Convert from line level to mic level, without worsening the sound quality
You need to add an AC coupling capacitor before your signal goes into the iPad.  I'm thinking that about 30dB attenuation should get you in the ballpark level-wise.  

Also be careful of the TRRS pinout both coming out of the iPhone and going into the iPad.  Check online sources for the pinout - my recollection is that Apple does things differently from anybody else.







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