Schiit Audio Magnius back panel photo courtesy of Schiit Audio. The larger 3 pin sockets are the AES balanced input and output connectors.
As with other topics in audio, balanced audio connections are shrouded in myth and mystery. This article introduces some of the basics in a non-technical way. Reference  from Benchmark Media takes a more detailed look at pro verses home differential connections and some of the myths surrounding differential mode connections and amplifiers. Reference  also from Benchmark Media talks about differential headphone amplifiers.
Read Carefully and Be Careful What You Read
This is engineering, not politics. Alternative facts don’t exist, but fried equipment does. Miss-wiring an amplifier’s output stages can activate the protection or destroy the output stages. Transistors have a disconcerting ability to protect the output fuses of classic amplifiers. Modern amplifiers like Schiit Audio’s Vidar use electronic protection of the output stages. Be sure to follow amplifier user guide wiring instructions.
I’ve listed all of the references I used to create this article. I encourage you to at least read the Benchmark Media articles from their knowledge base. Benchmark’s audience is pro recording and mastering studios and the creatives who own and run them. Benchmark is all about getting its clients to put their money where it matters to build a high value mastering system that is clear and transparent until it needs to be colored to make judgements about how a mix will sound over typical audience playback systems.
- https://en.wikipedia.org/wiki/Differential_signaling (math here)
- https://en.wikipedia.org/wiki/Differential_amplifier (math here)
- https://www.ortofon.com/mc-cadenza-red-p-485-n-4557 moderately priced moving coil cartridge description
- https://benchmarkmedia.com/search?q=balanced%20headphones*https://benchmarkmedia.com/search?q=balanced%20headphones* Balanced connections for power amplifier outputs not needed.
- 25 August 2020: Revised to add material about use of differential connections and differential amplifiers in power amplifiers. When is differential mode useful. When it is not. Differential with headphones. Wiring cautions for DIY.
When Balanced Connections are Used
Pro audio makes extensive use of balanced connections to manage electromagnetic noise and to transmit low level microphone signals from the stage to the mix position or to a digital mixer stage box.
Microphone balanced connections are always twisted pair and have differential mode amplifier as a destination. External magnetic fields affect both sides of the twisted pair and untwisted pair identically. So the noise is in phase (common mode) on both conductors while the signal is out of phase (differential mode). The differential line receiver cancels the common mode noise while amplifying the differential signal. The advantage of twisting is that it makes the connection loop area smaller reducing the electromagnetic noise picked up by the conductor run.
AES: Audio Engineering Society, the boffins who develop audio engineering standards. AES-3 is their standard for digital audio interconnects.
Balanced connection: A connection is said to be balanced when it connects identical source and load impedance. When a connection is balanced, external electromagnetic interference will induce equal noise voltage in the inverting and non-inverting conductors.
Common Mode: A signal that is in phase on both the non-inverting (plus) and inverting (minus) conductors of a differential connection.
Differential Mode: A signal that is 180 degrees out of phase on the inverting conductor relative to that on the non-inverting conductor.
Differential Amplifier: An amplifier that amplifies the difference in voltage (the differential mode voltage) between its two input terminals while rejecting the common mode voltage.
Shield Conductor: The shield conductor is the third conductor in a balanced connection. The shield conductor or foil wraps the signal pair to intercept some of the interference energy and redirect it to ground. The shield is grounded to the earth ground at one or both ends of the circuit. If the two ends of the connection have different ground potentials a ground loop is present. It is possible to have circulating currents between the two grounds. Some equipment, especially performance amplifiers, have ground lift switches to break the loop. Ground loops often present as hum but large ground potential mismatches can cause damaging circulating currents.
Twisted pair conductors: By twisting the conductors together, the area of of the loop formed by the non-inverting and inverting conductors is minimized. The reduced area reduces the sensitivity of the connection to external electromagnetic interference.
Use of Balanced Connections in Home Audio
In general, home audio systems can be assembled with a minimum of balanced connections. Because signals are robust (2 Volts) and runs are short, there is little advantage to be gained using the more expensive balanced connection.
Balanced differential connections affect component cost in several ways.
- Plus and minus supply rails are required
- Input and output transformers may be required
- Differential mode input and output stages are required (easy with integrated circuit amplifiers).
- The amplifier is differential end to end making the topology more complex, increasing development time, and increasing build cost.
- The patch panel has to make room for XLR connectors increasing panel size.
Do Single Ended and Differential Sound Different?
When properly designed and built, no.
In my system I can’t tell the difference between single ended and differential connections. The balanced connection is 3 db louder than the unbalanced connections requiring a level adjustment when switching to and from the balanced input.
The balanced connection is louder because the differential signal is two copies of the single ended signal, one of which has been inverted. There is no difference in tonality or temporal coherence between the balanced and unbalanced outputs, just the level difference. Perceptively, the level difference masks any differences in tonality.
When is Balanced Connection of Benefit?
