Quick answer: A balanced pair is two conductors carrying equal and opposite signals, twisted together so both see the same noise, with a shield around them tied at one end. The twist keeps the loop area small, the differential input subtracts the noise, and the shield handles the electrostatic field. The star-quad adds two conductors and buys roughly ten to twenty decibels more rejection at higher capacitance.
Balanced Pair: The Construction Behind the Rejection
A balanced pair is two conductors that carry the same signal in opposite polarity, and the construction around them is what makes the rejection work. The two wires are twisted together so that they see the same electromagnetic field along their length, and the twist is what converts the differential input’s arithmetic into real noise rejection. The star-quad is the next step, with four conductors in two parallel pairs, and it buys more rejection at the price of capacitance. This guide covers the twist, the star-quad, the capacitance and impedance numbers, the shield practice, and the construction to choose for the studio and the stage.
The Twisted Pair
The twist is the oldest noise control in the cable business, and the mechanism is geometric rather than electrical. The two conductors are wound around each other, so along any short length the pair occupies nearly the same position in space, and an external field induces nearly the same voltage in both. The difference between the two voltages is what the input sees, and if the induced voltages are equal the difference is zero. The twist also reduces the loop area, which is the area that picks up the magnetic field, and the tighter the twist the smaller the area.
Why the Twist Works
The mechanism is the common-mode induction, and the arithmetic of the differential input is the other half. The field induces the same voltage in both conductors, the input subtracts the two, and the induced part cancels while the signal part, which is opposite in the two conductors, adds. The cancellation is never perfect, because the conductors are not in exactly the same place, and the residual is the common-mode rejection that the cable and the input together achieve. The twist is what keeps the residual small.
The Lay Length
The lay length is the distance the pair takes to complete one turn, and it is the number that describes how tight the twist is. A shorter lay length puts the two conductors closer together in space for a given length of cable, which improves the induction match and reduces the loop area. The cost is the extra copper and the extra capacitance, because the conductors run at an angle and face each other more closely. The lay length is a design compromise, and the good cables state it or at least state the result in the capacitance figure.
The Star-Quad
The star-quad puts four conductors in the cable, arranged as two pairs that are wired in parallel, and the geometry is what gives it the name. The four conductors sit at the corners of a square, so each conductor has two neighbours at right angles and one diagonally opposite, and the field that reaches them is more nearly equal across the group than in a twisted pair. The result is a rejection figure roughly ten to twenty decibels better than the twisted pair, and the price is a capacitance that is roughly double.
Four Conductors, Two Pairs
The four conductors are wired as two pairs in parallel: the two opposite corners carry the hot leg and the other two carry the cold. The arrangement means each leg has two conductors sharing the current, which lowers the resistance and the inductance without lowering the capacitance. The pairs are not separate channels, they are two halves of one channel, and the wiring at the connector has to combine them correctly. A star-quad wired as two channels is a fault, and the connector is where the mistake happens.
The Rejection Gain and the Cost
The rejection gain is the reason the star-quad exists, and it is largest where the interference is magnetic rather than electrostatic. The stage and the studio near mains transformers and lighting dimmers are the places it shows, and the improvement is audible as a lower residual hum. The cost is the capacitance, which is about twice the twisted pair’s figure, and the capacitance matters where the source impedance is high. On a low impedance line output it makes no difference, and on a high impedance source it rolls off the top end.
The Numbers
The numbers describe the pair better than the adjectives, and three of them matter. The capacitance between the conductors is quoted in picofarads per foot or per metre, and it sets the top end loss with the source impedance. The characteristic impedance is around a hundred ohms for the audio pair, and it matters at radio frequencies and at the digital rates rather than at audio. The resistance per conductor sets the length the pair can run before the level drops, and it is small in the audio band.
Capacitance and Impedance
The capacitance of a twisted pair is typically fifteen to thirty picofarads per foot, and the star-quad is thirty to sixty. The figure matters through the source impedance: a hundred ohm output driving thirty picofarads per foot has a corner far above the audio band, and a ten kilohm source driving the same cable has a corner inside it. The characteristic impedance of a hundred ohms is the number for the digital and radio use, and the audio band does not depend on it. The resistance is the third number and it is the one that sets the loss over a long run.
The Shield and the Drain
The shield is the layer that handles the electrostatic field, and the drain wire is the conductor that connects it to the connector. The shield is tied at one end in the standard practice, because tying both ends creates a path for the shield current that a ground loop drives. The drain wire is what makes the termination possible in a foil shield, and the braid shield can be terminated directly. The combination of the twist and the shield is what the cable does, and the input does the rest.
