Phone Jacks and Plugs

In theory, phone jacks/ plugs are some of the simplest connectors around.  They are generally low-cost, and they're used on countless pieces of equipment, from guitar amps (where they are invariably used for inputs, and often for speaker outputs as well) to mobile (cell) phones and many other common audio items.  You can (of course) find some that are rather expensive, but that doesn't ensure higher quality (or 'better' audio).

They were originally used in early manual telephone exchanges (central offices) to connect the caller to the desired party, and these were always TRS (tip, ring and sleeve) types.  They were used because the phone system (POTS - plain old telephone system) is balanced, so tip and ring were used for the phone connection, with the sleeve grounded or floating.  The most common jack for guitar and other musical instruments is just a TS (tip and sleeve) mono type.

There are many different sizes, with the 1/4" (6.35mm) being the original standard, but now we also have 3.5mm (commonly referred to as 1/8").  2.5mm versions are also used, although these are less common (and far easier to damage) than the larger types.  It appears that the 3.5mm types have been 'converted' into an imperial measurement (albeit wrong) by those who don't speak metric.  A reader alerted me to this, and a measurement confirms that they really are 3.5mm.

In all cases, the sleeve is also the main support structure for the jack plug, and it was always intended that this would be earth/ ground.  For reasons unknown, Apple (being the pack of bastards they are) changed that for TRRS jacks, making the sleeve the mic connection (or video connection where appropriate), and using the first ring the ground.  To say that this is uninspiring (and IMO bloody stupid) is to put it very mildly indeed, and all it achieved was to make Apple accessories incompatible with other devices.  Most mobile phone makers have adopted the Apple convention - probably not because they wanted to, but to make accessories interchangeable.

Figure 1 - The Three Standard Jack Plugs

Fig 1 shows the three standards, commonly available in 3.5mm and 6.35mm sizes.  The drawing shows how the sections are supposed to be used, but as noted below the TRRS plug and jack wiring has been hijacked so it doesn't make sense.  With metal plugs, the housing is always electrically connected to the sleeve, so with the mangled wiring scheme first implemented by Apple, that would make the housing connected to the mic wiring.  That makes absolutely no sense however you look at it, but it became the defacto 'standard' and Android phones (and tablets) adopted the same idiotic scheme.

Figure 2 - Phone Plug and Dismantled Jack

The jack is useful for more things than you may think.  When stripped down to the basics, the threaded collar, washer and nut make a fine 'bearing' for pot or rotary switch extension shafts.  When it was available, the ESP extension shaft used the collar from a phone jack as the 'bearing', so the shaft wasn't just rotating in a hole in the front panel.  However, they aren't especially easy to get apart - I used my lathe as other methods are hit-and-miss.  The cut end also has to be re-rivetted to hold together, requiring tooling that you'd probably have to make for yourself (as I did).

The internal structure of the plug is not so easily accessed.  There used to be plugs that had a screw-on tip, and they could be dismantled easily.  Unfortunately, it was common for the tip to unscrew and disappear, and all modern versions are rivetted.  The connections for the rings are generally concentric tubes with insulation between each.  The standard of construction isn't always as one might hope, but failures (other than wear and tear after years of use) are (surprisingly) uncommon.

Many jacks (sockets) are simple types with no switching or other 'fancy' stuff.  Fig. 2 shows the most basic - a mono TS plug and jack.  These are common for basic connections, where no switching is required.  This connector has been used for decades for musical instruments and amplifiers - generally as the speaker output.  IMO it's not at all suitable for speakers, but it's so common that it's pretty much impossible to change.  A major disadvantage when used on a speaker cabinet is that the tip and sleeve are shorted during insertion - not good for solid-state amplifiers!

Note:  The sleeve should always be ground.  This is the way these connectors were designed, and the way they are supposed to be wired.  Unfortunately, Apple, in its 'wisdom' changed that for TRRS types (see below) and other manufacturers followed suit.  The 'alternative' connection is stupid and makes no sense.  The sleeve has a heavy termination designed for anchoring the shield, but it's not used for its intended purpose when the wiring scheme is changed by idiots!  Stupidest decision I've come across for a long time.

Figure 3 - Mono Tip-Sleeve Plug and Jack Socket

Most of these sockets are not insulated from the chassis, so the sleeve is forced to be at earth/ ground because it's attached to the chassis by default.  Insulated types are also available, but they have a plastic housing that isn't very strong and may be easily broken.  Many of the plastic sockets include some switching.

