A phone connector, also known as phone jack, audio jack, headphone jack or jack plug, is a family of electrical connectors typically used for analog audio signals.
The phone connector was invented for use in telephone switchboards in the 19th century and is still widely used.
The phone connector is cylindrical in shape, with a grooved tip to retain it. In its original audio configuration, it typically has two, three, four and, occasionally, five contacts. Three-contact versions are known as TRS connectors, where T stands for "tip", R stands for "ring" and S stands for "sleeve". Ring contacts are typically the same diameter as the sleeve, the long shank. Similarly, two-, four- and five- contact versions are called TS, TRRS and TRRRS connectors respectively. The outside diameter of the "sleeve" conductor is 6.35 millimetres ( inch). The "mini" connector has a diameter of 3.5 mm (0.14 in) and the "sub-mini" connector has a diameter of 2.5 mm (0.098 in).
Specific models, and connectors used in specific applications, may be termed e.g. stereo plug, headphone jack, microphone jack, aux input, etc. The 3.5 mm versions are commonly called mini-phone, mini-stereo, mini jack, etc.[not in citation given]
In the UK, the terms jack plug and jack socket are commonly used for the respective male and female phone connectors. In the US, a stationary (more fixed) electrical connector is called a jack. The terms phone plug and phone jack are sometimes used to refer to different genders of phone connectors, but are also sometimes used to refer to RJ11 and older telephone plugs and the corresponding jacks that connect wired telephones to wall outlets.
Phone plugs and jacks are not to be confused with the similar terms phono plug and phono jack (or in the UK, phono socket) which refer to RCA connectors common in consumer hi-fi and audiovisual equipment. The 3.5 mm connector is, however, sometimes--but counter to the connector manufacturers' nomenclature--referred to as mini phono.
Modern phone connectors are available in three standard sizes. The original inch (6.35 mm) version descends from as early as 1877, when the first-ever telephone switchboard was installed at 109 Court Street in Boston in a building owned by Charles Williams, Jr.; or 1878, when an early switchboard was used for the first commercial manual telephone exchange in New Haven, Connecticut created by George W. Coy. The 1877 switchboard was last known to be located in the lobby of 185 Franklin Street, Boston.
In February 1884, C.E. Scribner was issued US Patent 293,198 for a "jack-knife" connector that is the origin of calling the receptacle a "jack". Scribner was issued U.S. Patents 262,701, 305,021, and 489,570 relating to an improved design that more closely resembles the modern plug. The current form of the switchboard-plug was patented prior to 1902, when Henry P. Clausen received a patent on an improved design. It is today still used on mainstream musical equipment, especially on electric guitars.
Western Electric was the manufacturing arm of the Bell System, and thus originated or refined most of the engineering designs, including the telephone jacks and plugs which were later adopted by other industries, including the U.S. Military.
By 1907, Western Electric had designed a number of models for different purposes, including:
By 1950, the two main plug designs were:
Several modern designs have descended from those earlier versions:
U.S. military versions of the Western Electric plugs were initially specified in Amendment No.1, MIL-P-642, and included:
The 3.5 mm or miniature size was originally designed in the 1950s as two-conductor connectors for earpieces on transistor radios, and remains a standard still used today. This roughly half-sized version of the original, popularized by the Sony EFM-117J radio (released in 1964),[not in citation given] is still commonly used in portable applications. It became very popular with its application on the Walkman in 1979, as unlike earlier transistor radios, these devices had no speaker of their own; the usual way to listen to them was to plug in headphones. There is also an EIA standard for 0.141-inch miniature phone jacks.
The 2.5 mm or sub-miniature sizes were similarly popularized on small portable electronics. They often appeared next to a 3.5 mm microphone jack for a remote control on-off switch on early portable tape recorders; the microphone provided with such machines had the on-off switch and used a two-pronged connector with both the 3.5 and 2.5 mm plugs. They were also used for low-voltage DC power input from wall adapters. In the latter role they were soon replaced by coaxial DC power connectors. 2.5 mm phone jacks have also been used as the headset jacks on mobile telephones (see § PDAs and mobile phones).
