Balanced audio: how it cancels noise, and when it helps

How a balanced line differs from an unbalanced one

An unbalanced connection, the kind behind every RCA and 3.5 mm plug, has one signal conductor and a return that also serves as the shield and the ground shared by both boxes. Anything the cable picks up on the way, and any small voltage difference between the two chassis, adds straight onto the signal. The input at the far end has no way to tell them apart.

On DAC and headphone amp spec sheets, "single-ended" means the same unbalanced connection. In amplifier design the phrase also names an output stage with one device handling the whole waveform, which is a different idea, so check which sense a spec sheet intends.

A balanced connection uses two signal conductors, usually twisted together, plus a separate ground. Both legs present the same impedance to ground, and the receiving input is differential: it measures the voltage between the two legs and ignores whatever they share. Interference induced in a twisted pair arrives on both legs at almost the same level, so it cancels. How much an input rejects is its common-mode rejection ratio, or CMRR, quoted in dB.

Most balanced outputs also drive the two legs in opposite polarity, called hot and cold. That doubles the signal the differential input receives while the shared noise still cancels. The cancellation itself comes from the matched impedances, though. An output that drives only the hot leg and ties the cold leg to ground through a matching resistance is still a balanced output.

Where the noise rejection pays off

Studios and stages are where balanced wiring pays for itself: long microphone runs, stage boxes, lighting dimmers and several mains circuits feeding one system. Each of those makes interference and ground-voltage differences more likely, and a balanced input discards most of them. That's why microphones and professional line equipment use XLR.

At home, a DAC half a meter from its amplifier and plugged into the same mains strip gives a balanced input very little to reject. It can still help with hum from a ground loop. The small voltage between two chassis grounds lands on both legs of a balanced cable and is rejected, where an RCA cable puts it directly in series with the music.

Balanced outputs also tend to run louder. Consumer DACs usually put about twice the RCA voltage on their XLR sockets, and the Denafrips Enyo 15th lists 2.0 Vrms on RCA and 4.0 Vrms on XLR. Twice the voltage is 6 dB, and the louder of two sources tends to be heard as the better one, so match levels before you decide the XLR side sounds cleaner.

What a balanced headphone output does

A headphone has no differential input, so nothing cancels at the driver. On a headphone jack, balanced describes how the amplifier drives each driver. An unbalanced output drives one end of each driver and connects the other end to a ground return shared by both channels. A balanced output gives each driver its own pair of amplifier outputs working in opposite polarity, so a stereo headphone takes four amplifier channels.

Two things change. The voltage across each driver can swing twice as far from the same supply, which allows up to four times the power. Four times is a ceiling, because an amplifier driving a low impedance often runs out of current before it runs out of voltage. The HIFIMAN EF499 is rated at 4.35 W balanced and 1.28 W single-ended into 32 Ω, a ratio of about 3.4. The HIFIMAN Serenade makes 4 W balanced and 2.8 W single-ended into the same load, so its single-ended jack gives away much less.

The second change is the ground return. With a shared return, current from each channel flows through the same conductor, and the small voltage it develops across that conductor's resistance leaks into the other channel as crosstalk. Separate returns close that path, which shows up as better channel separation on a measurement.

There is a cost. Two amplifiers per driver means two sources of output noise, so a very sensitive IEM can hiss slightly more on a balanced jack than on the single-ended jack of the same device. The noise floor explainer covers why sensitivity decides whether you hear it.

XLR, 4.4 mm and the other balanced plugs

Line-level balanced connections use three-pin XLR: pin 1 for the shield and ground, pins 2 and 3 for the two legs, with one cable per channel. Headphones use four-pin XLR, the 4.4 mm Pentaconn or the older 2.5 mm plug, and each keeps separate conductors for both ends of both drivers.

• Four-pin XLR carries left plus, left minus, right plus and right minus in one large locking plug. Many desktop amplifiers carry one.
• The 4.4 mm plug carries the same four signals plus a ground contact, in a size that fits portable players and dongles. Desktop units now fit it too, and the Serenade has both it and four-pin XLR.
• The 2.5 mm four-pole plug came first on portable players. It is thin and easy to bend, and newer gear has mostly moved to 4.4 mm.
• The 6.35 mm and 3.5 mm headphone jacks are unbalanced, with one return shared by both channels.

A socket tells you how an output is wired and nothing about the amplifier behind it. Read the power figure at your headphone's impedance, and check whether a maker quotes it from the balanced or the single-ended output, because the two numbers can be far apart.

When a balanced connection is worth having

Use balanced interconnects when the runs are long, when the gear is professional and expects XLR, or when an RCA connection hums and a ground loop is the cause. In a short, quiet desktop chain you can keep RCA and lose nothing you would hear.

Choose a balanced headphone output when your headphones need more voltage than the single-ended jack can supply, which is common with low-sensitivity planars and high-impedance dynamic headphones. The headphone power calculator shows how much you need at your listening level. With sensitive IEMs the extra swing buys nothing, and the single-ended jack may be the quieter choice.

Check the loading too. Measured across both legs, a balanced output's impedance is the sum of the two, and both Denafrips units list the XLR figure at twice the RCA one: 625 Ω and 1250 Ω on the Enyo 15th, and 200 Ω and 400 Ω on the Denafrips Hades 12th preamp. The usual rule of thumb asks for an input impedance at least ten times the output impedance feeding it, so 1250 Ω wants 12.5 kΩ or more downstream.

A balanced cable cannot make an unbalanced source balanced. Plug a 4.4 mm headphone lead into an adapter on a single-ended jack and you have an unbalanced connection with one more joint in it. The headphone amplifier guide ranks the headphone amps reviewed here by score.