Their first major hit single was "You've Lost That Lovin' Feelin'" in 1965. Produced by Phil Spector, the record is often cited as one of the peak expressions of Spector's Wall of Sound production techniques. It was one of the most successful pop singles of its time, despite exceeding the standard length for radio play. Indeed, "You've Lost That Lovin' Feelin'" remains the most played song in radio history, estimated to have been broadcast over 8 million times to date. A little known fact about this song was that Spector utilized Sonny and Cher as back-up singers