As opposed to line level, there are weaker audio signals, such as those from microphones and instrument pickups, and stronger signals, such as those used to drive headphones and loudspeakers. The strength of these various signals depends on the output voltage of the source device, along with its output impedance.
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 output and line in receives a signal input. The line in/out connections on a consumer-oriented computer sound card are unbalanced, with a 3.5 mm (1/8") 3-conductor TRS minijack connector providing ground, left channel, and right channel. The connections on most other consumer equipment use RCA jacks. Professional equipment commonly uses balanced connections on 6.5 mm (1/4") phone jacks or XLR connectors. In most cases changing the volume setting on the source equipment does not vary the strength of the line out signal.
Nominal levels 
A line level describes a line's nominal signal level as a ratio, expressed in decibels, against a standard reference voltage. The nominal level and the reference voltage against which it is expressed depend on the line level being used. While the nominal levels themselves vary, only two reference voltages are common: decibel volts [dBV] for consumer applications, and decibels unloaded [dBu] for professional applications.
The reference voltage for the decibel volt (0 dBV) is 1 VRMS, which is the voltage required to produce 1 milliwatt [mW] of power across a 1 kilohm [kΩ] load. The reference voltage for the decibel unloaded (0 dBu) is the voltage required to produce 1 mW of power across a 600 Ω load (approximately 0.7746 VRMS).
The most common nominal level for consumer audio equipment is −10 dBV, and the most common nominal level for professional equipment is 4 dBu. By convention, nominal levels are always written with an explicit sign symbol. Thus 4 dBu is written as +4 dBu.
Expressed in absolute terms, a signal at −10 dBV is equivalent to a sine wave signal with a peak amplitude of approximately 0.447 volts, or any general signal at 0.316 volts root mean square (VRMS). A signal at +4 dBu is equivalent to a sine wave signal with a peak amplitude of approximately 1.737 volts, or any general signal at approximately 1.228 VRMS.
'Peak-to-peak' amplitude (sometimes abbreviated as 'p-p') refers to the total voltage swing of a signal, which is double the peak amplitude of the signal. For instance, a signal with a peak amplitude of +/-0.5 V has a p-p amplitude of 1.0 V.
|Use||Nominal level||Nominal level, VRMS||Peak Amplitude, VPK||Peak-to-Peak Amplitude, VPP|
|ARD, Germany||+6 dBu||1.550 (approximate)||2.192 (approximate)||4.384 (approximate)|
|Professional audio||+4 dBu||1.228 (approximate)||1.737 (approximate)||3.474 (approximate)|
|Consumer audio||−10 dBV||0.316||0.447||0.894|
The line level signal is an alternating current signal without a DC offset, meaning that its voltage varies with respect to signal ground from the peak amplitude (for example +2.192 V) to the equivalent negative voltage (-2.192 V).
As cables between line output and line input are generally extremely short compared to the audio signal wavelength in the cable, transmission line effects can be disregarded and impedance matching need not be used. Instead, line level circuits use the impedance bridging principle, in which a low impedance output drives a high impedance input. A typical line out connection has an output impedance from 100 to 600 Ω, with lower values being more common in newer equipment. Line inputs present a much higher impedance, typically 10 kΩ or more.
The two impedances form a voltage divider with a shunt element that is large relative to the size of the series element, which ensures that little of the signal is shunted to ground and that current requirements are minimized. Most of the voltage asserted by the output appears across the input impedance and almost none of the voltage is dropped across the output. The line input acts similarly to a high impedance voltmeter or oscilloscope input, measuring the voltage asserted by the output while drawing minimal current (and hence minimal power) from the source. The high impedance of the line in circuit does not load down the output of the source device.
Line out 
Line-out symbol. PC Guide color Lime green.
The signal out or line out remains at a constant level, regardless of the current setting of the volume control. Recording equipment can be connected to line out without having to monitor it through the device's speaker, and without the loudness of the recording changing if the volume control setting of the device is modified whilst 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).
Connecting other devices 
Connecting a low-impedance load such as a loudspeaker (usually 4 to 8 Ω) to a line out will essentially short circuit the output circuit, as such loads are around 1/1000 the impedance a line out is designed to drive. The line out is thus usually not designed to source as much current as such a load would draw at normal line out signal voltages. The result will be very weak sound from the speaker and possibly a damaged line out circuit.
Headphone outputs and line outputs are sometimes confused. Different make and model headphones have widely varying impedances, from a common low of 32 Ω to a few hundred ohms; the lowest of these will have results similar to a speaker, while the highest may work acceptably if the line out impedance is low enough and the headphones are sensitive enough. Conversely, a headphone output generally has an impedance of only a few ohms (to provide a bridging connection with 32 ohm headphones) and will easily drive a line input.
For similar reasons, "wye"-cables (or "Y-splitters") should not be used to combine two line out signals into a single line in. Each line output would be driving the other line output as well as the intended input, again resulting in a much heavier load than designed for. This will result in signal loss and possibly even damage. An active mixer, using for example op-amps, should be used instead.
Line in 
Line-in symbol. PC Guide color Light blue.
It is intended by designers that the line out of one device be connected to the line input of another. Line inputs are designed to accept voltage levels in the range provided by line outputs. Impedances, on the other hand, are deliberately not matched from output to input. The impedance of a line input is typically around 10 kΩ. When driven by a line output's usual low impedance of 100 to 600 ohms, this forms a "bridging" connection in which most of the voltage generated by the source (the output) is dropped across the load (the input), and minimal current flows due to the load's relatively high impedance.
Although line inputs have a high impedance compared to that of line outputs, they should not be confused with so-called "Hi-Z" inputs (Z being the symbol for impedance) which have an impedance of 470 kΩ to over 1 MΩ. These "Hi-Z" or "instrument" inputs also have much more gain than a line input. They are designed to be used with, for example, electric guitar pickups and "direct-in" boxes. Some of these sources can provide only minimal voltage and current and the high impedance input is designed to not load them excessively.
Line level in traditional signal paths 
Acoustic sounds (such as voices or musical instruments) are often recorded with transducers (microphones and pickups) that produce weak electrical signals. These signals must be amplified to line level, where they are more easily manipulated by other devices such as mixing consoles and tape recorders. Such amplification is performed by a device known as a preamplifier or "preamp", which boosts the signal to line level. After manipulation at line level, signals are then typically sent to a power amplifier, where they are amplified to levels that can drive headphones or loudspeakers. These convert the signals back into sounds that can be heard through the air.
Most phonograph cartridges also have a low output level and require a preamp; typically, a home stereo integrated amplifier or receiver will have a special phono input. This input passes the signal through a phono preamp, which applies RIAA equalization to the signal as well as boosting it to line level.
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.
See also 
- Tangible Tech Audio Basics
- Glenn M. Ballou, ed. (1998). Handbook for Sound Engineers: The New Audio Cyclopedia, Second Edition. Focal Press. p. 761. ISBN 0-240-80331-0.
- Oscilloscoped measurement for line level signal
- Dennis Bohn (May 1996). "Practical Line-Driving Current Requirements". RaneNotes. Rane Corporation. Retrieved 2012-07-15. "Practically speaking, electrical engineering transmission line theory does not apply to real world audio lines. ... This paves the way for simple R-C modeling of our audio line."
- Dennis Bohn (April 2004). "Why Not Wye?". RaneNotes. Rane Corporation. Retrieved 2012-07-15. "Outputs are low impedance and must only be connected to high impedance inputs -- never, never tie two outputs directly together -- never. If you do, then each output tries to drive the very low impedance of the other, forcing both outputs into current-limit and possible damage. As a minimum, severe signal loss results."