Balanced audio
When a phone connector is used to make a balanced connection, the two active conductors are both used for a monaural signal. The ring, used for the right channel in stereo systems, is used instead for the inverting input. This is a common use in small audio mixing desks, where space is a premium and they offer a more compact alternative to XLR connectors. Another advantage offered by TRS phone connectors used for balanced microphone inputs is that a standard unbalanced signal lead using a TS phone jack can simply be plugged into such an input. The ring (right channel) contact then makes contact with the plug body, correctly grounding the inverting input.
A disadvantage of using phone connectors for balanced audio connections is that the ground mates last and the socket grounds the plug tip and ring when inserting or disconnecting the plug. This causes bursts of hum, cracks and pops and may stress some outputs as they will be short circuited briefly, or longer if the plug is left half in.
This problem does not occur when using the 'gauge B' (BPO) phone connector (PO 316)[28] which although it is of 0.25 in (6.3 mm) diameter has a smaller tip and a recessed ring so that the ground contact of the socket never touches the tip or ring of the plug. This type was designed for balanced audio use, being the original telephone 'switchboard' connector and is still common in broadcast, telecommunications and many professional audio applications where it is vital that permanent circuits being monitored (bridged) are not interrupted by the insertion or removal of connectors. This same tapered shape used in the 'gauge B' (BPO) plug can be seen also in aviation and military applications on various diameters of jack connector including the PJ-068 and 'bantam' plugs. The more common straight-sided profile used in domestic and commercial applications and discussed in most of this article is known as 'gauge A'.
XLR connectors used in much professional audio equipment mate the ground signal on pin 1 first.
Unbalanced audio
Phone connectors with three conductors are also commonly used as unbalanced audio patch points (or insert points, or simply inserts), with the output on many mixers found on the tip (left channel) and the input on the ring (right channel). This is often expressed as "tip send, ring return". Other mixers have unbalanced insert points with "ring send, tip return". One advantage of this system is that the switch contact within the panel socket, originally designed for other purposes, can be used to close the circuit when the patch point is not in use. An advantage of the tip send patch point is that if it is used as an output only, a 2-conductor mono phone plug correctly grounds the input. In the same fashion, use of a "tip return" insert style allows a mono phone plug to bring an unbalanced signal directly into the circuit, though in this case the output must be robust enough to withstand being grounded. Combining send and return functions via single 1⁄4 in TRS connectors in this way is seen in very many professional and semi-professional audio mixing desks, due to the halving of space needed for insert jack fields which would otherwise need two jacks, one for send and one for return. The tradeoff is that unbalanced signals are more prone to buzz, hum and outside interference.
In some three-conductor TRS phone inserts, the concept is extended by using specially designed phone jacks that will accept a mono phone plug partly inserted to the first click and will then connect the tip to the signal path without breaking it. Most standard phone connectors can also be used in this way with varying success, but neither the switch contact nor the tip contact can be relied upon unless the internal contacts have been designed with extra strength for holding the plug tip in place. Even with stronger contacts, an accidental mechanical movement of the inserted plug can interrupt signal within the circuit. For maximum reliability, any usage involving first click or half-click positions will instead rewire the plug to short tip and ring together and then insert this modified plug all the way into the jack.
The TRS tip return, ring send unbalanced insert configuration is mostly found on older mixers. This allowed for the insert jack to serve as a standard-wired mono line input that would bypass the mic preamp. However tip send has become the generally accepted standard for mixer inserts since the early-to-mid 1990s. The TRS ring send configuration is still found on some compressor sidechain input jacks such as the dbx 166XL.
In some very compact equipment, 3.5 mm TRS phone connectors are used as patch points.
Some sound recording devices use a three-conductor phone connector as a mono microphone input, using the tip as the signal path and the ring to connect a standby switch on the microphone.
