Amplifier Hiss

Symptom: Constant hissing sound from tweeters or midrange. Present even with no music playing and head unit volume at zero. Does not change with engine RPM.

Root cause: Amplifier internal noise floor, or gain set too high (amplifying the noise floor of the head unit or source).

Diagnostic:

Step 1: Turn head unit volume to zero. If hiss disappears: noise is from head unit preamp output. Gain too high.

Step 2: Disconnect RCA from amplifier. If hiss disappears: noise entering from source. Gain too high.

Step 3: If hiss persists with RCA disconnected: amplifier internal noise. Normal at very low levels; excessive if loud.

Fix:

1. Set gain correctly — Use target voltage method (Chapter 4.4). Correct gain setting eliminates amplification of source noise floor.
2. Use higher-output head unit — A 4–6V preamp output versus 2V means 6–10 dB more signal relative to noise floor.
3. Improve power supply to amplifier — Switching amplifiers can generate their own switching noise. Clean power helps.
4. Check input impedance matching — Some amplifiers have low input impedance that loads down high-impedance sources.