Iphone Xs

my understanding is that all the I-phones have an onboard DAC but that it’s not up to much. When listening to tidal on the headphones without an external DAC it’s not very good quality. I have the older sennheiser hd700’s and they expose any deficiencies in the source. Even the arc am made the sound less harsh but at the same time revealed more detail. My ultimate objective is to achieve that again but not spend a fortune doing it. 

Hi im new to the forum. I have a solo ultra linear headphone amp that has been locked away in a cupboard for ages. In the past i connected my iphone via an arcam DAC docking station into the phono in sockets. however since those days I have changed my phone many times and it is no longer compatible with the arcam docking station. checking teh arcam site they no longer make this type of simple device so im thinking a simple lightning to phono lead mught do the trick? however this would mean no DAC in between the phone and the amp so i imagine this would give poorer results? Any suggestions would be appreciated. Be gentle with me as I am absolutely clueless about this stuff.