Headphones or earphones or even headsets are a frequent accessory for eBook Readers. In this article the names are used interchangeably.
Headphones can serve for TTS use or provide music in the background while we are reading. Some reading devices can even synchronize audio recordings of a book with the pages. This is especially useful for multilingual eBook reading as an aid in learning a language. In this case the audio is usually a different language from the text.
 Headphone jack
In older devices that supported sound there was always a universal 3.5mm headphone jack that was intended to provide a connection to the device. This was generally true even if the device also had internal speakers or Bluetooth support. The jack provided three connections, stereo sound left and right plus a ground connection. Some have a fourth connection that provides for microphone input, allowing cell phone style headphones to be used. Headphones are generally inexpensive and are not powered. They depend on the source device to provide the analog signal they need.
However, lately the headphone jack is missing from many devices. This is most likely on devices that advertise being waterproof. The standard 3.5mm jack can be a source of water egress and dust egress to the internals of the device. It is possible to to find a 3.5mm jack that is waterproof but it costs more money than the standard one.
One solution is to only provide Bluetooth support for audio. A number of eBook readers have taken this approach. An adapter such as the Bluetooth KN321 can be used with regular headphones or earphones.
Another solution is to provide an adapter for audio use that uses the existing USB or equivalent charging port. This approach is used on some Kindle devices. The sound data is sent to the device as digital information that is converted to analog inside the adapter. This is done with a DAC, Digital to Analog Converter. Any required amplification was also inside the adapter. Power is usually supplied by the Reader although an internal battery could also be used.
The lightning connector on Apple devices and the new USB-C can both provide a way to directly support headphones. The Apple devices have a DAC inside the source Reader and provides headphones that can use the connector directly. A simple adapter to go to 3.5mm connector is also available for the user to provide their own headphones. However, some USB-C devices do not have a DAC inside and send the audio as digital data to the port like was done on the older USB ports. These devices depend on the special adapter to contain the DAC to convert the signal and then have a 3.5mm connector at the end; or they may depend on the DAC to be inside the headphones themselves that they offer. This means that these headphones are no longer interchangeable between devices. The user needs to be aware of this change which is primarily on Android devices and they may not be able to use their favorite headphones.