The 3.5 mm and 2.5 mm sizes are sometimes referred to as in and in respectively in the United States, though those dimensions are only approximations. All sizes are now readily available in two-conductor (unbalanced mono) and three-conductor (balanced mono or unbalanced stereo) versions.
Four-conductor versions of the 3.5 mm plug and jack are used for certain applications. A four-conductor version is often used in compact camcorders and portable media players, providing stereo sound and composite analog video. It is also used as a combined stereo headphone and microphone connector (i.e. a headset connector) on some laptop computers, most mobile phones, and on some handheld amateur radio transceivers from Yaesu. Some headphone amplifiers have used it to connect "balanced" stereo headphones, which require two conductors per audio channel as the channels do not share a common ground.
By the 1940s, broadcast radio stations were using Western Electric Code No. 103 plugs and matching jacks for patching audio throughout studios. This connector was used because of its use in AT&T's Long Line circuits for distribution of audio programs over the radio networks' leased telephone lines. Because of the large amount of space these patch panels required, the industry began switching to 3-conductor plugs and jacks in the late 1940s, using the WE Type 291 plug with WE type 239 jacks. The type 291 plug was used instead of the standard type 110 switchboard plug because the location of the large bulb shape on this TRS plug would have resulted in both audio signal connections being shorted together for a brief moment while the plug is being inserted and removed. The Type 291 plug avoids this by having a shorter tip.
Professional audio and the telecommunication industry use a 0.173 in (4.4 mm) diameter plug, associated with trademarked names including Bantam, TT, Tini-Telephone, and Tini-Tel. They are not compatible with standard EIA RS-453/IEC 60603-11 1/4-inch jacks. In addition to a slightly smaller diameter, they have a slightly different geometry. The three-conductor TRS versions are capable of handling balanced line signals and are used in professional audio installations. Though unable to handle as much power, and less reliable than a 6.35 mm (0.250 in) jack, Bantam connectors are used for professional console and outboard patchbays in recording studio and live sound applications, where large numbers of patch points are needed in a limited space. The slightly different shape of Bantam plugs is also less likely to cause shorting as they are plugged in.
Less commonly used 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, such as 0.210 inch inside diameter jacks for fire safety communication jacks in public buildings, the same size found in discontinued Bell & Howell 16 mm projector speaker jacks.
US military phone connectors include both 0.25 in (6.35 mm) and 0.21 in (5.34 mm) diameter plugs, which both mate with the M641-series open frame jacks, exemplified by Switchcraft C11 and C12 jacks.
Military specifications and standards relating to phone connectors include MIL-STD 202, MIL-P-642/*, and MIL-J-641.
Commercial and general aviation (GA) civil airplane headset plugs are similar, but not identical. A standard in monaural plug, type PL-55 (both two-conductor phone plugs, a.k.a. PJ-055B, which mate with JK-24 and JK-34A jacks) is used for headphones. On many newer GA aircraft the headphone jack is a standard in phone connector wired in the standard unbalanced stereo configuration instead of the PJ-055 to allow stereo music sources to be reproduced.
Aviation headphones are paired with special tip-ring-sleeve, 0.206 inch/5.23 mm diameter plug, type PJ-068 (PL-68, for the microphone. The PJ-068 mates with a JK-33 jack (Switchcraft C-12B), and is similar to the Western Electric plug WE-109.
In the microphone plug the Ring is used for the microphone 'hot' and the sleeve is common or microphone 'Lo'. The extra (tip) connection in the microphone plug is often left unconnected but is also sometimes used for various functions, most commonly an optional push-to-talk switch, but on some aircraft it carries headphone audio and on others a DC supply.