Figure 4 - Stereo (TRS) Plug and Socket

Headphones are almost always used with TRS plugs, and the Tip is always intended to be the left channel.  The same wiring is used for 6.35mm and 3.5mm connectors, and adapters are readily available to convert from 3.5mm to 6.35mm, since most fixed stereo systems use the larger version.  The stereo version is often used for balanced connections, and 'combo' connectors are available that will accept either XLR or stereo (6.35mm) phone plugs for the input.  The terms 'hot' and 'cold' refer to common terminology for balanced circuits, where 'hot' is the positive signal and 'cold' is the negative signal, or no signal in the case of pseudo-balanced circuits.  See the article Balanced Audio Interfaces for info on this topic.

Figure 5 - TRRS Plug and Socket (Correct Wiring Scheme)

When used with a stereo headset (stereo headphones plus a mic connection), the above shows how it was intended that the connectors should be wired.  Unfortunately, Apple decided that this was too sensible and they changed it so the sleeve is the mic connection, and the second ring is used for earth/ ground.  Android phones eventually did likewise so that headsets would be interchangeable.  This non-standard (and IMO just plain stupid) connection scheme conveys zero actual benefit (quite the reverse in fact, as it's hard to connect the shield so it adds strain relief).  The idea of using non-sensible wiring schemes isn't just on phones - many manufacturers (e.g. Sony, Panasonic, Toshiba) also used the sleeve inappropriately.  This is something that will often happen whenever any connector has more than 2 wires - some lunatic will decide to rearrange them rather than follow accepted standards.

Figure 6 - TRRS Plug and Socket (Mated)

When the plug is fully inserted, the Tip connection in the socket is meant to engage with the notch on the plug.  This provides some resistance against pull-out, and it requires a deliberate effort to disengage the connectors.  It's not a 'true' latching system though, and accidental disconnection is fairly easy.  There are true latching phone plug/ jack combos, but they're not common.

TRS sockets often have basic switching that disconnects internal speakers (for example) when a set of headphones is plugged in.  This is very common, and the connections are shown next.

Figure 7 - TRS Speaker/ Headphone Switching

Simple switching such as that shown is very common.  The signal will rarely be the actual speaker output, as connecting headphones directly to a power amp would not be sensible.  This is because their sensitivity is measured in dB SPL/ mW, and even a 10W (8Ω) amp will deliver around 2.5W into 32Ω headphones (anyone care for 134dB SPL peak, assuming the 'phones survive?).  If you need indirect switching, then other options are available.  Not all can be obtained for all sizes though, with 6.35mm sockets usually having the most (and most robust) switching options.

The way the switching is performed varies with different designs.  Some use a fully isolated switch, while others have one contact wired to a socket terminal.  The version shown is the simplest arrangement, using simple SPST switches for Tip and Ring.  Where isolated switching is required, a more complex mechanism is needed.

Figure 8 - TRS Switching Option

The switching that's included ranges from a simple normally closed (NC) switch to SPDT (single-pole double-throw) as shown above or DPDT (double-pole double-throw).  In some cases the switches are connected to one or more of the contacts, and in others they are separate (I've shown a separate isolated SPDT switch).  This allows the user to use the switching circuits independently of the contacts.  This is often used to apply power to the device when a jack plug is inserted - particularly if the product is battery powered.

Switching is generally available with both 6.35mm and 3.5mm connectors, although the type of switching can be more limited with small connectors as there's less space for complex contact assemblies.  One of the difficulties isn't making the switches themselves, but ensuring that they are mechanically rugged enough to withstand normal 'abuse' - especially where the connector is expected to last for a long time.

In some products, the switch may be used to detect that something has been plugged in, and this is common for computers.  When a plug is inserted, software detects that 'something' has been plugged in, and you may be asked to identify if it's a 'line' input (typically ~100mV) or a microphone (1-5mV).  Circuitry is switched accordingly under software control.

Phone plugs and jacks are one of the most common audio connectors around, and they serve a need in many different pieces of equipment.  They are compact, fairly reliable and easily wired by a hobbyist with reasonable soldering skills.  They are not intended for high power (although 6.35mm [¼"] types carry the current for 100W from a guitar amp quite happily), and they're available anywhere - at least in their basic form for sockets.  2.5mm types aren't recommended unless you already have equipment that uses this size.  Because these are so small they are physically weaker than their larger brethren and are more easily damaged.

As mentioned above, the sleeve is intended to be ground, and the plug has a strong anchor to hold the shield (and the cable via the clamp at the end) and provide strain relief for the smaller and more delicate internal wire(s).  The fact that most jacks have a grounded sleeve connection shows that this was always the intent.  However, if you need a TRRS plug for stereo headphones and a mic (for a phone or a tablet) then you are stuck with the stupid arrangement that's now used for most (if not all) modern devices.

This article is intended as a quick look into phone plugs and jacks, and not every combination can be covered.  For example, 5-pole (TRRRS) types are made, but they're not readily available and decidedly non-standard.  If you need more than 4 connections (TRRS) then I'd suggest that you choose a different type of connector.

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