Back in 1971, Jim Keltner closed his eyes as he laid down the drums to John Lennon’s bittersweet ballad “Jealous Guy.” He knew he shouldn’t do it blind -- the song was just so mesmerizing. He almost forgot that he was collaborating with a former Beatle.

“But when I opened my eyes and saw John singing on the microphone…” he trails off, recalling the moment to Billboard. “That’s something I’ll never forget. It still gives me that same feeling today.”

On Friday (Oct. 5), the music that enchanted a young Keltner nearly 50 years ago receives its most lavish reissue yet with Imagine: The Ultimate Collection (Geffen/UMe).

Beatles fans may know these songs by heart, but over the course of six discs, this box set dives deep into Lennon’s first Billboard 200 No. 1 solo album and brings listeners right into the studio. Throughout it all, however, one element stands the test of time and studio trickery: Lennon’s voice.

It all started at Ascot Studios, the London recording space built by Lennon and Yoko Ono on the grounds near their country home, Tittenhurst Park. Finally freed from public pressure, bureaucracy and bad vibes, the newly emancipated Beatle could work on his own terms.

Ono sees the studio as a mini-rebellion against the trappings of a mainstream pop band -- and for a holistic “us.” “Both of us understood that it was very important to be honest and open,” she remembers. “Not for other people, but for ourselves.”

Lennon’s first solo album, John Lennon/Plastic Ono Band, was tortured, didactic and consumed with grief. With its follow-up, Imagine, he was ready to deliver similar messages in a more accessible package. The result was Lennon’s most commercially successful album, hung on its startling title track, which became a modern standard by earnestly depicting a world without hierarchy or dogma.

And its satellite songs were nearly as good: the elegant ballad “How?”, the apologetic “Jealous Guy,” and the bile-filled indictment of manipulative political figures “Gimme Some Truth."

But even as those songs have saturated the public consciousness over nearly five decades, it turns out there were still ways to flatter the sound. Specifically, Lennon was notoriously insecure about his vocal ability, leading to lots of double-tracking and tape delay on the 1971 mixes.

And Phil Spector’s production, while forward-thinking, sometimes bore his mark a little too much; “I Don’t Wanna Be A Soldier Mama,” in particular, swims in reverb and buries its powerful live-in-the-studio performance between Lennon, Keltner and bassist Klaus Voormann.

Engineers Paul Hicks and Rob Stevens took a unique, double-faced perspective to The Ultimate Collection. Hicks, a long-time Beatle comrade who worked on projects like Cirque du Soleil's Love show, the remix album Let It Be… Naked and the Beatles’ 2009 remaster campaign, handled Disc 1’s Ultimate Mixes. He simply shone the original 1971 mix to a 2018 clarity without making it overly slick or modernized.

Stevens, who co-produced 1998’s The John Lennon Anthology with Ono, took Disc 3, called Raw Studio Mixes, which feature no strings, overdubs or decoration: simply the barest, driest version of the music, as if the listener was sitting in with the band. According to Stevens, the only way to access this ultra-pure version of Imagine was to drop his own ego.

“Let’s bring up these faders, let’s forget who it is, let’s forget it’s a legend,” he remembers of the process behind Raw Studio Mixes. “Otherwise, you’re listening through a filter of ‘This is freaking John from the Beatles. This is John who sang 'No one, I think, is in my tree,' and made me sit there with my jaw dropped.’”

While Hicks’ presentation of Imagine is simmering and subtle, recommended for acolytes of the original mix, it’s by design that Stevens would go deepest into what actually happened in the room. Lennon’s living bandmates on Imagine -- Keltner, Voormann and drummer Alan White -- all hear John’s sweet-and-sour voice, accompanied by guitar or piano, as all you need from the man.

“It was always a mistake,” says Voormann, now one of John’s oldest friends and collaborators in the visual and musical realms. “He hated his voice. He told me he didn’t like his voice. But you can get much more into John’s feelings when you don’t have those effects.”

Keltner, who drummed on “Jealous Guy” and “I Don’t Wanna Be a Soldier,” agrees that less is more with Lennon: “He had one of the greatest voices ever. But he was a searcher. He loved to have things not how they were.” ?

Despite being apprehensive about his voice, Lennon was, by all accounts, more relaxed than ever at Ascot. Alan White, who drums on most of Imagine, recalls a “homey” atmosphere where all involved shared meals around a big wooden table.

“It was informal,” White says, “But there was a sense of the meaningfulness of the songs. John would give us the lyrics beforehand to make sure we knew what they meant and what we were saying to the world.”

The Ultimate Collection is not the first re-release of Imagine, but it is by far the most extensive, spanning four CDs and two Blu-Ray discs in a visually arresting, Ono-curated package.

“This is it for Imagine, as far as I can… imagine,” Hicks says with a chuckle. “There’s nothing else. We’ve gone through all the tapes.”

Stevens also sees this box set as the logical finish line for an album that has continued to ensnare new fans over the last several decades. “The reason the word ‘ultimate’ was used was because that was the intention,” he says. “If you wanted to put out an Imagine that was more comprehensive and artistic than this one, good luck.”