Military aircraft and civil helicopters have another type termed a U-174/U. These are also termed 'NATO plugs' or Nexus TP120 phone plugs. They are similar to in (6.35 mm) plug, but with a 7.10 mm (0.280 in) diameter short shaft with an extra ring, i.e. four conductors in total, allowing two for the headphones (mono), and two for the microphone. There is a confusingly similar four pole (or four conductor) British connector with a slightly smaller diameter and a different wiring configuration used for headsets in many UK Military aircraft and often also referred to as a NATO or 'UK NATO' connector.
The original application for the 6.35 mm ( in) phone jack was in manual telephone exchanges. Many different configurations of these phone plugs 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, loudspeakers, and many other items of audio equipment.
When a three-conductor version of the 6.35 mm 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 of the amplifier. This attempt has long been abandoned, and now the convention is that all plugs fit all sockets of the same size, regardless of whether they are balanced or unbalanced, mono or stereo. Most 6.35 mm 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.
The results of this physical compatibility are:
Because of a lack of standardization in the past regarding the dimensions (length) given to the ring conductor and the insulating portions on either side of it in 6.35 mm ( in) phone connectors and the width of the conductors in different brands and generations of sockets, there are occasional issues with compatibility between differing brands of plug and socket. This can result in a contact in the socket bridging (shorting) the ring and sleeve contacts on a phone connector.
The most common arrangement remains to have the male plug on the cable and the female socket mounted in a piece of equipment: 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 15 amperes for certain heavy duty in versions intended for loudspeaker connections.
Some common uses of phone plugs and their matching sockets are:
Personal computer sound cards, such as Creative Labs' Sound Blaster line, use a 3.5 mm phone connector as a mono microphone input, and deliver a 5 V polarizing voltage on the ring to power electret microphones. Sometimes termed phantom power, this is not a suitable power source for microphones designed for true phantom power and is better termed bias voltage. (Note that this is not a polarizing voltage for the condenser, as electrets by definition have an intrinsic voltage; it is power for a FET preamplifier built into the microphone.) Compatibility between different manufacturers is unreliable.
The Apple PlainTalk microphone jack used on some older Macintosh systems is designed to accept an extended 3.5 mm three-conductor phone connector; in this case, the tip carries power for a preamplifier inside the microphone. If a PlainTalk-compatible microphone is not available, the jack can accept a line-level sound input, though it cannot accept a standard microphone without a preamp.
Normally, 3.5 mm three-conductor sockets are used in computer sound cards for stereo output. Thus, for a sound card with 5.1 output, there will be three sockets to accommodate six channels: "front left and right", "surround left and right", and "center + subwoofer". 6.1 and 7.1 channel sound cards from Creative Labs, however, use a single three-conductor socket (for the front speakers) and two 4-conductor sockets. This is to accommodate rear-center (6.1) or rear left and right (7.1) channels without the need for additional sockets on the sound card. (Note that Creative's documentation uses the word "pole" instead of "conductor".)
Some portable computers have a combined 3.5 mm TRS-TOSLINK jack, supporting stereo audio output using a TRS connector, or TOSLINK (stereo or 5.1 Dolby Digital/DTS) digital output using a suitable optical adapter. Most iMac computers have this digital/analog combo output feature as standard, with early MacBooks having two ports, one for analog/digital audio input and other for output. Support for input was dropped on various later models
Some newer computers, such as Lenovo laptops, have 3.5 mm TRRS headset sockets, which are compatible with phone headsets and may be distinguished by a headset icon instead of the usual headphones or microphone icons. These are particularly used for Voice over IP.
Equipment requiring video with stereo audio input/output sometimes uses 3.5 mm TRRS connectors. Two incompatible variants exist, of 15 millimetres (0.59 in) and 17 mm (0.67 in) length, and using the wrong variant may either simply not work, or could cause physical damage.
Attempting to fully insert the longer (17 mm) plug into a receptacle designed for the shorter (15 mm) plug may damage the receptacle, and may damage any electronics located immediately behind the receptacle. However, partially inserting the plug will work as the tip/ring/ring distances are the same for both variants.
