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 - 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.