Choosing the Construction
The construction follows the environment and the source impedance, and the two cases are the studio and the stage. The studio run is short and the interference is mostly electrostatic, so the twisted pair with a good shield is enough, and the star-quad is the choice for the rack that sits near a transformer. The stage run is long and the interference is magnetic as well, so the star-quad earns its place, and the capacitance is not a problem because the source is a low impedance output.
The Studio Run
The studio run is usually under ten metres, and the source is a low impedance output at line level. The twisted pair with a foil shield and a drain wire is the standard cable, and it is the one in the patch bay and behind the rack. The star-quad is the upgrade for the specific position near the power amplifier or the lighting controller, where the magnetic field is strong. The capacitance of the star-quad is not an issue at line level, and the extra cost is the only trade.
The Stage Run
The stage run is the long one, and the environment is the hostile one. The multicore carries thirty or forty channels beside the mains, and the magnetic field from the dimmers and the amplifiers is the interference that the twist alone does not remove. The star-quad is the standard for the microphone lines in that environment, and the improvement is the lower residual hum on the quiet passages. The cable is heavier and less flexible, which is the trade the stage accepts.
Pair Constructions Compared
| Construction | Conductors | Capacitance | Rejection | Best case |
|---|---|---|---|---|
| Twisted pair, foil shield | 2 plus drain | 15 to 30 pF per ft | Good | Studio rack and patch |
| Twisted pair, braid shield | 2 | 15 to 30 pF per ft | Good, more robust | Stage and touring |
| Star-quad | 4 in two pairs | 30 to 60 pF per ft | 10 to 20 dB better | Near dimmers and transformers |
| Coaxial unbalanced | 1 plus shield | 20 to 30 pF per ft | No differential rejection | Short runs |
| Twin conductor, no twist | 2 parallel | 15 to 25 pF per ft | Poor | Not for audio lines |
Real-World Application Scenarios
The studio rack is the case the twisted pair serves. The patch cables run a metre or two between the preamps and the converters, the source is a low impedance output, and the foil shield handles the electrostatic field from the mains wiring in the wall. The stage multicore is the second, and there the star-quad is the standard for the microphone lines, because the dimmers and the amplifier transformers put a magnetic field on the cable that the twist alone does not remove. The location rig is the third, where the cable is coiled and moved and the braid shield survives the handling. The home studio is the fourth, and the twisted pair with a decent shield is enough for the short runs in a quiet room.
Related Guides on This Site
The related guides on this site cover the wiring and the connectors. The pin assignment at the connector is set out at XLR wiring, the connector configuration is at XLR pin configuration, and the judgment call on a finished cable is at how to tell a good XLR cable.
Frequently Asked Questions (FAQ)
Q: What is a balanced pair?
A: It is two conductors carrying the same signal in opposite polarity, twisted together and usually shielded, so that noise induced in both cancels at the differential input. The pair is the physical basis of every balanced cable. The twist keeps the two conductors in the same field, and the input subtracts what is common to both.
Q: Is star-quad better than a twisted pair?
A: It rejects more noise, typically ten to twenty decibels more, and it has about twice the capacitance. The gain is largest where the interference is magnetic, such as near dimmers and transformers. The cost matters only with a high impedance source, which is not the case on a line output. On a low impedance line it is the better cable.
Q: Does the shield need to be connected at both ends?
A: Not in the standard audio practice. Tying both ends gives the shield current a path, and that path is the ground loop that produces hum. The shield is tied at one end, usually the source end, and the drain wire makes the connection. The exception is the radio frequency case, where both ends are tied for the shield’s own sake.
Q: What capacitance should a balanced cable have?
A: Fifteen to thirty picofarads per foot for a twisted pair, and thirty to sixty for a star-quad. The figure matters through the source impedance, because the two form a low pass filter. On a low impedance line output the corner is far above the audio band, and on a high impedance source it can fall inside it. The number is on the specification of the better cables.
Q: Why does a star-quad cable have four conductors?
A: Because the four conductors sit at the corners of a square, and two opposite corners are wired in parallel for each leg. The geometry puts every conductor at a similar distance from the interference, so the induced voltages match more closely. The parallel wiring lowers the resistance without lowering the capacitance. The four conductors are two halves of one channel, not two channels.