Balanced interconnects are of benefit in several circumstances
- Long distances (stage to mixer, mixer to amplifiers). Here, shielded twisted pair minimizes EM noise pick up and the differential input stage stops the common mode noise dead.
- Low levels (microphones and LP record playback transducers). Having the signal in differential mode and the noise in common mode allows the differential input stage to short stop the noise.
- Uncertain chassis grounds (multiple power panels, multiple runs to ground). Signal ground and earth ground are separate. Earth ground noise is kept out of the signal circuitry.
Venue audio systems encounter all of these complexities. Home audio less so, especially since few play phonograph records today.
Phonograph transducers produce signals on the order of 5 millivolts or less (even lower if moving coil). The transducer is inherently differential but the amplifier input may be differential, or more likely, single ended with one terminal of the cartridge connected to signal ground.
Balanced differential inputs are a feature of better separate phonograph stages and control amplifiers. Price sensitive devices are single ended.
Moving magnet transducers typically have a 47 kilo-ohm source impedance while the amplifier has a matching 47 kilo-ohm input impedance. This standardization and the higher level makes it somewhat easier to design moving magnet phonograph stages.
Moving coil cartridges typically have a 0.5 milli-volt output level. Among moving coil cartridge makers, there is no agreement on source impedance or load impedance making design of a moving coil input stage more difficult. Typical recommended input stage load impedance lies in the range of 50-500 ohms. Typically, a termination resistor is used to provide the recommended cartridge load with the high impedance input amplifier connected to it in either single-ended or differential mode. Some more expensive amplifiers provide switchable moving coil input termination resistors. The lower signal level requires more gain and more attention to circuit noise.
Use of a proper balanced interconnect and differential input amplifier would reduce hum in the stage and provide the chassis ground connection that most cartridges require.
Long interconnects turn up in a couple of contexts, amplifiers at the loudspeakers and a chair-side headphone amplifier. Differential interconnects reduce the likelihood of hum introduction in these applications.
Differential Power Amplifiers?
Benchmark Media, a respected pro audio equipment producer, recommends against the use of balanced amplifiers with headphones and loudspeakers in the general case. Benchmark Media notes that differential amplifiers are useful when more power is needed. As explained above the signal is amplified and the signal is also inverted and amplified to make the A and -A lines leaving the amplifier. In single ended mode, the two lines are A and ground. So differential mode offers four times the power. (Power is Esquared/R).
Differential Mode Useful for High Power
When a stereo amplifier like Schiit Audio Vidar is run in mono mode, the balanced signal A is applied to one stage’s + input and -A to the other stage’s + input. The output is taken from the two amplifier + terminals. The – terminal is not needed. Because the amplifier is differential end to end, its output power can be quadrupled for the cost of adding a Neutrik XLR connector to the panel.
This is a very cost effective route to high power. 400 watts peak power per channel for twice the price of 100 per channel. Schiit notes that Vidar, though robust at 100 watts per channel, will be working hard in mono mode and will go into protection on some program material, particularly compressed rock or loud pipe organ works.
Differential Mode not Needed With Headphones
Benchmark Media recommends single ended amplifiers for headphone driving duties, primarily because the increased build cost outweighs the benefits of the power increase. Leave the differential amplifier to speaker amp applications where the power may be needed.
Benchmark notes that only a few milliwatts average are needed in headphone amplifications and that single ended amplifiers easily achieve the peak headroom needed by uncompressed music.
When selecting a headphone amplifier, it is important to consider headphone impedance. High impedance headphones require larger output swings than low impedance phones (power = E**2/R) to achieve a given power in the load. Schiit Audio, in its amplifier specs, notes the range of load impedances for which its headphone amplifiers are suitable. Please observe the manufacturer’s recommendations.
All headphones can be balanced
It just takes the proper cable. Each ear has its individual source and return lines. Common headphones share the return line to use a 3 wire cable and to use tip-ring-shank phone plugs for connection. By moving up to a 4 pin XLR connector, and using a 4 conductor cable, the headphones may be used with a differential amplifier in difference mode.
Benchmark Audio recommends that headphones used for critical mix evaluations be XLR connected because the XLR connector offers superior connection to that afforded by the phone plug. The difference in distortion between the two is measurable. This is the real benefit of the 4 pin XLR connection. This benefit accrues to both single ended and differential amplifiers.
Commercial headphone amplifiers like the Schiit Magnius above use a phone plug for regular headphones and an 4 pin XLR plug for differential wired (individual returns) headphones. This prevents accidental connection of amplifier outputs in parallel which will be dramatic.
If you have adequate level with a regular 3-wire connection there is no benefit of rewiring as 4-wire. If you don, your need to verify that your amplifier is rated to drive higher impedance headphones. Changing from from a 3-wire to a 4-wire connection offers a two times increase in swing when more may be needed.
Note that your cans will play a little louder with a balanced connection but they’ll have essentially the same sound with either a single ended or a balanced connection. It is not worth the bother to DIY them to be balanced.
So yes, balanced headphones are a marketing gimmick.