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:

  • A two-pin version, consisting of two mono 6.5mm jack plugs at a centre spacing of 1". The socket versions of these can be used with normal jack plugs provided the plug bodies are not too large, but the plug version will only mate with two jack sockets at 1" centre spacing, or with line sockets, again with sufficiently small bodies. These connectors were commonly used on early stereo tape recorders.
  • A short-barrelled version, once used on high-impedance mono headphones, and in particular those used in World War Two aircraft. 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. These are still manufactured but are now regarded as a non-standard size.

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:

  • If a two-conductor plug of the same size is connected to a three-conductor socket, the result is that the ring (right channel) of the socket is grounded. This is deliberately used in several applications, see tip ring sleeve. However it may also be dangerous to the equipment if the result is to ground (short circuit) the output of the right channel amplifier.
  • If a three-conductor plug is connected to a two-conductor socket, normally the result is to leave the ring of the plug unconnected (open circuit). In the days of valves this was also potentially dangerous to equipment but most solid state devices tolerate this condition well. A stereo socket could be wired as a mono socket but to ground the ring in this situation, but the more conventional wiring in this case is to leave the ring unconnected, exactly simulating a mono socket.


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

  • Headphone and earphone jacks on a wide range of equipment.
  • Microphone inputs on tape and cassette recorders, sometimes with remote control switching on the ring, see tip ring sleeve.
  • Patching points on a wide range of equipment, see tip ring sleeve.
  • Personal computer sound cards. Stereo 3.5mm jacks are used for:
    • Line in (stereo)
    • Line out (stereo)
    • Headphones/loudspeaker out (stereo)
    • Microphone input (mono, sometimes with 5v power available on the ring, see tip ring sleeve).
  • Electric guitars. Almost all electric guitars use a 6.5mm mono jack (socket) as their output connector. Some makes (e.g. Shergold) use a stereo jack instead for stereo output, but more commonly a second mono jack is provided (e.g. Rickenbacker).
  • Instrument amplifiers for guitars, basses and similar amplified musical instruments. 6.5mm jacks are overwhelmingly the most common connectors for:
    • Inputs. A shielded cable with a mono 6.5mm jack plug on each end is commonly called a guitar cord or a patching cord, the first name reflecting this usage, the second the history of the jack plug's development for use in manual telephone exchanges.
    • Loudspeaker outputs.
    • Line outputs.
    • Foot switches. Stereo plugs are used for double switches (e.g. by Fender). There is little compatibility between makers.
    • Effects loops, which are normally wired as patch points, see tip ring sleeve.
  • Electronic keyboards use jacks for a similar range of uses to guitars and amplifiers, and in addition
    • Sustain pedals.
  • Some compact and/or economy model audio mixing desks use stereo jacks for balanced microphone inputs, see tip ring sleeve.
  • Much professional audio equipment uses mono jacks as the standard unbalanced input or output connector, often providing a 6.5mm unbalanced line connector alongside and as an alternative to an XLR balanced line connector.
  • Some small electronic devices such as audio cassette players, especially in the cheaper price brackets, use a two-conductor 3.5mm or 2.5mm jack as a DC power connector.

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.

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