Rubin proposed that Cash sit down with him in the living room of the Sunset Strip house or at Cash's house near Nashville and sing every song he ever loved. The process went on for weeks, Rubin putting it all on tape. Eventually, the producer started bringing in songs for Cash, such as Trent Reznor's "Hurt." The rewards included three Grammys.

"Most important, Rick made me have faith in myself again," Cash said after the first album in 1994. "He made me believe in myself and my music, which I thought was gone forever."

Cash spoke in equally glowing terms nearly a decade later, a period when the singer was so ill — from asthma, various bouts with pneumonia and more — that he sometimes had to rest between lines of a song in the recording studio.

In an interview just before the release of his 2002 CD with Rubin, Cash said he couldn't imagine anyone wanting to keep recording him, especially when Rubin's albums with the Peppers and others sold far more.

So after the last vocal on the record, Cash said he walked over to Rubin, shook hands and said, "It's been fun." It was his way of saying he understood if Rubin wanted to call it quits.

"But Rick immediately asked what I wanted to do next," Cash continued, his eyes moistening at the loyalty. "I mentioned [an album of] black gospel, and then I mentioned an album of songs that would show my musical roots, and Rick said, 'Let's do them both.' I was dumbfounded. I thought I might finally be at the point where I would only be singing for myself."

The scenario with the Chili Peppers wasn't at all similar to the Cash story, but the ending touched similar emotions.

Rubin was impressed when he saw the Peppers on stage at the Greek Theatre in 1989, but he was even more impressed by their potential. "They were always incredible musicians, but that doesn't necessarily make a great band," Rubin says, looking back.

The producer felt that the Peppers were limited in the way they saw themselves — a funk group with rap lyrics, which can get old pretty quick. He could see them playing rock on a much wider scale if he could get them to change their idea of what the Chili Peppers were about. He "needed to break down the walls in their own imagination."

While encouraging the band to think in wider terms, Rubin focused with lyricist Anthony Kiedis on material. The breakthrough came in the late-'90s when Rubin glanced through a book of Kiedis' poetry and notes and found what became the lyrics to the song "Under the Bridge," a tale of drugs and alienation that spoke with more depth and universality than the group's previous tunes.

"It was something that cried for singing, not rapping, and for a melody, not just funk beat, and I said, 'What's this?' and he said, 'Oh, that's not for the Chili Peppers, it's not what we do.' "

But Rubin kept encouraging Kiedis, who eventually sang it for guitarist John Frusciante, who put chords to it, and they played it for bassist Flea and drummer Chad Smith, who were open to it. It was a breakthrough hit that greatly upgraded the future of the band.

As with Cash, Rubin helped the Chili Peppers to believe in themselves.

And now U2 too

It's a few weeks later and Rubin is back home after working with U2 in England on two tracks for the band's latest greatest-hits album and three weeks vacationing in Hawaii with his fiancée, model Amanda Santos.