Quick answer: An unbalanced cable has one signal conductor and one return, and the return is usually the shield. It works over short runs and it hums when the two devices sit on different earths. The coaxial form is the common one for RCA and instrument cables, and the two-conductor form for the older jack leads. The length limit is set by the source impedance and the noise environment.
Unbalanced Cable: Construction, Hum and Real Limits
An unbalanced cable carries the signal on one conductor and the return on the other, and the return is usually the shield. The arrangement is the oldest in audio and it is still the right one for most domestic connections, because the runs are short and the levels are high enough that the noise does not matter. The weakness appears when the two devices are earthed through different paths, because the shield then carries a current that the signal reads as hum. This guide covers the two-conductor principle, the coaxial form, the hum mechanism, the runs where the unbalanced cable is correct, and the practice that keeps it quiet.
The Two-Conductor Principle
The principle is the reference, and the reference is the ground. The signal is a voltage measured against the return conductor, so the receiver needs the return to know what the signal is. In a balanced cable the return is a driven copy of the signal, and in the unbalanced cable it is the ground. The arrangement is simpler and it needs one conductor fewer, and it makes the ground part of the signal path. Everything that follows, including the hum, comes from that one fact.
Signal and Return in One Shield
The shield has two jobs in the unbalanced cable, and the second is the one that causes the trouble. It keeps the electrostatic field off the signal conductor, and it carries the return current back to the source. The two jobs are compatible in a short run and they conflict in a long one, because the return current flows in the same conductor that is meant to be a screen. The voltage that the current develops along the shield appears in series with the signal, and the receiver reads it as part of the programme.
The Coaxial Form
The coaxial form is the unbalanced cable at its best. The centre conductor carries the signal, the shield surrounds it completely, and the geometry gives the cable a defined characteristic impedance and a shield that works at radio frequencies. The RCA and the instrument cable are both coaxial, and the form is why they are reasonably immune to the electrostatic noise even though they are unbalanced. The coaxial construction is the reason the unbalanced cable works as well as it does.
The Hum Mechanism
The hum is a current in the shield, and the current comes from a loop. Two devices plugged into the mains are both connected to the earth, and the signal cable gives the earth a second path between them. The mains earth carries a small current from the building wiring, and the loop divides that current between the two paths, so a part of it flows in the signal shield. The voltage that the current develops along the shield is in series with the signal, and the amplifier reproduces it as a hum at the mains frequency and its harmonics.
The Ground Loop
The ground loop is the name for the second path, and it is the reason the hum appears when a new device is added rather than when the system is designed. The loop is between the two earth connections, and the signal cable is the second path that completes it. The current in the loop is small and it is enough to be audible, because the shield resistance is small and the amplifier gain is large. The loop is broken by isolation rather than by a different cable.
The Shield Current
The shield current is what the loop drives through the return, and the voltage it develops is the hum. The voltage depends on the current and on the shield resistance, so a lower resistance shield carries more current for the same loop voltage and develops less voltage per unit length. That is the reason a heavy braid shield hums less than a foil in the same loop, and it is not a cure. The cure is the transformer or the ground lift, and the shield is the symptom.
Where the Unbalanced Cable Is Right
The unbalanced cable is right in the cases where the run is short and the levels are high, and those cases are the majority of domestic audio. The one metre cable between a streamer and an amplifier, the instrument cable from the guitar to the pedal, and the cable from the turntable to the phono stage are all unbalanced by design, and the equipment at both ends expects it. The short run means the loop area is small and the induced noise is small, and the high level means the noise is far below the signal.
The Short Domestic Run
The short domestic run is the case the RCA cable serves, and the run is usually under a metre. The shield does its job, the loop area is small, and the level is high enough that the residual hum is inaudible. The cable quality that matters is the shield coverage and the connector fit, and the length is short enough that the capacitance does not roll off the top end. The ordinary coaxial cable is the right part and the premium is not buying a property that the run needs.
The Instrument Cable
The instrument cable is the case where the unbalanced interface is a design choice rather than a compromise. The guitar pickup is a high impedance source with one output, and the amplifier’s input is designed for it, so a balanced connection would need a converter at the guitar. The cable’s capacitance is part of the instrument’s sound, and the players choose the cable for that reason. The unbalanced connection is the interface the instrument was designed around.
