Line level
The line in/out connections on consumer-oriented audio equipment are typically unbalanced, with a 3.5 mm (0.14 inch, but commonly called "eighth inch") 3-conductor TRS minijack connector providing ground, left channel, and right channel, or stereo RCA jacks.
Professional equipment commonly uses balanced connections on 6.35 mm (1/4 inch) TRS phone jacks or XLR connectors.
[2] This awkward unit is a holdover from the early telephone standards, which used 600 Ω sources and loads, and measured dissipated power in decibel-milliwatts (dBm).
The most common nominal level for professional equipment is +4 dBu (by convention, decibel values are written with an explicit sign symbol).
A typical line out connection has an output impedance from 100 to 600 Ω, with lower values being more common in newer equipment.
The frequency response of most modern equipment is advertised as at least 20 Hz to 20 kHz, which corresponds to the range of human hearing.
Line outputs are intended to drive a load impedance of 10,000 ohms; with only a few volts, this requires only minimal current.
[6] A large resistor in series with each output can be used to safely mix them together, but must be appropriately designed for the load impedance and cable length.
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.
Some of these sources can provide only minimal voltage and current and the high impedance input is designed to not load them excessively.
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.
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.
