A combination of readers (MrPooHead and slaphead to be precise) have suggested it's Mel Torme ('the Velvet Fog') singing 'You've Made Me So Very Happy', originally a hit for Blood, Sweat and Tears. And I'll add that it was written by Motown founder Berry Gordy Jr. with Brenda and Patrice Holloway and Frank Wilson. And that BS&T featured the phenomenal Al Kooper on keyboards, who also plays the sweeping intro to 'Like A Rolling Stone' for Dylan.