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Balanced vs unbalanced connections—what does it mean and what are the benefits?
If you’ve ever looked closely at microphones, professional audio gear, or high-end headphone amplification, you’ve likely seen the term balanced connection in reference to the equipment’s output, input, or both. If you’ve ever wondered exactly what “balanced” means, and how it works exactly, you’re in the right place!
What are unbalanced connections?
Typical analog electrical connections such as those between your turntable and amplifier, or the cable you use to plug your phone into a speaker are called single-ended (SE), or unbalanced connections. This means that a single conductor—plus a shield that provides reference to the ground, acting as a current return to complete the circuit—carries each audio signal (one for the left channel, and one for the right). The standard plug for this type of connection is the RCA for home audio. You might also see the more rugged 1/4-inch tip-sleeve (TS) jacks used in semi-professional applications and for electric musical instruments, or the 3.5mm tip-ring-sleeve (TRS) jack, which carries two unbalanced signals in stereo applications, commonly called an “aux” cable.
What are balanced connections?
Used in professional applications, balanced connections carry audio signals over cable runs that are typically much longer than those in home audio, and likely to be exposed to lots of potential sources of interference. Balanced connections use cables and connectors with at least three conductors: one for ground, as well as a matched, twisted pair of conductors—one for the “hot” signal, and one for the “cold” signal. Since the technology is primarily used for professional applications such as live music performance, recording studios, and film sets, the connectors are more rugged (and more expensive), typically either XLR connectors or 1/4-inch tip-ring-sleeve (TRS) jacks.
Why should you use balanced cables?
Professionals route signals via balanced cables in an effort to reduce noise created by interference from electromagnetic (EM) and radio frequency (RF) sources like AC mains, fluorescent lights, motors, and cell phones. These can all induce noise in a cable’s conductors. The goal is noise immunity, to reduce the influence of EMI on the wanted audio signal, and keep the audio clean and free from hums and buzzes.
How do balanced connections work?
See also: The ultimate guide to audio connections
The principle used here is called differential signalling. The concept relies on the balanced input stage of the receiving equipment amplifying only the difference between the hot and cold lines, meaning anything common to both lines is rejected. How well this is achieved is defined by the common-mode rejection ratio (CMRR). Since the cable’s two conductors are twisted together, it means they effectively occupy the same space, averaged over their length, so the noise induced should be equal in both conductors—and thus will be removed by the differential amplifier.
Think of balanced audio connections as incorporating a kind of active noise cancelling, but rather than cancelling ambient acoustic noise, it cancels noise caused by electrical interference
You can think of balanced audio connections as incorporating a kind of active noise cancelling (ANC), but rather than cancelling ambient acoustic noise like ANC headphones, this technology cancels noise caused by induced electrical interference picked along the cable run. Originally this technique was achieved with balancing transformers at the input and output of each piece of gear, but is more likely to be implemented with differential amplifiers (op-amps) in most cases due to cost.
Balanced connections are absolutely essential for microphone level signals, which are only small numbers of millivolts. The large electrical gain applied to the signal in a microphone pre-amplifier will amplify any induced noise along with the wanted signal, making it a significant problem. The same balancing approach is used for line level signals in professional applications, using the same cables and connectors.
Connections between home audio components are usually short, so using a balanced connection is unlikely to provide much benefit.
While it’s not as critical as it is for microphone level signals, if the cable run is more than a couple of feet, or is likely to encounter sources of interference like power cables, it’s necessary. In the home environment, connections between audio components are usually short, so using a balanced connection is unlikely to provide much benefit, besides the more reliable, professional XLR connectors.
What’s a balanced headphone output for?
More recently, higher tier headphone amplifiers have started to offer “balanced” outputs to drive appropriately wired headphones. Here, equivalent and opposing (positive and negative phase) versions of the same audio signals are sent to the terminals of the headphone drivers. It’d be more accurate to call this differential drive, rather than “balanced,” but the name seems to have stuck.
Ordinarily, headphones are wired with a three-pole jack: either a 1/4-inch or 3.5mm TRS type. Since each headphone driver has a positive and negative input terminal, for a total of four, the two negative terminals of the drivers are connected together at the jack’s sleeve, to share the return path to the amplifier. This is referred to as single-ended, as only the positive terminal of the connector carries any voltage swing, just like in the unbalanced connection described at the top of the article.
For a headphone to work with differential or “balanced” drive signals, it requires two independent conductors for each ear, so a 4-terminal connector and 4-conductor cabling are needed. Currently, there are four(!) standards for balanced headphone connectors:
- 2.5mm 4 pole jack
- 4.4mm 5 pole jack, aka “Pentacon”
- 4 pin XLR
- (2x) 3 pin XLR
To be compatible with a “balanced” amplifier output, a headphone needs to ship with one of these connectors attached (and you still need an adaptor if it doesn’t match the amp’s connector). If not, you’ll be at a better starting point if your headphones have a removable cable, and an aftermarket one exists with one of the above connector types. You will also find that many headphones can’t be driven differentially without extensive modifications due to the way they are wired internally. But please read to the end of this article before you start thinking about that.
Many headphones can't be driven differentially without extensive modifications... but please read to the end of this article before you start thinking about that.
So some headphone amplifiers deliver differential signals to your headphones if they’re wired correctly. However, the problem of induced noise is not an issue even with traditional single-ended headphones, because the low output impedance of the amplifier prevents significant RF interference noise. If it doesn’t help with noise, what’s the benefit?
What are the benefits of balanced headphone outputs?
Theoretically, the differential drive delivers an increase in audio performance due to the doubling of the amp’s voltage slew rate (how quickly the amp responds to input signals), lower noise, and reduced crosstalk due to the elimination of the shared audio ground between the left and right channels. But you’re highly unlikely to notice much improvement in listening. Actually, the only real tangible benefit from this is that some amplifiers can output a higher drive voltage through their “balanced” output than they can through the single-ended one. This is only really a benefit if you’re not getting sufficient level to your conventionally wired headphones.
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