Line out
The signal out of line out remains at a constant level, regardless of the current setting of the volume control. You can connect recording equipment to line out and record the signal, without having to listen to it through the device's speaker, and without the loudness of the recording changing if you change the volume control setting of the device while you are recording.
The impedance is around 100 Ω, the voltage can reach 2 volts peak-to-peak with levels referenced to -10 dBV (300 mV) at 10 kΩ, and frequency response of most modern equipment is advertised as 20 Hz - 20 000 Hz (although other factors influence frequency response).[citation needed]
This impedance level is much higher than the usual 4 - 8 Ω of a speaker or 32 Ω of headphones, such that a speaker connected to line out essentially short circuits the op-amp. Even if the impedances would match, yielding the theoretical maximum power transfer of 50%, the power supplied through line out is not enough to drive a speaker.
Headphone amplifier
headphone amplifier is a small power amplifier that can be connected to a standard headphone jack or (usually) the line output of an audio source. The headphone amplifier improves the sound quality by increasing the amount of power available to move the transducer, increasing the control that the source has over just where the transducer is in space.
Many headphone amplifiers have an output impedance in the range of 20 - 50 Ohms. The 1996 IEC 61938 standard recommended an output impedance of 120 Ohms.
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