I'm seeing Fiio E18 in its official website:
It says that E18 would work on any device with USB OTG function. Just what is USB OTG? It also says by means of Micro USB cable, so all android devices with Micro USB are supposed to work with it?
What about Android FTP/MTP connection? Some smartphones can only act as one.
OTG - on the go usb which turns your smartphone (if supported) to a "host" for connection of peripheral devices like mouse, keyboard, external usb storage, etc. Although it looks like a short usb cable (micro-usb on one side to connect to your phone and female full size usb to attach other devices) it has two internal pins shorted to tell your phone its otg cable which sets "host" mode. The easiest way is to google your phone model and if it supports OTG.