Looking to get involved in professional music production? Or simply finding the best way to operate your club’s PA system? Then you need to learn more about the concept of balanced audio. Sound engineers everywhere are always discussing about the best ways to utilize balanced audio, whether in the studio or live. So what is balanced audio exactly?
In today’s article, we will be covering the core aspects, surrounding the subject of balanced audio. You will be able to find out more on how this method is implemented throughout various applications in the professional audio world. Learn how this concept can be a huge benefit to not only sound engineers, but also to your own home productions!
Introduction – The concept
The method that is used to interconnect various audio equipment using “balanced lines”, is known as balanced audio. This concept of audio connection is very important and universally practised in sound recording and music production as it permits the use of long audio cables without much loss in terms of the electrical audio signal’s strength.
Connections for balanced audio require the use of three-conductor connectors, typically the XLR or TRS phone connector. The XLR connectors for example, are almost always used with professional microphones due to their durable and rugged construction, while TRS jack plugs are widely used on mixing consoles for signal inputs and outputs due to their compact profile.
Most microphones that are used in professional audio operate at low voltage levels, with some having high output impedance (hi-Z), making long microphone cables more prone to electromagnetic interference. Hence, microphone cables (with XLR connectors) are ideal for balanced interconnections, which will cancel out most of this induced noise caused by the electromagnetic interference.
In a live sound situation, it is very common to also use balanced lines for the routing of electrical audio signal from the main outputs of the mixer to power amplifiers, regardless of the distance between these two components. Many other signal processors, such as graphic equalizers and reverb effects units, are also designed with balanced inputs and outputs for interconnections.
Let us look at the various aspects that we will discuss further in this article:
- Interference reduction
- Differential signalling
- Internally balanced audio design
A balanced cable will often contain two identical wires, that are twisted together. The twisted wires will then have a third conductor (foil or braid) wrapped around it, which functions as a shield. Ultimately, the two wires form a circuit which carries the electrical audio signal – one wire is in phase in relation to the source signal, the other wire is reversed in polarity.
When you take a look inside of most balanced cables, you will find that “in-phase” wires are often referred to as “non-inverting”, “positive” or “hot” while wires that are “out-of-phase” normally called “inverting”, “phase-inverted”, “anti-phase”, “negative” or “cold”. On circuit diagrams, the hot and cold connections are often identified as “In+” and In−” (“in plus” and “in minus”).
The term “balanced” is derived from the method of connecting each wire to both source (output) and load (input) that have the same impedance. This results in an equal noise voltage in each wire, induced by electromagnetic interference. The amplifier at the receiving end only amplifies the “difference” in voltage between both signal lines, thus, identical noise on the two wires are rejected.
Other than noise that will be induced by electromagnetic interferences, there can also be random noise or “white noise” with no coherence, or coherent noise introduced by a device’s processing algorithms. By having the inverted line (180 degrees out-of-phase) to be flipped back into phase with the first “in-phase” line, it will cancel out these types of noise. This whole process that is known as differential signal recombination is often implemented with a “differential amplifier”.
The wires in a balanced cable are almost always twisted together, to minimize the effects of electromagnetic induction. By using this twisting method, the loop area between the conductors will be very small, thus ensuring that equal levels of noise will be induced on both lines (by a magnetic field passing equally through adjacent loops). This induced noise is then cancelled out by the differential amplifier.
If by any chance that the noise source (such as power cables or electronic devices) is extremely close to the cable, then there is a possibility that the noise will be induced on one of the lines more than the other. Take note that this “excess” noise will not be cancelled either, but cancelling will still occur to the extent of the amount of noise that is equal on both lines.
The outer shield
Unlike an “unbalanced” audio cable, a balanced audio connection will always include a separate shield (that is essentially a conductor as well) which provides a noise rejection advantage as compared to an unbalanced two-conductor design (typically found in home stereo systems), where the shield has to also function as the signal return wire at the same time.
Another major benefit to having this outer shield is that, any noise currents induced into this balanced cable shield, will not be directly modulated onto the signal (carried by the two “hot” and “cold” conductors). However in a two-conductor system (unbalanced) they will be. This also helps to prevent ground loop issues, by having the shield or chassis to be separated from signal ground.
When using balanced connections, audio signals are often transmitted in “differential mode” over balanced lines. This means that the wires carry signals that are in opposite polarity to each other (Take an XLR connector for instance, pin 2 carries the normal polarity signal, while an inverted version of that is carried by pin 3). Contrary to what most engineers think, this concept is not vital for noise rejection.
What’s crucial is that the impedances are balanced, so noise will be induced equally into the two wires (ultimately rejected by the differential amplifier), regardless of the signal strength on them. A typical and simple method of establishing a balanced line, is by routing the signal into the “hot” wire via a known source impedance, and connecting the “cold” wire to the signal’s local ground reference via an identical impedance.
Differential signalling does however, still have its minor benefits:
- Electromagnetic field surrounding a differential line will ideally be zero, thus reducing “crosstalk” into adjacent cables, useful for telephone pairs.
- If both wire carries half the signal voltage (as in fully differential outputs) then having longer cable runs without losing high frequencies would be possible.
- Correlated noise caused between two power amplifiers (due to imperfect power supply rejection, for example), would be cancelled out.
Internally balanced audio design
Almost every professional audio equipment (studio, live sound etc.) provides differential balanced inputs and outputs, mostly using XLR or TRS phone connectors. It is good to take note that a balanced input signal will be internally converted to a single-ended signal (usually via a differential amp). After signal processing, this signal is converted back to a balanced signal before reaching the output.
There are some professional audio products out there, that have been engineered to have an entirely differential balanced signal circuit, from its input to output (meaning the audio signal never unbalances). Very simply, this design is established by creating identical (mirrored) internal signal paths for both pin 2 and pin 3 signals (known as “hot” and “cold” audio signals).
Here’s a great video illustrating how balanced and unbalanced audio works!
Other than the 3-pin XLR connector, which is a widely used balanced connector, the quarter-inch (¼” or 6.35 mm) TRS phone (tip-ring-sleeve) connector is also very common. You can find many hybrid jacks now, that are designed to accept either XLR or TRS phone plugs. For the TRS phone plugs, the tip is “hot” (positive), the ring is “cold” (negative), and the sleeve is ground (earthed or chassis).
If you send a stereo or binaural signal into such a jack (balanced), one channel (typically the right) will be rejected leaving only the left channel. This results in an undesirable L − R (left minus right) signal, instead of the regular monophonic L + R. If the polarity is reversed in a balanced audio system, it will also lead to the same outcome at some point, when it is later mixed-down with its other channel.
Unbalanced signals can very easily be converted to balanced signals by an electrical component called a “balun”, which is found in a DI unit (an electrical device often used in professional audio).
In situations where balanced audio signals have to be routed into unbalanced connections, the electronic circuit design used for the balanced output stage must be identified. Most of the time, the negative output can be tied to ground, but in some cases the negative output is best left disconnected.
We have come to the end of this article. I hope you have found this presentation to be educational and beneficial. Now go and start using balanced connections for all your recording projects!
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