If you use a standard iPod/iPad/iPhone connector to 3.5mm to the E7, your iPad will bypass its usage of the internal amplifier and instead send the sound signal to the E7. The E7 in turn acts as the amplifier, which is what you want. This is suppossed to provide the cleanest signal and sound to the E7 as the iPod/iPad/iPhone's internal amp is disabled when the LOD cable is used.
Fiio has two LOD cables (standard iDevice connector -> 3.5mm plug) to handle this duty:
- L1 / L9 (standard cable - L9 is right angled)
- L3 (better quality cable)
In the absence of the LOD cable, you can use a 3.5mm interconnect to use with the E7 as an amp, but I imagine the signal isn't as clean.
Hope this helps!