• Balanced and unbalanced connections

    There are two basic ways to carry an electrical audio signal.

    • The first one is unbalanced. The signal is carried on a two-conductor cable. Unbalanced signal connectors have two pins, such as the RCA (also called Phono and Cinch, commonly used by home hi-fi equipment) and the unbalanced 1/4" (used in musical instruments and semi-professional audio). Multi-pin connectors can also carry unbalanced signal, although they will not use all pins). For example, a three-pin XLR (Cannon) might carry an unbalanced signal, leaving one pin unused. Home computers use almost all unbalanced connections.

      Unbalanced connections are very simple, and are commonly and easily used for the connection of many musical instruments. The reason that these types of connections are not considered "professional" is that they are very susceptible to contamination from electro-magnetic interference, particularly when the cable distances are long.

    • The other way is balanced. The signal is carried twice, one of them with reverse polarity. This is known as signal balancing. To carry a balanced signal we need three-pin connectors and a three-conductor cable, one of which is the screen mesh (sleeve) of the cable. Electro-magnetic interference that does not reject the cable shield will affect the two wires carrying the signal in the same way. The input of the device to which we carry the signal does what is known as unbalancing, which consists of adding the two signals that reach it after inverting one of them. Since one signal is inverted with respect to the other in the cable, the balancing process manages to reinforce (double) the original signal and cancel the interference produced in the cable. In practice the attenuation of interference is very complex and the expected results are not always achieved, although in any case the balanced signal transport is preferable for professional applications. The CMRR (Common Mode Rejection Ratio) parameter expresses the attenuation of an interference that is introduced in equal amounts into the conductors carrying the signal, and usually ranges between 60 and 80 dB, which are given by the tolerances of the input unbalancing circuit, and which define the accuracy of the sum of the signals. The following illustration graphically explains the balancing process: the output device produces two copies of the same signal, one of which is inverted; if there is interference, it is produced in the same way in the two signals carried by the cable; in the target device, the signals are added together after one of them is inverted, cancelling the interference.

      Three-pin connectors are required to carry a balanced signal, such as XLR and stereo 1/4" connectors. The terminals are usually named positive or hot, negative or cold, and ground (sleeve).

      In a 1/4" connector, typically the positive terminal is connected to the tip, negative to the middle ring, and ground to the sleeve (mesh) of the cable. The 1/4" connector with three pins is referred to as TRS (tip-ring-sleeve). In any case it is sometimes convenient to make sure that the manufacturers of our devices follow the usual pin assignment conventions, whatever the connector.

      Typical pin assignment for 1/4" connectors

      Nowadays, for XLR connectors, the most common practice is to assign the terminals according to the AES standard, so that pin 2 is connected to positive, pin 3 to negative and pin 1 to sleeve. In the past, many manufacturers connected the 2 and 3 the other way round (coincidentally this was the way described by the original manufacturer, Cannon), so that the interconnection of equipment could cause mismatch problems, although almost all manufacturers seem to have now adopted the AES polarity convention (although older models that are still in production, such as a DN360 graphic equalizer, may maintain the opposite polarity, as well as discontinued models still in use).

      Typical pin assignment for XLR connectors

      Within balanced connections, we can differentiate between transformer-balanced and electronically balanced connections.

      Electronically balanced. For this we need a device with a balanced output and one with a balanced input. Often the ground is lifted at the input to prevent ground loops, which cause hum. This is the most common form of balancing, usually implemented in professional equipment.

      Transformer balanced. For this we need an input or output transformer. Normally it doesn't make sense to use a transform in both the input and the output, since with just one we have electrical isolation that will allow us to avoid hum from ground loops. The main disadvantage of transformers is that it is difficult to find them with linearity and distortion characteristics approaching those of an electronically balanced system. Those of sufficient quality are usually very expensive. The Jensen brand is the reference in transformers for signal balancing, a niche market. In general, transformer balancing is not used very often, and only very high-end equipment incorporates it, often only as an option. Perhaps the most rational option when using transformer balancing is to use it at the input only, which combines the advantages of electronic balancing with those of transformer balancing, providing ground isolation. In addition, input transformers are less bulky and lighter than output transformers.