Using the shorter plug in a socket designed for the longer connector will result in the plug not 'locking in', and may additionally result in wrong signal routing and/or a short circuit inside the equipment (e.g. the plug tip may cause the contacts inside the receptacle - tip/ring 1, etc. - to short together).
The shorter 15 mm TRRS variant is more common and fully physically compatible with 'standard' 3.5 mm TRS and TS connectors.
Many small video cameras, laptops, recorders and other consumer devices use a 3.5 mm microphone connector for attaching a (mono/stereo) microphone to the system. These fall into three categories:
Plug-in power is supplied on the same line as the audio signal, using an RC filter. The DC bias voltage supplies the FET amplifier (at a low current), while the capacitor decouples the DC supply from the AC input to the recorder. Typically, V=1.5 V, R=1 k?, C=47 µF.
If a recorder provides plug-in power, and the microphone does not need it, everything will usually work ok, although the sound quality may be lower than expected, as the microphone may not work optimally with a constant DC current flowing through it. In the converse case (recorder provides no power; microphone needs power), no sound will be recorded. Neither misconfiguration will damage consumer hardware, but providing power when none is needed could destroy a broadcast-type microphone.
Three- or four-conductor (TRS or TRRS) 2.5 mm and 3.5 mm sockets are common on cell phones, providing mono (three conductor) or stereo (four conductor) sound and a microphone input, together with signaling (e.g., push a button to answer a call). Three-conductor 2.5 mm connectors are particularly common on older phones, while four-conductor 3.5 mm connectors are more common on newer smartphones. These are used both for handsfree headsets (esp. mono audio plus mic, also stereo audio plus mic, plus signaling for call handling) and for (stereo) headphones (stereo audio, no mic). Wireless (connectorless) headsets or headphones usually use the Bluetooth protocol.
There is no recognised standard for TRRS connectors or compatibility with three conductor TRS. The four conductors of a TRRS connector are assigned to different purposes by different manufacturers. Any 3.5 mm plug can be plugged mechanically into any socket, but many combinations are electrically incompatible. For example, plugging TRRS headphones into a TRS headset socket (or vice versa) or plugging TRRS headphones from one manufacturer into a TRRS socket from another may not function correctly, or at all. Mono audio will usually work, but stereo audio or microphone may not work, depending on wiring. Signaling compatibility depends both on wiring compatibility and the signals sent by the hands-free/headphones controller being correctly interpreted by the phone.[original research?] Adapters that are wired for headsets will not work for stereo headphones and conversely. Further, as TTY/TDDs are wired as headsets, TTY adapters can also be used to connect a 2.5 mm headset to a phone.
3.5 mm TRRS (stereo-plus-mic) sockets became particularly common on smartphones, and have been used e.g. by Nokia since 2006; they are often compatible with standard 3.5 mm stereo headphones. Two different forms are frequently found, both of which place left audio on the tip and right audio on the first ring (mirroring the configuration found on stereo connectors). Where they differ is in the placement of the microphone and return contacts. The first, which places the ground return on the second ring and the microphone on the sleeve, is used by Apple's iPhone line (through the iPhone 6), HTC devices, recent Samsung, Nokia, and Sony phones, among others. The second, which reverses these contacts, is used by older Nokia mobiles, older Samsung smartphones, and some Sony Ericsson phones. Some adapters switch the last two conductors to allow a device made to one standard to be used with a headset made to the other.
Some computers now include a TRRS headset socket, compatible with headsets intended for smartphones. One such pin assignment, with ground on the sleeve, is standardized in OMTP and has been accepted as a national Chinese standard YDT 1885-2009.
