Paul McCartney has paid a heartfelt tribute on social media following the sad death of his good friend, American recording engineer Al Schmitt.MORE: Paul McCartney reveals exciting family news - and it involves his children!The Beatles star took to Instagram to share a black-and-white photo of the pair in a recording studio laughing, alongside a personal message.The music icon wrote: "So sad to hear of the passing of my friend Al Schmitt.