The jack plug and socket are a very common form of connector. They may also be referred to as the phone plug. They have been in use for very many years, being originally designed in the 1878 for use with telephone circuits - they were widely used in telephone switchboards. In view of their long period of service, this makes them the oldest form of electronics connector in regular use today.
The jack, phone, or telephone connectors are able to provide very reliable service, although they are only suitable for low frequency operation and they are not suitable for connections above 20 kHz or so. Accordingly they are widely used for audio applications. Virtually every set of headphones or earpieces will use a jack connector of one size of another. While they may not represent the very latest in connector technology, their use is so embedded in consumer audio, that they are unlikely to be replaced for very many years to come.
Jack connector standards and specifications
Jack connector sizes
There are a number of standard sizes for jack connectors in use these days. They are:
- 1/4" jack (6.35 mm) or standard jack
- 3.5mm also known as the miniature jack
- 2.5mm also known as the subminiature jack
Mono and stereo variants
In addition to the basic size there are other variations to the jack plugs and jack sockets. The main one of that there are variants to cater for mono and also for stereo applications. Mono and stereo variants of jack plugs and sockets are available in all sizes.
The basic mono format was the first to be used. This only required the use of a line or signal connection and an earth or return. Mono jack connectors therefore only possess two connections, the tip and the body. Mono jack connectors were the first types to be designed and manufactured.
With the introduction of stereo in the 1950s and 1960s it became necessary to be able to modify the basic mono jack connector to be able to carry stereo signals. Stereo requires the use of an earth or return as well as one signal line for each of the two stereo channels. Thus a total of three connections are required instead of the two provided by the original "mono" jack connector. The additional connection is accommodated by providing a "ring" between the connection at the tip, and the body or earth connection.
Jack connectors with switches
Many jack sockets incorporate switches so that when the plug is inserted some switch contacts are made or broken. The original requirement for this arose so that when the jack plug was inserted to enable headphones to be used, then the speaker output was disconnected. For switched jack sockets, there is one set of contacts for each channel, i.e. one for a mono socket, and two for a stereo socket.
Standard jack connections
In view of the huge number of these connectors used for applications such as headphones, etc where it is anticipated that they can be plugged in and work, it is essential that a common connection standard is adopted. The connections that are invariably used are given in the table below. By adopting this convention it is possible to plug almost any item using one of these connectors in to a relevant item of equipment and it will work.
i.e. 'hot' signal
|Ring||Not present||Signal -
i.e. negative phase
|Sleeve||Ground / return||Ground /shield||Ground / shield|