Balanced Vs Unbalanced Audio XLR Ground-Live-Return vs RCA Connectors
Balanced audio connections are extremely important in sound recording and production because they allow for the use of very long cables with reduced introduction of outside noise. The most common balanced connector is the 3-pin XLR, which is used with microphones because of its durable construction. Many microphones have low impedance (low-Z), which makes long microphone cables susceptible to some forms of outside noise, and a perfect application for a balanced line, which cancels out most of this outside noise.
A balanced audio connection has two wires, one of inverted polarity to the other. (For instance, in an XLR connector, pin 2 carries the signal with normal polarity, and pin 3 carries an upside-down version of the same signal.) However, an XLR plug also carries a third connection – pin 1 is used as an earth to shield the other two. The received signal is the difference between the two signal lines. This signal recombination can be implemented with a differential amplifier where the negated signal is tied to the negative terminal of the operational amplifier. A balun may also be used instead of an active differential amplifier device. Much of the noise induced in the cable is induced equally in both signal lines, so this noise can be easily rejected – the noise received in the second, negated line is applied against the first, upright signal, and cancelled out when the two signals are added together.
The separate shield of a balanced audio connection also yields a noise rejection advantage over a typical two-conductor arrangement such as used on domestic hi-fi where the shield is actually one of the two signal wires and is not really a shield at all, but relies on its low, but in practice not zero, impedance to signal ground. Any noise currents induced into a balanced audio shield will not therefore be directly modulated onto the signal, whereas in a two-conductor system they will be. This also prevents ground loops.
If the power amplifiers of a public address system are located at any distance from the mixing console, it is also normal to use balanced lines for the signal paths from the mixer to these amplifiers. Many other components, such as graphic equalizers and effects units, have balanced inputs and outputs to allow this. In recording and for short cable runs in general, a compromise is necessary between the noise reduction given by balanced lines and the noise and distortion introduced by the extra circuitry they require.
Most domestic audio equipment has unbalanced audio inputs and outputs. This means that the audio output (left, right, or mono) appears on a single wire, and is referenced to “0V” or “Ground”. Typical connectors used are RCA “phono” connectors, DIN plugs/sockets, and 0.25″ (6.3mm) or 3.5mm jack plugs/sockets.
Unbalanced audio is fine for the domestic environment, and for line-level signals in a typical broadcast studio. Problems start to occur if the signals are being carried over long distances, especially if the source and destination have separate mains supplies.Balanced Audio
A lot of professional equipment will have balanced audio inputs and outputs, usually on 3-pin “XLR” connectors. A balanced audio signal consists of a pair of wires carrying the audio signal in anti-phase with each other. Relative to 0V, if one wire carries a positive voltage, the other carries an equal and opposite negative voltage.
The 3 wires used in a typical XLR lead are often referred to as Ground, Live and Return, or Ground, Hot and Cold. “Live” and “Return” carry the “in-phase” and “out-of-phase” versions of the audio respectively. Calling the signals “Live” and “Return” makes it nice and easy to remember which goes on which pin of the XLR plug/socket:
X = Ground
L = Live
R = Return
The advantage of balanced audio over unbalanced audio is it’s ability to reject external interference added as the signal is carried over the wire. The receiving equipment takes the voltage difference between the two wires as the input signal. Interference will usually get added to both wires equally, and so gets ignored by the receiving equipment.
Often, it is necessary to connect a balanced output to an unbalanced input, or vice-versa. The 100% correct and official way to do this is via a transformer or an electronic converter made from an op-amp. There are also some reasonably-priced ICs especially designed for the task – check out the SSM2141/2/3 from Analog Devices for example.
Assuming you are within the relatively benign surroundings of somewhere such as a radio studio however, there are simpler ways to connect balanced and unbalanced inputs and outputs. All result in unbalanced signals on the wires, so don’t do this if you fear interference!