|Standard||Tip||Ring 1||Ring 2||Sleeve||Devices using this standard|
|OMTP||Left audio||Right audio||Microphone||Ground||Old Nokia (and also Lumia starting from the 2nd generation), old Samsung (2012 Chromebooks), old Sony Ericsson (2010 and 2011 Xperias), Sony (PlayStation Vita), OnePlus One, all phones sold in China after 2009.|
|CTIA, AHJ||Left audio||Right audio||Ground||Microphone||Apple, HTC, LG, Blackberry, latest Nokia (clarification needed]), latest Samsung, Jolla, Sony (Dualshock 4), Microsoft (including Surface and Xbox One controller with chat adapter), most Android phones.[|
|CTIA-style AV||Left audio||Right audio||Ground||CVBS video||Apple iPod (up to 6th generation), Alpine iLX-702D, Raspberry Pi (2014 onwards), Xbox 360 E, Zune, clarification needed].[|
|Video/audio 1||Left audio||CVBS video||Ground||Right audio||Sony and Panasonic camcorders. On some early Sony camcorders, this socket doubled up as a headphone socket. When a headphone plug was inserted, ring 2 was shorted to the sleeve contact and the camcorder output the right audio on ring 1.|
|Video/audio 2||CVBS video||Left audio||Right audio||Ground||Unknown camcorders, portable VCD and DVD players, Western Digital TV live!, some newer LG TVs.|
|Video/audio 3||CVBS video||Left audio||Ground||Right audio||Toshiba TVs|
New TRRRS standard for 3.5 mm connectors was developed and recently approved by ITU-T. The new standard, called P.382 (formerly P.MMIC), outlines technical requirements and test methods for a 5-pole socket and plug configuration. Compared to legacy TRRS standard TRRRS provides one extra line that can be used for connecting a second microphone or external power to/from the audio accessory. P.382 requires compliant sockets and plugs to be backwards compatible with legacy TRRS and TRS connectors. Therefore, P.382 compliant TRRRS connectors should allow for seamless integration when used on new products. TRRRS connectors enable following audio applications: active noise cancelling, binaural recording and others, where dual analogue microphone lines can be directly connected to a host device.
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Panel-mounting jacks are often provided with switch contacts. Most commonly, a mono jack is provided with one 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). Some designs of jack also have such a connection on the sleeve. As this contact is usually ground, it is not much use for signal switching, but could be used to indicate to electronic circuitry that the socket was in use.
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. 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 (or vice versa), thus forming a patch point.
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 the tip of the other, 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 3.5 mm or 2.5 mm 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.
A standard stereo jack is used on most battery-powered guitar effects pedals to eliminate the need for a separate power switch. In this configuration, the internal battery has its negative terminal wired to the sleeve contact of the jack. When the user plugs in a two-conductor (mono) guitar or microphone lead, the resulting short-circuit between sleeve and ring connects an internal battery to the unit's circuitry, ensuring that it powers up or down automatically whenever a signal lead is inserted or removed. A drawback of this design is the risk of inadvertently discharging the battery if the lead is not removed after use, such as if the equipment is left plugged in overnight.
|Pin||Unbalanced mono||Balanced mono
|Tip||Signal||Send or return signal||Positive, hot||Left channel|
|Ring||Ground, or no connection||Return or send signal||Negative, cold||Right channel|
When a phone connector is used to make a balanced connection, the two active conductors are both used for a monaural signal. The ring, used for the right channel in stereo systems, is used instead for the inverting input. This is a common use in small audio mixing desks, where space is a premium and they offer a more compact alternative to XLR connectors. Another advantage offered by TRS phone connectors used for balanced microphone inputs is that a standard unbalanced signal lead using a TS phone jack can simply be plugged into such an input. The ring (right channel) contact then makes contact with the plug body, correctly grounding the inverting input.
A disadvantage of using phone connectors for balanced audio connections is that the ground mates last and the socket grounds the plug tip and ring when inserting or disconnecting the plug. This causes bursts of hum, cracks and pops and may stress some outputs as they will be short circuited briefly, or longer if the plug is left half in.
