Line (electrical engineering)

In electrical engineering, a line is, more generally, any circuit (or loop) of an electrical system. This electric circuit loop (or electrical network), consists of electrical elements (or components) connected directly by conductor terminals to other devices in series.


Consumer electronic devices concerned with audio (for example Sound cards) often have a connector labeled "line in" and/or "line out". Line out provides an audio signal and line in receives one. The connection is typically unbalanced using a TRS connector of 6.35mm (1/4"), 3.5mm (1/8" miniature) or 2.5mm (3/32" subminiature).

For example, while recording from the line output of a stereo FM receiver, you can turn up the speaker volume for your favorite song, then turn the volume back down after it ends while you record. When you play back the recording, having only changed the speaker volume, all the songs will be at the same loudness; you will not hear the volume increase where you turned up your favorite song (assuming that the radio station was broadcasting all the songs at the same volume level). In contrast, if you recorded by connecting to the speaker or headphone connector, the recording's volume would follow the volume knob's setting.

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 ohms, the voltage can reach 2 volts peak-to-peak with levels referenced to -10 dBV (300 mV) at 10k ohms, and frequency response of most modern equipment is advertised as 20Hz-20kHz (although other factors influence frequency response). This impedance level is much higher than the usual 4 or 8 ohms of a speaker, 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.

Line in

Line in expects the kind of voltage level and impedance that line out provides. You can typically connect the line out connector of one device with the line in of another. However, doing this with a straight cable directly connected to both devices and having both devices on AC power, you may run into a ground loop; although some devices provide isolation by using an opto-isolator, which does not create a physical connection of the devices.

A line input has a high impedance of around 10,000 ohms, as is often labeled as "Hi-Z" input (Z being the designator for impedance).


The desired effect is to have the impedances far from matched (10,000 ohm to 100 ohm) so that very little power is transferred and the line in circuit does not load down the output of the other device. When a line out signal, with its impedance of around 100 ohms, is connected to the high impedance line in of 10k ohms, most of the voltage appears across the input resistance and almost none of the voltage is dropped across the output.

Information transfer

These are voltage signals (as opposed to current signals) and it is the signal information (voltage) that is desired, not power to drive a transducer, such as a speaker or antenna. The actual information that is exchanged between the devices is the variance in voltage; it is this alternating voltage signal that conveys the information, making the current irrelevant.

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