Connecting your Equipment

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This article covers connection of audio sources to a computer.


USB audio devices connect to the computer with digital data channels, and are covered separately here and here.


Connectors

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" jack
  • 3.5mm jack
  • 2 pin DIN
  • Mobile phone sockets
  • Professional only connector types
  • Bantam jack
  • 5 pin DIN
  • 2.5mm jack

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.

1/4" Jack / TRS

1/4" TRS Jackplugs

These date from the 19th century, and are very popular on professional and semi-professional equipment. 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.

3.5mm Jack / TRS

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

Most popular audio connector on portable equipment and computers. Its small & cheap, but not as robust or reliable as larger connectors, and with higher contact resistance. 3.5mm jacks are best avoided when professional studio standards are required.

2 Pin DIN

2 pin DIN socket and plug

Widely used as speaker plugs on domestic hifi. One pin is round, one is flat.

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!

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.

5 pin DIN

5 Pin DIN plug

Sometimes found on domestic (non-portable) stereos. The DIN connector system carries upto 4 signal paths in one connector, so only a single lead is needed to connect a cassette deck. (Compare to phono connectors, of which 4 are needed, all of which must go in the correct sockets.) Inherent issues with the design of this system make it unpopular today.

2.5mm Jack / TRS

Occasionally used on portable equipment for audio.

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

2.5mm plugs are fairly weak.

Others

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

Notes

Terminology

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. Pictures can be useful.


About TRS / Jack Plugs & Sockets

TRS jackplugs & sockets of all sizes come in 2 versions, mono and stereo.

  • Mono plugs have 2 connector areas separated by a ring of plastic.
  • Stereo plugs have 3 connector areas with 2 rings of plastic separating them.
1: Ground 2: Right 3: Left or mono 4: Insulation rings

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 & 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.


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. However there are some gotchas. Be sure to read about the safety issues below in the 'Never Connect' section if you're considering this.

Safety First

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

CRT TVs contain EHT (normally 19-25kV) 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.

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 3 wire connection to get a stereo signal.

  1. The ground wire goes to the device's chassic.
  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 soundcard. 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 designs were standard in the 1980s, and 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.

Its ok to connect to a TV's scart socket.

Some Vintage Radios

Some vintage radios have no isolation transformer for their 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 LT supply only.

These sets are only safe to connect 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 were the standard output connectors on 1920s and 1930s radios. 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 radio from 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 people.

  • 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, prividing an 8 ohm feed and 5k primary load.

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

Note that plug-in power supplies that produce dc 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 amps also used output tranformers until the 1970s.

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

PA Amplifiers

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.