Phone connector (audio)

Modern phone connectors are available in three standard sizes. The original 1⁄4 inch (6.35 mm) version descends from as early as 1877, when the first-ever telephone switchboard was installed at 109 Court Street in Boston in a building owned by Charles Williams, Jr.;[8] or 1878, when an early switchboard was used for the first commercial manual telephone exchange[9][10] in New Haven, Connecticut created by George W. Coy.[11][12] The 1877 switchboard was last known to be located in the lobby of 185 Franklin Street, Boston.[8]

In February 1884, C. E. Scribner was issued US Patent 293,198[13] for a "jack-knife" connector that is the origin of calling the receptacle a "jack".[14] Scribner was issued U.S. Patents 262,701,[13] 305,021,[15] and 489,570 relating to an improved design that more closely resembles the modern plug.[16] The current form of the switchboard-plug was patented prior to 1902, when Henry P. Clausen received a patent on an improved design.[17] It is today still used on mainstream musical equipment, especially on electric guitars.

Western Electric was the manufacturing arm of the Bell System, and thus originated or refined most of the engineering designs, including the telephone jacks and plugs which were later adopted by other industries, including the U.S. military.

By 1907, Western Electric had designed a number of models for different purposes, including:[18]

• Code No. 47 2-conductor plugs for use with type 3, 91, 99, 102, 103, 108, and 124 jacks—used for switchboards
• Code No. 85 3-conductor plugs for use with type 77 jacks—used for the operator's head telephone
• Code No. 103 twin 2-conductor plugs for use with type 91, and type 99 jacks—used for the operator's head telephone and chest transmitter (microphone)
• Code No. 109 3-conductor plugs for use with jack 92 on telephone switchboards (with the same basic shape as the modern Bantam plugs)
• Code No. 110, 3-conductor plug for use with jacks 49, 117, 118, 140, and 141 on switchboards
• Code No. 112, twin 2-conductor plug for use with jacks 91 and 99—used for the operator's head telephone and chest, with a transmitter cutout key (microphone mute)
• Code No. 116, 1-conductor plug for use with cordless jack boxes
• Code No. 126, 3-conductor plug for use with type 132 and type 309 jacks on portable street railway sets

By 1950, the two main plug designs were:

• WE-309 (compatible with 3⁄16-inch jacks, such as 246 jack), for use on high-density jack panels such as the 608A
• WE-310 (compatible with 1⁄4-inch jacks, such as the 242)

Several modern designs have descended from those earlier versions:

• B-Gauge standard BPO316 (not compatible with EIA RS-453)
• EIA RS-453: Dimensional, Mechanical and Electrical Characteristics Defining Phone Plugs & Jacks standard of 0.206 in (5.2 mm) diameter, also found in IEC 60603-11:1992 Connectors for frequencies below 3 MHz for use with printed boards – Part 11: Detail specification for concentric connectors (dimensions for free connectors and fixed connectors).

U.S. military versions of the Western Electric plugs were initially specified in Amendment No.1, MIL-P-642, and included:

• M642/1-1
• M642/1-2
• M642/2-1
• M642/2-2
• M642/4-1
• M642/4-2
• MIL-P-642/2, also known as PJ-051. (Similar to Western Electric WE-310, and thus not compatible with EIA RS-453)
• MIL-P-642/5A: Plug, Telephone (TYPE PJ-068) and Accessory Screws (1973),[19] and MIL-DTL-642F: Plugs, Telephone, and Accessory Screws (2015),[20] with 0.206 in (5.2 mm) diameter, also known by the earlier Signal Corps PL-68 designation. These are commonly used as the microphone jack for aviation radios, and on Collins S-line and many Drake amateur radios. MIL-DTL-642F states, "This specification covers telephone plugs used in telephone (including telephone switchboard consoles), telegraph, and teletype circuits, and for connecting headsets, handsets, and microphones into communications circuits."

The 3.5 mm or miniature size was originally designed in the 1950s as two-conductor connectors for earpieces on transistor radios, and remains a standard still used today.[21] This roughly half-sized version of the original, popularized by the Sony EFM-117J radio (released in 1964),[22][23] is still commonly used in portable applications. The three-conductor version became very popular with its application on the Walkman in 1979, as unlike earlier transistor radios, these devices had no speaker of their own; the usual way to listen to them was to plug in headphones. There is also an EIA standard for 0.141-inch miniature phone jacks.

