There are a number of different devices that can be referred to as in-ear amplifiers, including monitor equipment for musicians and devices that are used to improve hearing. In-ear amplifiers that are used as monitors do not technically fit into ears, though some of them are small enough to be worn on a belt. This type of in-ear amplifier has one or more audio outputs that can be connected to headphones, providing a musician with a personal monitor. Another type of device that can be referred to as an in-ear amplifier typically consists of an earpiece that is connected to a box that houses the amplification circuitry. These amplifiers are similar to early hearing aids, which are technically a type of in-ear amplifier as well.
In a musical and sound mixing context, an in-ear amplifier is an amp that is specifically designed for use with headphones. These amps can be wireless or hard wired, depending on the particular application. Musicians who move around a lot sometimes opt for wireless in-ear amps, while hardwired versions can be better for drummers, keyboardists, and orchestral musicians who are more likely to stay in one place. Since these amps are often used as personal monitors, they typically have one or more tip-ring-sleeve (TRS) jacks and can be used with regular headphones or custom designed earmold that fits into the musician's ears.
Another type of in-ear amplifier is used primarily to help a listener hear distant or quiet sounds. These devices usually have a single, non-custom earbud that is connected by a wire to a box that contains the electronics necessary for sound amplification. A microphone located on the box receives sounds, the circuitry amplifies it, and then the earbud passes it directly into the listener's ear. In some cases, the amplification electronics are much more compact and are connected directly to the earbud. These devices are sometimes used to spy on conversations, listen to televisions or radios on low volume settings, or as an inexpensive hearing aid substitute.
Hearing aids are technically in-ear amplifiers as well, as they are designed to amplify sounds for the hard of hearing. There are many different kinds of hearing aids, ranging from large behind the ear (BTE) units to advanced cochlear implants. Most hearing aids consist of a custom earmold that fits into the outer bowl of the ear or the ear canal. BTE units have a component that contains the amplification circuitry, sits behind the ear and is hooked to the earmold by a tube. The other designs typically contain the amplification circuitry and power source inside the earmold.