I use the NuForce iDo. They are no longer available new, but you might find a used one, or NOS, on eBay or Amazon. I use mine to provide bit-perfect digital output from various iPods, iPhones, etc., to my DAC. It is also a very good headphone amp if you need that.
I've used mine with Apple gen 4, 5 and 6 with no issues.
I've used mine with Apple gen 4, 5 and 6 with no issues.