Quick answer: A balanced line output is a low impedance differential output referenced to +4 dBu in pro equipment and -10 dBV in consumer equipment, about twelve decibels apart. The headroom above the nominal is typically twenty decibels. The low output impedance lets the cable run long, and the gain staging at the receiving end is what reconciles the two references.
Balanced Line Output: Level, Headroom and Gain Staging
A balanced line output is a low impedance, differential output at a defined reference level, and the reference is where the pro and the consumer worlds part. The pro reference is +4 dBu at the nominal level, and the consumer reference is -10 dBV, and the difference between them is close to twelve decibels. That difference is the reason a consumer source into a pro input records quietly and a pro source into a consumer input clips. This guide covers the two references, the headroom above them, the impedance and the cable length, and the gain staging that makes the two worlds talk.
The Line Level Reference
The reference is the level that the meters call zero, and it is not the same in the two markets. The pro reference of +4 dBu is about 1.23 volts for a sine wave, and the consumer reference of -10 dBV is about 0.316 volts, and the ratio between the two is a little under twelve decibels. The numbers come from different histories, one from the telephone and broadcast practice and the other from the consumer equipment of the nineteen sixties, and they have stayed separate because the equipment in each market is built around its own convention.
+4 dBu and -10 dBV
The two units are not the same kind of decibel, which is the detail that makes the comparison confusing. The dBu is referenced to 0.775 volts, and the dBV is referenced to 1 volt, so a level quoted in dBu is not directly comparable with one quoted in dBV without the conversion. The practical difference is still about twelve decibels, which is what the gain stage has to make up. The unit difference is worth knowing because the specifications mix the two.
Why the Numbers Differ
The numbers differ because the two markets had different constraints. The pro equipment had the mains-powered headroom for a higher nominal level, and the higher level pushed the noise floor further below the signal. The consumer equipment ran on smaller supplies and needed a lower nominal to leave room for the peaks in the same circuit. The convention stuck, and the equipment of each market is designed around it, which is why the interface between them needs the gain.
Headroom
The headroom is the room above the nominal level before the output clips, and it is typically twenty decibels in the pro equipment. The nominal is the level the meters treat as zero, and the peaks of real programme sit well above it, so the headroom is the margin that keeps the peaks clean. The consumer equipment often has less headroom, which is one of the reasons the pro gear costs more. The number matters at the interface, because a source with twenty decibels of headroom into an input with ten will clip before the source does.
The Nominal and the Peak
The nominal level is a reference for the meter, and the peak level is what the electronics actually has to pass. A programme with a nominal zero and peaks at plus eighteen is common, and the output has to pass those peaks without clipping. The headroom figure describes the margin, and the clipping point is the nominal plus the headroom. The receiving input has its own clipping point, and the interface works when the two are compatible or when the gain is set to make them so.
Where the Headroom Goes
The headroom goes into the transients, which are the parts of the programme that the meters do not show. A drum hit or a consonant is a short peak well above the average, and the headroom is what passes it without distortion. The listener hears the loss as a hardening of the sound rather than as obvious clipping, which is why the gain staging is worth the attention. The output with twenty decibels of headroom gives the transients the room they need.
Driving a Long Cable
The output impedance is what lets the balanced line drive a long cable, and the figure is low. A line output is typically fifty to six hundred ohms, and the input it drives is ten kilohms or more, so the cable capacitance forms a corner far above the audio band. The balanced pair also rejects the noise that the cable picks up, which is the other half of the long run capability. The combination is why the pro installation can run thirty metres between the rack and the stage.
Output Impedance and Load
The output impedance and the input impedance form a divider, and the ratio between them sets the level loss and the loading. A fifty ohm output into a ten kilohm input loses a fraction of a decibel, which is why the bridging practice works. The input impedance is deliberately high so that several inputs can be paralleled without loading the source. The cable capacitance sits in parallel with the input, and the low source impedance keeps the corner frequency high.
The Length the Level Supports
The length is set by the capacitance and the source impedance rather than by the level, and the balanced pair removes the noise limit that the unbalanced cable has. A line output into a twisted pair runs thirty metres without a problem and a hundred metres with a good cable and a clean environment. The limit at the far end is the capacitance and the rejection rather than the level, and the level is the part that the gain stage adjusts. The long run is the case the balanced line was designed for.
