V of the battery, and above the amp's low-voltage cutoff (check the manual) | Fix connections and fuse holders, upgrade the wire, check the battery and alternator |
| 2. Ground | Voltage drop from the amp ground terminal to battery negative while playing. | 0.2 V or less | Move the ground to clean bare metal, shorten it, match the power wire size |
| 3. Remote | DC volts from REM to ground with the source on. | Close to battery voltage and steady | Use a relay if one remote feeds several amps; check the source's remote output |
| 4. Impedance | Disconnect the speakers and measure the resistance of the whole load at the amp leads. Also measure each lead to chassis. | Load at or above the amp's minimum rating; no continuity to chassis | Rewire for a safe load (Woofer Wizard), repair pinched wires, replace damaged speakers |
| 5. Heat | Feel the heatsink after it shuts down; look at mounting and airflow. | Warm, not too hot to touch | Give it airflow, don't mount under carpet, lower the gain, raise the load impedance |
Reading the impedance numberA meter reads DC resistance, which is a bit lower than the rated impedance. A 4 Ω voice coil often measures around 3–3.6 Ω. Use Woofer Wizard to see what your wiring should total.
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Check 6: the bench test
- Disconnect the speakers and the RCAs, leaving only power, ground and remote connected.
- Turn the system on. If the amp still goes into protect, the problem is power, ground, remote or the amplifier itself.
- If the amp now powers up normally, reconnect the RCAs. Protect now points to the signal source or a ground loop.
- Reconnect speakers one channel at a time. The channel that triggers protect has the short or the low-impedance load.
- Still in protect with only power, ground and remote? Contact the manufacturer about warranty or repair.
Safety firstNever bypass the protection circuit or use a larger fuse to stop a shutdown. Protect mode prevents a fire or a failed amplifier.
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