The 2.5 mm or sub-miniature sizes were similarly popularized on small portable electronics. They often appeared next to a 3.5 mm microphone jack for a remote control on-off switch on early portable tape recorders; the microphone provided with such machines had the on-off switch and used a two-pronged connector with both the 3.5 and 2.5 mm plugs. They were also used for low-voltage DC power input from wall adapters. In the latter role, they were soon replaced by coaxial DC power connectors. 2.5 mm phone jacks have also been used as headset jacks on mobile telephones (see § PDAs and mobile phones).

The 3.5 mm and 2.5 mm sizes are sometimes called 1⁄8 in and 3⁄32 in respectively in the United States, though those dimensions are only approximations.[24] All sizes are now readily available in two-conductor (unbalanced mono) and three-conductor (balanced mono or unbalanced stereo) versions.

Four-conductor versions of the 3.5 mm plug and jack are used for certain applications. A four-conductor version is often used in compact camcorders and portable media players, providing stereo sound and composite analog video. It is also used for a combination of stereo audio, a microphone, and controlling media playback, calls, volume and/or a virtual assistant on some laptop computers and most mobile phones,[25] and some handheld amateur radio transceivers from Yaesu.[26] Some headphone amplifiers have used it to connect "balanced" stereo headphones, which require two conductors per audio channel as the channels do not share a common ground.[27]

By the 1940s, broadcast radio stations were using Western Electric Code No. 103 plugs and matching jacks for patching audio throughout studios. This connector was used because of its use in AT&T's Long Line circuits for the distribution of audio programs over the radio networks' leased telephone lines. Because of the large amount of space these patch panels required, the industry began switching to 3-conductor plugs and jacks in the late 1940s, using the WE Type 291 plug with WE type 239 jacks. The type 291 plug was used instead of the standard type 110 switchboard plug because the location of the large bulb shape on this TRS plug would have resulted in both audio signal connections being shorted together for a brief moment while the plug was being inserted and removed. The Type 291 plug avoids this by having a shorter tip.[28]

Professional audio and the telecommunication industry use a 0.173 in (4.4 mm) diameter plug, associated with trademarked names including Bantam, TT, Tini-Telephone, and Tini-Tel. They are not compatible with standard EIA RS-453/IEC 60603-11 1/4-inch jacks. In addition to a slightly smaller diameter, they have a slightly different geometry.[29] The three-conductor TRS versions are capable of handling balanced line signals and are used in professional audio installations. Though unable to handle as much power, and less reliable than a 6.35 mm (0.250 in) jack,[30] Bantam connectors are used for mixing console and outboard patchbays in recording studio and live sound applications, where large numbers of patch points are needed in a limited space.[29] The slightly different shape of Bantam plugs is also less likely to cause shorting as they are plugged in.

A dual 310 patch cable, two-pin phone plug

A two-pin version, known to the telecom industry as a "310 connector", consists of two 1⁄4-inch phone plugs at a centre spacing of 5⁄8 inch (16 mm). The socket versions of these can be used with normal phone plugs provided the plug bodies are not too large, but the plug version will only mate with two sockets at 5⁄8 inches centre spacing, or with line sockets, again with sufficiently small bodies. These connectors are still used today in telephone company central offices on "DSX" patch panels for DS1 circuits. A similar type of 3.5 mm connector is often used in the armrests of older aircraft, as part of the on-board in-flight entertainment system. Plugging a stereo plug into one of the two mono jacks typically results in the audio coming into only one ear. Adapters are available.

A short-barrelled version of the phone plug was used for 20th-century high-impedance mono headphones, and in particular those used in World War II aircraft. These have become rare. It is physically possible to use a normal plug in a short socket, but a short plug will neither lock into a normal socket nor complete the tip circuit.

Less commonly used sizes, both diameters and lengths, are also available from some manufacturers, and are used when it is desired to restrict the availability of matching connectors, such as 0.210-inch (5.3 mm) inside diameter jacks for fire safety communication in public buildings.[lower-alpha 1]

Two different forms are frequently found, both of which place left audio on the tip and right audio on the first ring (for compatibility with stereo connectors). Where they differ is in the placement of the microphone and return contacts.