Making It Work
The practice that keeps an unbalanced cable quiet is short, and most of it is about the shield and the length. The shield is tied at both ends in this interface, because the return needs a path, and the loop is managed by the equipment rather than the cable. The length is kept as short as the layout allows, and the cable is routed away from the mains and the power supplies. Where the hum appears despite the practice, the isolation transformer is the answer rather than a heavier cable.
The Shield Practice
The shield practice in the unbalanced interface is the opposite of the balanced one, because the shield is the return. It is tied at both ends, and the connector shell is part of the circuit. The practice that follows is to keep the shield continuous and the connectors tight, because a broken shield is a broken return. The ground lift switch on a direct box is the exception, and it belongs to the balanced side of the box rather than to the unbalanced cable.
The Length Limit
The length limit is set by the source impedance and the noise environment rather than by a fixed number. A low impedance line output can drive ten metres of coaxial cable without a problem, and a high impedance guitar pickup is audibly darker at six metres. The environment sets the other half: the same ten metre cable that is quiet in a house hums in a room with dimmers. The practical rule is the shortest cable that reaches, and the balanced connection where the run has to be long.
Unbalanced Cable in Practice
| Case | Source impedance | Practical length | Main risk | Fix |
|---|---|---|---|---|
| Streamer to amplifier | Low, under 1 kohm | 1 to 3 m | None usually | Nothing needed |
| Turntable to phono stage | Moving magnet, high | 1 m | Capacitance and hum | Short low capacitance cable |
| Guitar to pedal | Several kohm | Up to 6 m | Top end loss | Shorter or lower capacitance |
| Line output to active speaker | Low | Up to 10 m | Ground loop | Isolation transformer |
| Long run to another room | Low to high | Beyond 15 m | Hum and noise | Balanced connection instead |
Real-World Application Scenarios
The domestic rack is the case the unbalanced cable is designed for. The streamer, the disc player, and the amplifier sit within a metre of each other, the levels are high, and the coaxial cables carry the signal without picking up anything the listener can hear. The guitar rig is the second, where the instrument cable is unbalanced by design and the capacitance shapes the tone. The turntable is the third, and there the short low capacitance cable matters because the cartridge is a high impedance source with a small output. The long run to the second room is the fourth, and there the unbalanced cable is the wrong part, because the length and the separate earth make the hum inevitable.
Related Guides on This Site
The related guides on this site cover the forms. The coaxial construction that the RCA cable uses is set out at audio coaxial cable guide, the general analog lead is at analog stereo cable guide, and the audio coaxial variant is at audio cable coaxial guide.
Frequently Asked Questions (FAQ)
Q: What is an unbalanced cable?
A: It is a cable with one signal conductor and one return, where the return is usually the shield and is tied to ground at both ends. The receiver measures the signal against that ground reference. It is the standard interface for consumer audio and for instruments, and it is limited in length by the source impedance and the noise environment.
Q: Why do unbalanced cables hum?
A: Because the shield carries the return current, and when two devices sit on different earths the shield becomes part of a ground loop. The loop current develops a voltage along the shield, and that voltage is in series with the signal. The hum is the loop rather than the cable, and the fix is isolation rather than a heavier shield.
Q: How long can an unbalanced cable be?
A: It depends on the source impedance and the environment. A low impedance line output can drive ten metres without trouble in a quiet room, and a high impedance guitar pickup is audibly darker at six metres. The environment sets the other half, because the same cable that is quiet at home can hum beside a dimmer. The balanced connection is the answer beyond about fifteen metres.
Q: Can I use a balanced cable as an unbalanced one?
A: Yes, if the wiring is right. A balanced cable has the two conductors and the shield, and using one conductor as the signal and the shield as the return gives an unbalanced connection that works. The second conductor is left unconnected at both ends. The cable is heavier and more expensive for the job, and the electrical result is the same as a coaxial cable of the same length.
Q: Does a thicker shield stop hum?
A: It reduces the voltage that the shield current develops, because the resistance is lower, and it does not remove the loop that drives the current. The improvement is real and it is partial. The fix is to break the loop with a transformer or a ground lift, and the shield is the symptom rather than the cause.