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Jack plug

A jack plug is an extremely common audio connector. See also jack (connector), tip ring sleeve.

The terms phone plug, and phone jack for the corresponding socket, are also used for these connectors, especially for the original 1/4" size. Note that this is in contrast to the terms phono plug and phono jack which normally refer to RCA connectors. The potential for confusion here is heightened as the RCA jack is also known as a 1/4" phono jack and is mainly used in applications for which the 1/4" jack plug was previously (and also continues to be) used.

Jack plugs are available in three standard sizes. The original 6.5mm or 1/4", really and also known as 6.3mm, was designed for use in manual telephone exchanges. The 3.5mm or miniature and 2.5mm or subminiature sizes were originally designed as two-conductor connectors for earpieces on transistor radios. All three sizes are now readily available in two-conductor (mono) and three-conductor (stereo or tip ring sleeve) versions.

Four and five conductor versions of the 3.5mm plug are used for certain applications. A four conductor version is becoming a de facto standard output connector for compact camcorders, providing stereo sound plus a video signal. This interface is also seen on some laptop computers. Proprietry interfaces using both four and five conductor versions exist, notably the audio connector on the iPod MP3 player, where the extra conductors are used to supply power for accessories.

Both two-conductor and three-conductor versions of the three standard sizes are readily available in male (plug) and female (socket or simply "jack") line versions, and panel-mounting female versions. Panel-mounting male versions of these also exist but are rare, as they are vulnerable to mechanical damage and therefore unreliable. Female line versions are also notoriously unreliable and are avoided by many users.

So, by far the most common arrangement remains to have the male plug on the cable, and the female socket mounted in a piece of equipment, which was the original intention of the design. A considerable variety of line plugs and panel sockets is available, including plugs suiting various cable sizes, right angle plugs, and both plugs and sockets in a variety of price ranges and with current capacities up to about 15 amperes for the 1/4" version.

Non-standard 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.

Several obsolete versions of the 1/4" jack plug exist, including:

Table of contents
1 Mono and stereo compatibility
2 Uses
3 Switch contacts

Mono and stereo compatibility

In the original application in manual telephone exchanges, many different configurations of 1/4" jack plug were used, some accommodating five or more conductors, with several tip profiles. Of these many varieties, only the two-conductor version with a rounded tip profile was compatible between different manufacturers, and this was the design that was at first adopted for use with microphones, electric guitars, headphones and many other items of audio equipment.

Old profile jack plugs. The leftmost is three conductor, all others are two.
At the top is a three-conductor jack from the same era.

When a three-conductor version of the 1/4" jack was introduced for use with stereo headphones, it was given a sharper tip profile in order to make it possible to manufacture jacks (sockets) that would accept only stereo plugs, to avoid short-circuiting the right channel amplifier. This attempt has long been abandoned, and now the normal convention is that all plugs fit all sockets of the same size, regardless of whether they are mono or stereo. Most 1/4" plugs, mono or stereo, now have the profile of the original stereo plug, although a few rounded mono plugs are also still produced. The profiles of stereo miniature and subminiature plugs have always been identical to the mono plugs of the same size.

Modern profile 2-conductor 1/4" jack plugs.
''For a photo of the 3-conductor, miniature and subminiature versions see tip ring sleeve.

The results of this physical compatibility are:


Some common uses of jack plugs and their matching jacks are:

Switch contacts

Panel-mounting jacks are often provided with switch contacts. Most commonly, a mono jack is provided with a single normally closed (NC) contact, which is connected to the tip (live) connection when no plug is in the socket, and disconnected when a plug is inserted. Stereo sockets commonly provide two such NC contacts, one for the tip (left channel live) and one for the ring or collar (right channel live).

Less commonly, some jacks are provided with normally open (NO) or change-over contacts, and/or the switch contacts may be isolated from the connector.

The original purpose of these contacts was for switching in telephone exchanges, for which there were many patterns but two sets of change-over contacts, isolated from the connector contacts, were common. The more recent pattern of one NC contact for each signal path, internally attached to the connector contact, stems from their use as headphone jacks. In many amplifiers and equipment containing them, such as electronic organs, a headphone jack is provided that disconnects the loudspeakers when in use. This is done by means of these switch contacts. In other equipment, a dummy load is provided when the headphones are not connected. This is also easily provided by means of these NC contacts.

Other uses for these contacts have been found. One is to interrupt a signal path to enable other circuitry to be inserted. This is done by using one NC contact of a stereo jack to connect the tip and ring together when no plug is inserted. The tip is then made the output, and the ring the input, thus forming a patch point. See tip ring sleeve.

Another use is to provide alternative mono or stereo output facilities on some guitars and electronic organs. This is achieved by using two mono jacks, one for left channel and one for right, and wiring the NC contact on the right channel jack to connect the two connector tips together when the right channel output is not in use. This then mixes the signals so that the left channel jack doubles as a mono output.

Where a 3.5mm or 2.5mm jack is used as a DC power inlet connector, a switch contact may be used to disconnect an internal battery whenever an external power supply is connected, to prevent incorrect recharging of the battery. On some DI units and guitar effects units, a normally open switch contact on one of the signal connectors is used to connect an internal battery whenever a cord is connected. The desired effect is to switch the unit off whenever the cord is removed, a side effect is to flatten the battery if it is not, for example if equipment is left connected overnight.