Interfacing the Two Worlds
The interface between the two conventions needs a gain change in one direction and an attenuation in the other, and the change belongs at the receiving end. The consumer source into the pro input needs about twelve decibels of gain, and the pro source into the consumer input needs the same amount of attenuation. The cable cannot do either, and the adapter that claims to is a resistive pad at best. The gain stage on the interface or the mixer is the correct place, and the level match is set once.
The Pro to Consumer Direction
The pro output into the consumer input is the direction that clips, because the pro nominal is twelve decibels above the consumer one. The fix is the attenuator, either a resistive pad in the connector or the input trim on the consumer device. The pad is the passive answer and it costs a few decibels of noise performance, which is acceptable where the pro source has the headroom. The consumer device without an input trim is the case for the pad.
The Consumer to Pro Direction
The consumer output into the pro input is the direction that records quietly, and the fix is the input gain on the interface. The gain raises the signal and the noise floor together, so the clean result depends on the consumer source being quiet. The interface with a line input and a gain control is the right device, and the guitar input with its high impedance is the wrong one. The gain is set once and the level match holds.
Line Level References
| Reference | Voltage | Typical headroom | Common gear | Interfacing |
|---|
| +4 dBu | 1.23 V | 20 dB | Mixers and interfaces | Attenuate into consumer inputs |
| -10 dBV | 0.316 V | 10 to 15 dB | Consumer players | Add gain into pro inputs |
| +4 dBu, hot | 1.23 V nominal, peaks higher | 18 to 24 dB | Mastering equipment | Watch the input clip point |
| Mic level | Millivolts | Depends on the preamp | Microphones | Needs a preamp |
| Instrument level | Tens of millivolts | Depends on the amp | Guitars | Needs a DI or a hi-Z input |
Real-World Application Scenarios
The studio interface is the case the balanced line output is designed for. The interface output is +4 dBu into a ten kilohm input through a balanced pair, and the run to the monitor controller is five metres without a trace of noise. The live console is the second, where the output drives thirty metres of multicore to the amplifier rack at the pro level and the headroom carries the peaks. The consumer player into the interface is the third, and there the gain stage makes up the twelve decibels and the recording sits at the right level. The home system is the fourth, where the consumer output drives the consumer input at the matching level and the balanced question does not arise.
Related Guides on This Site
The related guides on this site cover the run and the cable. The length the balanced line supports is set out at long XLR cable, the cable families are compared at XLR cable selection, and the studio cable practice is at pro audio XLR cables guide.
Frequently Asked Questions (FAQ)
Q: What is a balanced line output?
A: It is a low impedance differential output at a defined reference level, +4 dBu in pro equipment and -10 dBV in consumer equipment. The two legs are driven in anti-phase, the impedance is low enough to drive a long cable, and the level is set by the reference. It is the standard output of a mixer, an interface, or a preamp.
Q: Why is +4 dBu louder than -10 dBV?
A: Because the two references are about twelve decibels apart, which is the difference between 1.23 volts and 0.316 volts. The units are also different, dBu against dBV, which adds to the confusion. The practical result is that a pro source into a consumer input clips unless it is attenuated, and a consumer source into a pro input records quietly unless the gain is raised.
Q: How far can a balanced line output run?
A: Thirty metres without difficulty and a hundred metres with a good cable and a clean environment. The limit is set by the cable capacitance and the rejection rather than by the level, and the low output impedance keeps the corner frequency high. The balanced pair handles the noise, and the level is adjusted at the receiving end.
Q: Do I need a converter between pro and consumer gear?
A: No, a gain change is enough in most cases. The pro to consumer direction needs an attenuator, and the consumer to pro direction needs input gain, and both belong at the receiving end. A converter is needed only where the interfaces differ, such as a digital to analogue conversion or a format change. The level match is a gain setting rather than a conversion.
Q: Does the cable length affect the level?
A: Only slightly, because the low output impedance and the high input impedance form a divider that loses a fraction of a decibel. The capacitance affects the top end rather than the level, and the corner frequency stays above the audio band at the line impedance. The level loss over a long run is not the problem; the noise and the capacitance are.