To connect an unbalanced output to a balanced input, simply connect the unbalanced signal to the “Live” side of the balanced input, and connect the Ground side of the output to both the “Return” and “Ground” of the input. Some unbalanced outputs assume they will be connected to a high-impedance input, so if your input is switchable between high-impedance and 600 Ohm, make sure it is set to high-impedance. If it is low-impedance only, you may be out of luck! If you’re desparate to get a signal, try using the headphone output to feed the equipment – just make sure you turn the levels down to start with, and turn them up slowly until you get a reasonable level. The sound quality may not be brilliant, but you will get half-decent sound – better than trying to cope with the very low input level caused by feeding a high-impedance output into a low impedance input!
A word of warning. XLR inputs on mixers are also used for Microphone inputs. These assume an input level of somewhere in the region of -60 to -40dB. Feeding them with a 0dB input level is not recommended. Make sure that you use only line-level inputs, or that the input has been switched into line-level mode. If you only have a mic-level input, you can make do by using a resistive divider to reduce the input level. An attenuation of 60dB is a factor of 1000, so put a 10k resistor and a 10 ohm (or 100k and 100 ohm) resistor in series between the output and ground. Now connect the live and return pins of the input across the 10 ohm resistor.
To connect a balanced output to an unbalanced input, there are a couple of choices.In both cases, the “Live” side of the output is connected to the signal input.The other connection depends on whether the output is transformer isolated (in which case the “Return” side is connected to the input ground) or electronically balanced using ICs (in which case the Ground pin of the output may be connected to the input ground instead). The latter will result in a 6dB (i.e. factor of 2) loss of amplitude, and perhaps reduced signal-to-noise ratio.
Why have we got two possibilities? If you try to get a signal between a transformer balanced output and ground you will get nothing – the signal is not referenced to ground at all. On the other hand, if you connect the “Return” output of an electronically-balanced output to the input ground, you will be shorting that output to ground, and the chip may not like it. From an audio fidelity point-of-view, it is obviously better to use the “Return” output rather than the “Ground”, so how do you know when it is safe to do so? If you’re being professional, you could open-up the equipment, find-out which IC is being used and then go and read the datasheet. The more practical solution is to “suck it and see” – try it out and see if the audio sounds distorted or looks distorted on an oscilloscope – you’re most unlikely to cause permanent damage to the chip as it will have built-in short-circuit protection.
If you regularly need to connect balanced outputs to unbalanced inputs, you may like to make up a lead with a switch in circuit to select whether the return or ground connection on the balanced output is connected to ground on the unbalanced input.
Reproduced from Nigel Dallard, Chief Engineer, Winchester Hospital Radio
9 October 1999
personally. it seems to many people worry to much about this. that to be it’s not worth worrying about. i’ll stick with unbalanced for my home theater and 2 channel rigs.
More on XLR to RCA if you need to wire it up.
More Info on Balanced Vs Unbalanced
input connections: Line 1 – 1/4 inch Line 2 – XLR
+ Tip Pin2
– Ring Pin3
Shield Sleeve Pin 1
LINE 1 INPUT This jack accepts a 1⁄4” TRS connector, wired either
balanced or unbalanced.
For balanced wiring, a 3-conductor 1⁄4″ TRS plug is necessary. See
the INPUT CONNECTIONS wiring chart below or on the amplifier
Unbalanced wiring works with either a 2 or 3-conductor 1⁄4” TRS
connector. A 2-conductor 1⁄4″ TS plug automatically grounds the
minus signal input, whereas a 3-conductor TRS plug wired
unbalanced provides the option of leaving the minus input open or
grounded. We recommend that you ground the unused input.
The LINE 1 INPUT connector is hardwired in parallel with the LINE 2
INPUT connector, so either may be used as an input or a pass-through
connection. Input specifications apply to both inputs.
LINE 2 INPUT This jack accepts XLR input connections, wired either
balanced or unbalanced. The LINE 2 INPUT connector is hardwired in
parallel with the LINE 1 INPUT connector, so either may be used as an input
or a pass-through connection. Input specifications apply to both inputs.