Audio cables come with many connector types, which may cause some confusion with users. Specifically with in-ear monitors (IEMs) and earphones, cable connectors can be classified into two groups: the terminal plug (the end you plug into your music device) and the connector (the end you plug into your earphone unit).

Terminal Plugs

The terminal plug is often available in 3.5mm, 4.4mm, 2.5mm, 6.35mm, and XLR. These numbers indicate the size of the plug, and must match the jack (output port) of the music device. Most mobile phones have 3.5mm headphone jacks, and must be used with a 3.5mm cable. 6.35mm (also called TRS) plugs are used in musical instruments and audio-engineering devices. 3.5mm to 6.35mm adaptors are commonly available for those using IEMs and headphones in the studio or stage. 2.5mm and 4.4mm headphone jacks are unique to consumer level audio devices, such as digital audio players (DAPs) and Hi-Fi DAC/Amps.

One thing to consider when choosing a cable plug type is the issue of single or balanced sources. Single/Balanced describes the ways in which the wires are wired within the music device and your earphone cable. Most consumer level devices (smartphones, older MP3 players, PC, Mac, tablets, etc) are 3.5mm single ended jacks. However, 2.5mm and 4.4mm jacks are almost exclusively balanced jacks.

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It is extremely important that you use the correct cable type (single/balanced) with your music device; failure to do so will result in device damage. The rule is: balanced cables can be used with both single and balanced jacks, but single-ended cables can ONLY be used with single-ended headphone jacks. For example, if you have a 2.5mm balanced plug cable, you can use a 2.5mm to 3.5mm adaptor to plug into your 3.5mm iPhone. However, you CANNOT use a 3.5mm single-ended cable to plug into a 2.5mm balanced DAP headphone jack. For this reason, balanced to single ended adaptors can be commonly found, but single to balanced adaptors are rare.

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Connector Types

For the IEM and earphone connectors, the common ones are: 2-pin, MMCX, and recessed 2-pin. As the name implies, 2-pin cables have 2 small pins extruding from the connector. MMCX connectors look cylindrical, and have one small pin within the cylinder.

2-pin connectors are available in 0.78mm and 0.75mm sizes. KZ and QDC manufacturers commonly use the 0.75mm 2-pin sizes, while most other IEM manufacturers stick to the standard 0.78mm 2-pin size. Furthermore, KZ and QDC manufacturers use the recessed 2-pin connector.

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MMCX connectors are often used by older audio commercial manufacturers such as Shure, Westone, and UE. 

MMCX connectors benefit from a very secure lock with the earphone unit. However, this can be a downfall as MMCX cables are more difficult to unhook, making it less suitable for switching cables frequently. 2-pin connectors have become more common for this reason, as they are easier to detach and swap around with other cables.

Ultimately, the connector type of your cable will depend on the connector type available on your earphone unit.

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