Headphone amplifiers with optical output
Optical
— digital audio output via TOSLINK fiber optic cable. It is similar in capabilities to the coaxial output (see the relevant paragraph), allows you to work with stereo and even multi-channel audio through one connector. Specifically, the optical fiber, on the one hand, is absolutely insensitive to electromagnetic interference, on the other hand, it does not tolerate pressure and bending, which is why such a cable requires care in handling.Note that specifically in headphone amplifiers, digital outputs (including optical) perform an auxiliary function: they are usually responsible for switching the digital signal coming to the corresponding input. So models with such outputs have not received much distribution, and it makes sense to specifically look for such an amplifier if you initially plan the appropriate application — for example, broadcasting music from a USB flash drive connected via USB OTG (see "Inputs") to a digital input of an external audio receiver.
It is also worth mentioning that this output can use a separate specialized connector or “share” the jack with another interface: for example, in some models, the optical output is built into a 3.5 mm mini-Jack jack, which requires the use of an appropriate cable / adapter.