The first, which places the ground return on the sleeve and the microphone on the second ring, is standardized in OMTP[45] and has been accepted as a national Chinese standard YDT 1885–2009. It is mostly used on older devices, such as older Nokia mobiles, older Samsung smartphones, and some Sony Ericsson phones,[46] and products meant for the Chinese market.[47][48] Headsets using this wiring may be indicated by black plastic separators between the rings.[49][48]

The second, which reverses these contacts, with the microphone on the sleeve, is used by Apple's iPhone line until the 6S and SE (1st), and has become the de facto TRRS standard, to maintain compatibility with these products.[50][51][52] It is now used by HTC devices, recent Samsung, Nokia, and Sony phones, among others. This is referred to as CTIA/AHJ, and has the disadvantage that the mic will be shorted to ground if the body of the device is metal, and the sleeve has a flange that contacts it. Headsets using this wiring may sometimes be indicated by white plastic separators between the rings.[49][48]

If a CTIA headset is connected to a mobile phone with OMTP interface, the missing ground will effectively connect speakers in out-of-phase series, resulting in no voice on typical popular music recordings where the singers are in the center; in this case, if the main microphone button is held down, shorting across the microphone and restoring ground, the correct sound may be audible.[48]

The 4-pole 3.5 mm connector is defined by the Japanese standard JEITA/EIAJ RC-5325A, "4-Pole miniature concentric plugs and jacks", originally published in 1993.[65] 3-pole 3.5 mm TRS connectors are defined in JIS C 6560. See also JIS C 5401 and IEC 60130-8.

The USB Type-C Cable and Connector Specification specifies a mapping from a USB-C jack to a 4-pole TRRS jack, for the use of headsets, and supports both CTIA and OMTP (YD/T 1885–2009) modes.[66] Some devices transparently handle many jack standards,[67][68] and there are hardware implementations of this available as components.[69] This is accomplished in some cases by applying a voltage to the sleeve and second ring to detect the wiring. The last two conductors may then be switched to allow a device made to one standard to be used with a headset made to the other.[70]

A TRRRS standard for 3.5 mm connectors was developed by ITU-T.[71] The standard, called P.382 (formerly P.MMIC), outlines technical requirements and test methods for a 5-conductor socket and plug configuration. Compared to the TRRS standard, TRRRS provides one extra conductor that can be used for connecting a second microphone or providing power to or from the audio accessory.

P.382 requires compliant sockets and plugs to be backward compatible with legacy TRRS and TRS connectors. Therefore, P.382-compliant TRRRS connectors should allow for seamless integration when used on new products. TRRRS connectors enable the following audio applications: active noise canceling, binaural recording and others, where dual analog microphone lines can be directly connected to a host device. It was commonly found on Sony phones starting with the Xperia Z1-XZ1 and Xperia 1 II.

Another TRRRS standard for 4.4 mm connectors following JEITA RC-8141C was introduced in 2015 and is used for balanced audio connections, in particular for headphone cables. This connector is often called a Pentaconn connector following the brand name of Nippon DICS (NDICS). It is used by some Sony products like the M1Z Walkman of their Signature series and by some Sennheiser products like the HD 820 headphone or the HDC 820 DAC headphone amplifier.[72][73]

When a phone connector is used to make a balanced connection, the two active conductors are used for differential versions of a monaural signal. The ring, used for the right channel in stereo systems, is used instead for the inverting input.

Where space is a premium, TRS connectors offer a more compact alternative to XLR connectors, and so are common in small audio mixing desks.

Another advantage offered by TRS 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 inverting input on the ring contact gets correctly grounded when it makes contact with the plug body.

The Sony Walkman NW-ZX300 has a balanced 4.4 mm Pentaconn output alongside a standard 3.5 mm unbalanced output.

When using non-switching phone connectors to make balanced audio connections, the socket grounds the plug tip and ring when inserting or disconnecting the plug, and the ground mates last. 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 with XLR or when using gauge B[82] which although it is of 0.25 in (6.35 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 are not interrupted by the insertion or removal of connectors. This same tapered shape used in the gauge B 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.

Alternatively, some switched audio jacks contain built-in isolated switches that only activate when the plug is fully inserted.[83] This can be used to avoid the insertion issue, for instance by wiring the connectors through a double pole, double throw switch that activates only upon full insertion. Or for instance by having the switch control a circuit that gracefully ramps up the audio once the plug is fully-inserted and mutes the audio when not fully-inserted.

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, because it halves the 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.[84]

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.