Here’s the JDS Labs link -
https://blog.jdslabs.com/2014/01/how-to-connect-android-ipad-or-iphone-to-a-dac/
You can avoid this problem by supplying external power to the DAC, thus offloading an otherwise heavy current drain from the iPad. In most cases (except the case of Lightning to USB3 Camera Adapter - see later notes) this is achieved by connecting your DAC via a powered USB hub.
Then the minimum requirement for an operating system which supports external DACs is iOS7 - so, in theory, if you update the operating system on your iPad, even as far back as the iPad 2 (from 2011) should work. Then to physically connect to your iPad;
i) for 30-pin connector models - require the offical Apple Camera Connection Kit - part number MC531ZM.
ii) for Lightning connector models - require the official Apple Lightning-to-USB Camera Adapter, of which there are 3 different models:
- part no. MD821ZM/A, then superceded by part no. MD821AM/A (USB2.0)
- part no. MK0W2AM/A (USB3) - this adapter will accept external power from the Lightning connector of an Apple power adapter - so in this case you don’t need a powered USB hub.
iii) for USB-C models (iPad Pro 3rd gen) - should be able to connect directly to your USB hub, although apparently there are some operating system bugs at this present time.
Regarding compatiblity - the XMOS USB interface, as used in the Khadas Tone Board, is definitely supported by iOS, and confirmed working with several XMOS-based DAC’s. That doesn’t necessarily guarantee compatibility, but it’s a promising indicator.