There are a few different devices that can be referred to as universal serial bus (USB) amplifiers, including desktop units, headphone amps, and more powerful home theater devices. In the most basic sense, a USB amplifier is a device with a USB input that can be used to drive loudspeakers. Desktop USB amplifiers sometimes include a built-in subwoofer, and usually power two additional speaker channels, though more complicated systems are also available. Headphone USB amplifiers are usually in-line devices that have a USB plug and tip-ring-sleeve (TRS) jack, though some include additional options. Home theater amplifiers that can be used with televisions and other devices usually have optical or coaxial connectors instead of USB ports, though some do include a universal serial bus input that is compatible with portable music players, laptops, and desktop computers.
Amplifiers are devices that are used to increase the power of signals, and they are found in many different applications. In the context of audio equipment, amplifiers are used to increase the power of audio signals in order to drive loudspeakers. The audio signal power provided by many input devices is not sufficient to power large loudspeakers, so these signals must be passed through amplification circuits. Since many computers and portable music players are capable of outputting digital audio signals through the universal serial bus interface, a USB amplifier can be used in conjunction with those devices.
One common type of USB amplifier is typically meant for desktop use. These amplifiers usually only have two channels of audio, and sometimes include additional input jacks for either digital or analog sources. Some desktop USB amplifiers contain built-in subwoofers, and have two or more TRS connectors for individual speakers. These systems sometimes have more speaker outputs, such as a 5.1 surround sound USB amplifier that is capable of producing five discrete channels in addition to a low frequency effects (LFE) channel.
Another similar type of USB amplifier is meant primarily for use with headphones. These amps sometimes also include a set of coaxial inputs and outputs, in addition to the mandatory USB input and TRS output. The more complex units that include additional coaxial outputs can often be used as desktop amps in addition to the primary headphone use. Some of these units can use batteries, though the power necessary to run these amps is usually provided by the USB connection.
Most home theater amplifiers that include digital outputs use coaxial or optical connectors, which are not directly compatible with USB. Some units do include a USB input though, which can allow a portable music player or computer to be connected. These USB amplifiers typically offer all of the same options and features as other home theater units, and can often be used to power a wider variety of speakers than other USB compatible amplifiers.