What is the output level of a car's CD player?

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The output is an AC waveform. At zero volume, the output voltage is zero volts AC. When you increase the volume, the output voltage increases. It will increase until the output driver reaches rail voltage (the power supply voltage for the output op-amp).

The maximum output is determined by the power supply voltage. For many of the low-end head uints, the power supply voltage must be less than the lowest voltage at which the head unit will operate. It's common for the op-amps to be powered by a 10 volt regulated power supply. Since the audio must swing both above and below a reference point (virtual ground), you only have about ±5v with which to work. Since most op-amps can't swing to within ~1.5v of rail, the maximum output swing is ~3.5v peak. This will allow approximately 2.5 volts RMS output.

Higher-end head units use switching power supplies that provide much more voltage for the op-amps (as much as ±15v) and allow the output to be as much as 8-10v RMS. These are rare.
 
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