Adapter cable

Overview

The term "Adapter cable" is used to describe an interconnect cable with different connectors on each end. This type of cable can be used to either connect two pieces of equipment that both have either balanced or unbalanced connections; or to connect balanced equipment to unbalanced equipment.

Basics

There are two basic types of adapter cables:

1. Cables that have connectors for balanced connection on both ends or cables that have connectors for unbalanced connections on both ends. These will be referred to as "Balanced or Unbalanced adapter cables."
2. Cable that have connectors for balanced connections on one end and unbalanced connections on the other end. These will be referred to as "Balanced to Unbalanced adapter cables."

Balanced or Unbalanced adapter cables

Balanced adapter cables

"Balanced" adapter cables must be constructed with a cable that has a "twisted-pair" of signal conductors. This means simply that there are two signal conductor wires of equal size and insulation that are twisted along the length of the cable. Twisting the conductors together both helps to insure they retain similar and consistent impedance characteristics and helps reduce the pick-up of interference induced on them by electromagnetic fields. Further isolation from interference is usually provided by an over-all "shield" in the form of a tubular conductor through which the twisted-pair runs. The shield is typically connected to some form of ground that is referenced to signal ground.

The connectors must have three conductors to provide connection for the twisted pair and shield. The two most common type of connector used for analog audio interconnection are the 3-pin XLR and 1/4" T.R.S. "phone" plug. The name "phone" refers to the original use of this type of connector in early telephone "switchboards."

Unbalanced adapter cables

"Unbalanced" adapter cables are usually constructed using "coaxial cable" which means simply that there is one central signal conductor surrounded by a tubular "shield" that serves as both the shield and signal return. The connectors must have two conductors to provide connection for the signal conductor and shield. The two most common type of connector used for analog audio interconnects are the "RCA" or "phono" plug and the 1/4" T.S. phone plug.

Balanced to Unbalanced adapter cables

For information on adapter cables made with XLR connectors; please see XLR. To make an unbalanced connection using an adapter or adapter cables in which one end is a T.R.S. 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; the sleeve and ring 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, the sleeve is NOT connected to the ring; 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 T.R.S. 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. 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 T.R.S. 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 T.R.S. connector (receiving end) and the pair is connected to tip and ring similar to a balanced connection.