Connecting your Equipment

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Revision as of 16:20, 15 November 2007 by NT (talk | contribs) (Early Radios: correct year)
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This article describes connection of audio sources (such as HI-FI, radios or TV) to a computer and the different type of connectors in use.
USB audio devices are covered separately in the USB turntables and USB microphones on Linux articles.


Related articles:

Transferring tapes and records to computer or CD

Recording Tips

Connector types

There are several types of socket used for audio signals. The following should help with picking the correct lead in each case.

The main types in use are, in approximate order of popularity:

  • RCA Phono
  • 1/4 inch (6.35 mm) jack/TRS
  • 1/8 inch (3.5 mm) jack/TRS
  • 2 pin DIN
  • Mobile phone sockets
  • Professional only connector types
  • Bantam jack
  • 5 pin DIN
  • 3/32 inch (2.5mm) jack/TRS

Other connectors are also found at times, but the above covers the great majority in use.

Terminology varies around the world, which can sometimes cause confusion on an international Wiki. 'Phone plugs' describes many different types of connector around the world. In the US it refers to TRS connectors. These are called jack plugs and jack sockets in Europe, but in the US 'jack' can describe various different types of socket. The pictures included below should help.


RCA Phono

RCA Phono plugs

Introduced in the 1940s, RCA sockets have been the most popular connector type on domestic Hi-Fi (non-portable) for many decades. The connectors are mono and are used in pairs for stereo.

Occasionally a lot of hum is produced when these are used. If this occurs, either:

  • Bend the plugs' outer connector leaves inward very slightly to ensure firm contact with the socket
  • Connect a separate wire from record deck chassis to the metal computer case.



Jack/TRS Plugs & Sockets

1: Ground 2: Right 3: Left or mono 4: Insulation rings

TRS jack plugs and sockets of all sizes come in two versions, mono and stereo.

  • Mono plugs have two connector areas separated by a ring of plastic.
  • Stereo plugs have three connector areas with two rings of plastic separating them.

A stereo lead may be used to connect a mono source to either mono or stereo input. Mono leads short out one signal channel on stereo outputs, so should not be plugged into many headphone and speaker outputs. Stereo leads are therefore more versatile on the whole. This does not apply to pro audio 1/4" patch leads where different considerations apply.


1/4 inch (6.35 mm) Jack/TRS

1/4 inch TRS Jackplugs

These were invented for use in telephone switchboards in the 19th century. They are very popular on professional and semi-professional equipment and are also the usual headphone connector on non-portable domestic Hi-Fi. These are called TRS connectors (international), phone plugs (US) or jack plugs (Europe).

Note the term 'phone plug' describes many different types of connector outside of the US.


1/8 inch (3.5mm) Jack/TRS

TRS Jackplugs: 2.5mm, 3.5mm, 3.5mm, 1/4 inch

This is the most popular audio connector on portable equipment and computers. It's small and cheap, but not as robust or reliable as larger connectors, and has higher contact resistance. 3.5mm jacks are best avoided when professional studio standards are required.


3/32 inch (2.5mm) Jack/TRS

Occasionally used on portable equipment for audio.

On equipment from the 1970s, 2.5mm sockets were routinely fitted next to the 3.5mm microphone jack and used for remote pause control, not for audio. 2.5mm plugs are fairly weak.


Bantam Jack

Also known as Tiny Telephone connector in the US, these are another type of TRS or jack plug. These are not seen on domestic equipment.


DIN

DIN is a family of multi-pin connectors. The connectors can be 13.2 mm diameter or 9.5 mm diameter (known as mini-DIN). Both are standards of the Deutsches Institut für Normung, the German standards body.


2 pin

2 pin DIN socket (left) and plug (right)

Widely used in domestic Hi-Fi for connecting a loudspeaker to a power amplifier, and can also be used to connect to other devices such as the earlier shoebox style tape recorders. The central pin is flat and the off-centre pin is circular. The two-pin DIN plug is considered inferior by some due to the lack of the outer sheath. This means far less force is required to disconnect the plug accidentally, making it more prone to bending or shifting of the pins during use. Worn two-pin speaker plugs on audio equipment are notorious for being very unreliable.


5 pin

5 Pin DIN plug

Sometimes found on domestic (non-portable) stereos. The DIN connector system carries up to four signal paths in one connector, so only a single lead is needed to connect a cassette deck. (Compare this to phono connectors, of which four are needed, all of which must go in the correct sockets.) Inherent issues with the design of this system make it unpopular today.



Mobile phone sockets

Mobile phones currently use proprietary connectors.


