Line- and Mic-Level, XLR

Line level (Source: Wikipedia)

is the specified strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles.

Line level sits between other levels of audio signals. There are weaker signals such as those from microphones (Mic Level/Microphone Level) and instrument pickups (Instrument Level), and stronger signals, such as those used to drive headphones and loudspeakers (Speaker Level). The “strength” of these various signals does not necessarily refer to the output voltage of the source device; it also depends on its output impedance and output power capability.

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 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. Professional equipment may also use unbalanced connections with (1/4 inch) TS phone jacks.



Voltage vs. time of sine waves at reference and line levels, with VRMS, VPK, and VPP marked for the +4dBu line level.


Use Nominal level Nominal level, VRMS Peak amplitude, VPK Peak-to-peak amplitude, VPP
Professional audio +4 dBu 1.228 1.736 3.472
Consumer audio −10 dBV 0.316 0.447 0.894



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 minimised. Most of the voltage asserted by the output appears across the input impedance and almost none of the voltage is dropped across the output.[5] 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.

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.

Line out

  Line out symbol.svg Line waves03-0-out.png Line waves03-1-out.png Line circle out.png PC Connector color  lime green.

Line outputs usually present a source impedance of from 100 to 600 ohms. The voltage can reach 2 volts peak-to-peak with levels referenced to −10 dBV (300 mV) at 10 kΩ. 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.

Line in

Line in symbol.svg Line in symbol.svg Line waves03-2-in.png Line waves03-3-in.png Line circle in.png PC Connector color   light blue. (Note: Microphone in has the PC Connector color pink, different audio-levels apply)

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 47 kΩ to over 1 MΩ. These “Hi-Z” or “instrument” inputs generally have higher gain than a line input. They are designed to be used with, for example, electric guitar pickups and “direct injection” boxes. Some of these sources can provide only minimal voltage and current and the high impedance input is designed to not load them excessively.

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. Such loads are around 1/1000 the impedance a line out is designed to drive, so the line out is usually not designed to source the current that would be drawn by a 4 to 8 ohm load 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 as little as 20 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 a source 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.[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.

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 equalisation to the signal as well as boosting it to line level.

Mic Level (Source: Sweetwater Sound, Inc.)

Microphones have comparatively small output voltages, on the order of thousandths of a volt (0.001V) ranging up to tenths of a volt (0.1V). Mic outputs can range from very low to very high depending on the mic type and design.

Low-output Mics: Some mics — particularly dynamic and ribbon mics (if you don’t know the difference, read this article about microphone fundamentals) — have extremely low output levels and need a lot of amplification, commonly called gain, which is the job of the mic preamp. Sometimes these low-output mics require as much as 50dB–70dB of gain, depending on the sound pressure level (SPL) generated by the sound source and the distance from the mic. Clearly a mic placed in front of a quiet singer will require more amplification than the same mic on a loud guitar amp. Examples of mics with low output levels are the AEA R84, Royer R121, or the Shure SM7B.

High-output Mics: Condenser mics such as the RODE NT1A, the Manley Reference Cardioid, or the AKG C414 XLII will have hotter outputs requiring drastically less amplification (less preamp gain) to achieve suitable signal levels, sometimes as little as 10dB–30dB of gain. The reason for this is that condenser mics have amplifiers built right into the mics (sometimes called head amps) that provide the voltage for the mic’s output.

What This Means to You: If you anticipate using microphones with low output (called low sensitivity) on quiet sources such as fingerpicked acoustic guitar, then you will need a mic preamp with a lot of gain. Preamps that only offer 40dB–50dB of amplification will not provide enough gain to record at optimum levels. Most importantly, mic output levels are too low to connect directly to inputs that expect to see our next level — line level.

XLR connector (Source: Wikipedia)

The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between three and seven pins. They are most commonly associated with balanced audio interconnection, including AES3 digital audio, but are also used for lighting control, low-voltage power supplies, and other applications. XLR connectors are available from a number of manufacturers and are covered by an international standard for dimensions, IEC 61076-2-103.[1] They are superficially similar to the smaller DIN connector range, but are not physically compatible with them.


EIA Standard RS-297-A describes the use of the three-pin XLR – known as XLR3 – for balanced audio signal level applications:


XLR pinouts.svg
Pin Function
1 Chassis ground (cable shield)
2 Positive polarity terminal for balanced audio circuits (aka “hot”)
3 Negative polarity terminal for balanced circuits (aka “cold”)
Note If you connect i.e. a Line-In or Line-Out via XLR bridge Pin 1 and 3 in the XLR Plug


The standard signal flow for audio in XLR connectors is that the output is a male connector and the input is female. In other words, the pins on the plug point in the direction of signal flow. This is the opposite of power connector standards which normally use female connectors for outputs, a convention influenced by the need to prevent accidental contact with dangerous voltages. However, the voltages of microphone and line level audio signals are not hazardous. The male XLR is usually incorporated in the body of a microphone.


If you want to connect a unbalanced, asymmetric line or microphone to an XLR male or female plug, use the above diagram for cabling. If you have a symmetric plug on the other end of the XLR cable, connect all cables through.