One convention that avoided this brief shorting in the professional audio world was to use the following convention for XLR and PJ-051/WE-310 patch bays, so that, "By making the tip of the jack the low side of the line, the high side is physically protected against momentary shorts when patching in and out of the jackfield.":
|XLR pin||TRS contact||Signal|
This problem does not occur when using the 'gauge B' (BPO) phone connector (PO 316) which although it is of 0.25 in (6.3 mm) diameter has a smaller tip and a recessed ring so that the ground contact of the socket never touches the tip or ring of the plug. This type was designed for balanced audio use, being the original telephone 'switchboard' connector and is still common in broadcast, telecommunications and many professional audio applications where it is vital that permanent circuits being monitored (bridged) are not interrupted by the insertion or removal of connectors. This same tapered shape used in the 'gauge B' (BPO) plug can be seen also in aviation and military applications on various diameters of jack connector including the PJ-068 and 'bantam' plugs. The more common straight-sided profile used in domestic and commercial applications and discussed in most of this article is known as 'gauge A'.
XLR connectors used in much professional audio equipment mate the ground signal on pin 1 first.
Phone connectors with three conductors are also commonly used as unbalanced audio patch points (or insert points, or simply inserts), with the output on many mixers found on the tip (left channel) and the input on the ring (right channel). This is often expressed as "tip send, ring return". Other mixers have unbalanced insert points with "ring send, tip return". One advantage of this system is that the switch contact within the panel socket, originally designed for other purposes, can be used to close the circuit when the patch point is not in use. An advantage of the tip send patch point is that if it is used as an output only, a 2-conductor mono phone plug correctly grounds the input. In the same fashion, use of a "tip return" insert style allows a mono phone plug to bring an unbalanced signal directly into the circuit, though in this case the output must be robust enough to withstand being grounded. Combining send and return functions via single in TRS connectors in this way is seen in very many professional and semi-professional audio mixing desks, because it halves the space needed for insert jack fields which would otherwise need two jacks, one for send and one for return. The tradeoff is that unbalanced signals are more prone to buzz, hum and outside interference.
In some three-conductor TRS phone inserts, the concept is extended by using specially designed phone jacks that will accept a mono phone plug partly inserted to the first click and will then connect the tip to the signal path without breaking it. Most standard phone connectors can also be used in this way with varying success, but neither the switch contact nor the tip contact can be relied upon unless the internal contacts have been designed with extra strength for holding the plug tip in place. Even with stronger contacts, an accidental mechanical movement of the inserted plug can interrupt signal within the circuit. For maximum reliability, any usage involving first click or half-click positions will instead rewire the plug to short tip and ring together and then insert this modified plug all the way into the jack.
The TRS tip return, ring send unbalanced insert configuration is mostly found on older mixers. This allowed for the insert jack to serve as a standard-wired mono line input that would bypass the mic preamp. However tip send has become the generally accepted standard for mixer inserts since the early-to-mid 1990s. The TRS ring send configuration is still found on some compressor sidechain input jacks such as the dbx 166XL.
In some very compact equipment, 3.5 mm TRS phone connectors are used as patch points.
Some sound recording devices use a three-conductor phone connector as a mono microphone input, using the tip as the signal path and the ring to connect a standby switch on the microphone.
Connectors that are tarnished, or that were not manufactured within tight tolerances, are prone to cause poor connections. Depending upon the surface material of the connectors, tarnished ones can be cleaned with a burnishing agent (for solid brass contacts typical) or contact cleaner (for plated contacts).
Also called a 3.5 mm or 1/8" connector, it is a plug and socket widely used for analog audio signals in portable devices.
(e.g.) 3.5mm female stereo mini phone jack to 1/4" male Stereo phone plug Adapter
Remember that sound cards use the smaller 1/8-inch mini-phono plug...
1/8-inch stereo mini-phono plug adapter.
3.5 mm Plug/Jack: Also referred to as a 1/8 inch, auxiliary input, mini stereo, and mini phono.
The 3.5mm mini-phono plug connector of the ATC Probe plugs into the 3.5mm mini-phono jack on the pH meter.
...instead of the headphone jack.... There will be a lightning-to-mini phono adapter included as well.
High power 2-conductor speaker jack carries 15A (continuous) audio speaker current levels.