Professional connectors

Connectors such as XLR and speakon are usually only found on professional equipment. If using such equipment, you won't be needing basic guidance!


When there is no Connector

Some equipment has no audio output connector. In most cases this can be successfully connected to a computer by connecting a socket or the wire ends of a lead to the built in speaker terminals of the equipment. However there are some problems. Especially, be sure to read about the following safety issues if you are considering this and check the Never Connect section for certain equipment you should never connect directly to the computer.

Safety First

Many types of equipment contains mains voltages which are often stored in capacitors after being unplugged.

CRT  TVs contain 19,000 - 25,000 volts  which is also stored after disconnection. CRT TVs also contain a picture tube, the rear of which is fragile. If broken these can explode. Many also contain a significant amount of metalwork connected to the mains.

Sockets and leads attached to mains equipment should be properly secured physically, otherwise coming adrift can result in touching live parts.

Internal wiring should be kept away from parts that can become hot in use.

There are sometimes additional safety issues with some equipment, particularly unusual, unsafe or historic equipment.

Warning! This article is for information only. It is not a recommendation for you to carry out works you are not able to execute safely. It is not intended to be a complete analysis of all possible risks nor professional advice.


How to Connect

Mono battery equipment will usually work fine with the audio output wires connected either way round. If hum, buzz or squeal occurs, reverse the output lead wires.

Mains powered equipment should have the chassis side of the speaker connected to the outer braid (ground) of the audio lead. Getting this the wrong way round can often short the amplifier and destroy it.

Stereo equipment needs a three-wire connection to get a stereo signal.

  1. The ground wire goes to the device's chassis
  2. One signal wire goes to the non-chassis terminal of one speaker
  3. The other signal wire goes to the non-chassis terminal of the other speaker


Never Connect

There are a few items that should never be connected to a computer sound card. The reason in all cases is the presence of dangerous or destructive voltages.

Many TVs

There are a lot of live chassis TVs in use. Live chassis design was still common in 1990s sets. If a TV has no headphone or SCART  socket, it is likely to be live chassis, and connecting to its internal speaker is likely to lead to shock, and possibly electrocution. However it's perfectly safe to connect to a TV's SCART socket.

Some Vintage Radios

Some vintage radios have no isolation transformer for their High Tension (HT) supply. These are called universal sets, live chassis sets, or neutral chassis sets.

The presence of a mains transformer in the set does not imply that a set is not of this type, as some of these sets used a transformer for the Low Tension (LT) supply only.

These sets should only be connected to a computer if a suitable isolation transformer is used either in the mains supply to the radio, or in the audio feed from radio to computer. In the latter case all wiring and connectors up to the transformer should be considered live/hot, making the use of standard audio connectors inappropriate.

Early Radios

Bare binding posts  and Fahnestock clips  were the standard output connectors on most radios until the 1940s. The audio output lines were usually connected direct to the HT supply, and such equipment should never be connected directly to a computer. Direct connection is likely to cause electric shock and/or burnt components.

As well as being live, the high impedance speaker output voltages are far too high for modern equipment. Both these issues can be addressed by using an audio output transformer to isolate the radio from the computer and transform the high voltage to a usable voltage. A small mains power transformer can also be used to do this, though the resulting quality is not as good as an audio transformer. Isolation transformers of all types must be connected correctly to avoid damage to equipment and the user.

  • A typical 1920s triode output set will drive a 110v:6v or 240v:15v transformer, giving 8 ohm output with 2k primary load.
  • A 1930s pentode output set will drive a 110v:4.5v or 240v:9v transformer, giving an 8 ohm feed and 5k primary load.

In both cases a load must be provided, because unloaded output transformers are destructive and unsafe. The load may be on primary or secondary.

Note that plug-in power supplies that produce direct current (DC) to power small appliances are not transformers, and are not suitable for this.

Electrostatic Speakers

These are occasionally encountered, usually in 1960s audio equipment. Such speakers run at very high voltages and should never be connected to a computer, or to standard audio connectors.

Valve amplifiers with no load

Almost all valve (tube) amplifiers use output transformers, and these can produce destructive voltages if not loaded. A minority of transistor amplifiers also used output transformers until the 1970s.

If you use any equipment with an output transformer, the amplifier should be connected to either speakers or a dummy load. Otherwise excessive voltages can be generated and destroy the equipment.

PA Amplifiers

PA (public address) amplifiers using 35v line, 70v line and 100v line outputs suffer the same issue as valve amplifiers due to also using output transformers. Again the output must be loaded to avoid destructive voltages that can otherwise occur. Also the signal voltages are too high to connect to a computer sound card without damage.