XLR

Overview

The term "XLR" is used to describe a latching cylindrical connector commonly used in professional audio electronics for balanced connections; both in analog interconnects and digital interconnects. Some "high-end" HiFi equipment also use XLR connections. The three contact version (contacts are also referred to as "pins") is typically used to carry one analog or two digital audio channels. Other versions with more than 3 pins are used to carry more than two channels of analog audio or for other uses such as power supply or simple "wired" remote control functions.

History

In the past, the XLR connector was sometimes referred to as a “Cannon” connector because it was originally invented by James H. Cannon who founded the Cannon Electric company. The designation “XLR” was a part number suffix derived from the original “X” series designation with “L” for “latch” and “R” for a rubber-like plastic used to surround the female connector’s contacts. Due in part to the popularity of the connector; other manufacturers such as Amphenol, Switchcraft, and Neutrik introduced compatible versions of the XLR in their own design.

Basics

The most common use of the 3 pin XLR is to make balanced analog audio connections. Pin 1 is used for the Shield “ground” connection; and as a safety feature, the original XLR was designed for Pin 1 to connect before the other Pins, which carry the audio signal. The most common form of cable used in balanced audio connections is shielded twisted-pair cable. The two “center” conductors are twisted together to increase noise-immunity in the balanced system, and these two conductors carry the “non-inverted” and “inverted’’ audio signals. A similar type of cable is used for professional AES/EBU stereo digital audio connections. The important difference is that, due to the much higher frequency of the digital audio information; the cable’s electrical characteristics are more critical and must be specifically designed for the application with a 110 Ohm impedance.

• The conductor that carries the “non-inverted” (normal polarity) audio signal is designated the “+,” “high,” or “hot” conductor and is typically connected to Pin 2. Lavry Engineering does not use the designation “Hot” in discussions of balanced signals because of the ambiguity due to the term also being used to describe the “non-ground” signal conductor in unbalanced connections.
• The conductor that caries the “inverted” (inverted polarity) audio signal is designated the “-,” “low,” or “cold” conductor and is typically connected to Pin 3. Lavry Engineering does not use the designation “Cold” in discussions of balanced signals because of the ambiguity due to the term also being used to describe the “grounded” signal conductor in unbalanced connections.

XLR Adapters and Adapter Cables

To make an unbalanced connection using an adapter or adapter cables in which one end is an XLR connector; there are multiple configurations of signal/pin connections.

1. In “standard” off-the-shelf adapters or adapter cables made with coaxial two-conductor cable; Pin 1 and Pin 3 are typically connected to each other; which results in the “-” signal conductor being held at ground voltage potential. Because electrical signals can exist as both a voltage and current; even though the voltage of the signal does not “appear” in the “-” conductor; the signal current does use this conductor as a signal return to the sending device. In this case, the shield of the coaxial cable acts as both a shield and a signal return.
2. In adapter cables that use shielded twisted-pair cable, Pin1 is NOT connected to Pin 3; the cable conductors are connected in the same manner as balanced connection; so the Shield and signal return are not the same conductor and the “-” conductor of the pair acts as the signal return while remaining protected by the shield. On the other end of the cable, the unbalanced (two conductor connector) can be connected in one of two manners:

(a) With the shield connected at the receiving end.

• If the sending end is the XLR and the receiving end is the unbalanced two-conductor connector; both the shield and the “-” conductor of the pair are connected to the “shield” connection of the two-conductor connector. The “+” conductor of the pair is connected to the signal conductor of the connector.
• If the sending end is the unbalanced two-conductor connector; both the shield and the “-” conductor of the pair are connected to the “shield” conductor of the connector. The “+” conductor of the pair is connected to the signal conductor of the connector.

(b) With the shield NOT connected on the receiving end to reduce hum and noise pick-up or to eliminate a ground loop. Please note that for safety reasons, it is essential that another form of “ground reference” connection MUST exist between two pieces of equipment that are connected with the shield disconnected at the receiving end. This can be in the form of both pieces having 3 prong (grounded) AC power cables plugged into a common AC power source.

• If the sending end is the XLR and the receiving end is the unbalanced two-conductor connector; the shield is connected to nothing on the receiving end; and the “-” conductor of the pair is connected to the “shield” connection of the two-conductor connector. The “+” conductor of the pair is connected to the signal conductor of the connector.
• If the sending end is the unbalanced two-conductor connector; both the shield and the “-” conductor of the pair are connected to the “shield” conductor of this connector. The “+” conductor of the pair is connected to the signal conductor of this connector. The shield is connected to nothing in the XLR connector (receiving end) and the pair is connected to Pin 2 & 3 similar to a balanced connection.

Although the wiring of the cable or adapter are similar for analog a digital applications; there are important difference in how a balanced analog output and digital output will react to these connections.

Analog

There are three types of balanced analog outputs:

1. Transformer-coupled- this type of output is effectively “floating” by nature unless one side or the other (“+” connection or “-” connection) is connected to ground. It is referred to as “floating” because it is not “tied” to ground electrically. This also means it is OK to ground one of the signal conductor in unbalanced connection. Due to the unique nature of this output, the level is the same whether the output is connected to balanced or unbalanced connections.
2. Electronic output with automatic sensing- this type of output is convenient because a “cross-coupled” circuit allows the amplifier to “sense” when one of the outputs is grounded. This both prevents an active output from trying to drive a “short circuit” caused by connecting it to ground, and boosts the gain of the active (un-grounded) output by 6dB to compensate for the signal level lost when one-half of the signal is removed by grounding one of the two outputs. Unless care is taken in how one of the two outputs is grounded; the distortion performance of this type of output can be significantly degraded. Even when carefully connected to the best available ground; this type of output cannot achieve the lowest distortion available from electronic outputs that are not “auto-sensing” in design. These outputs are “ground referenced” as versus “floating.”
3. Electronic output without automatic sensing- this type of output offers the advantage of the lowest distortion under all operating modes. With this type of output; it is very important to properly configure the output connection so that audio ground is connected to the Pin that will be grounded by the unbalanced connections (typically Pin 3). This is usually accomplished by changing a jumper setting inside the unit or via a setting. These outputs are “ground referenced” as versus “floating;” and by using only one of the two active outputs, the output level is reduced by 6dB for the same output level setting. Please note that all Lavry analog balanced outputs are of this design; so it is important to properly configure the output before making unbalanced connections to avoid increased distortion of the audio signal. The outputs are protected against short-circuits; so it is very unlikely that any damage would result if an output is improperly connected; unless operated in this manner for an extended period of time.

Digital

All Lavry AES/XLR digital inputs and outputs are transformer-coupled. This means that Pin 1 and Pin 3 can be (and should be) connected together in adapters or adapter cables used to make connections to coaxial S-PDIF inputs or outputs. The only exception would be where shielded twisted-pair cable is used in an adapter cable to connect an S-PDIF (unbalanced) output to an AES/XLR input. If the Shield is connected at the XLR end; Pin1 and Pin 3 would not be connected to each other.

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