Inside Studio B at Paisley Park in Chanhassen, Minn., Prince’s new co-producer Joshua Welton has just finished giving The Post an advance listen of the artist’s brand-new album, “HITNRUN,” when the phone rings. Welton passes the receiver to this reporter, and a voice says, “Hi, it’s Prince.”

Oddly, the 57-year-old is only a few yards away, in Studio A. Paisley Park is the $10 million complex that Prince calls home, and which he has used to record virtually all of his music since 1987.

From the outside, the building looks almost industrial, with high fencing to keep intruders away. Journalists are rarely permitted to enter, and those who do are banned from taking pictures or using recording devices. The unexpected phone call is almost certainly placed so that Prince can screen the guest in his home.

“I’m not interested in what happened yesterday,” he tells The Post bluntly.

He’s referring not only to his untouchable run of ’80s albums — including such classics as “1999” (1982), “Purple Rain” (1984) and “Sign o’ the Times” (1987) — but also to the constant stream of contemporary artists such as Miguel, the Weeknd and Tame Impala who crib from his back catalog.

Prince hasn’t had a Billboard hit single since the mid-’90s, and yet his brand of sexy funk is more dominant in pop music than ever.

“There might be music that sounds like me, but what good is that? You’re essentially in the feedback loop,” he says. “It’s a bad time for music in general. There’s not a lot of pop music in the mainstream that makes you feel scared, that makes you wonder what’s happening.”

Meanwhile, “HITNRUN” is the most aggressive and progressive set of songs the musician has put his name to in many years. In between glimpses of vintage Prince, there are blasts of hip-hop, electro-funk, house and even dubstep. “ ‘Brutal’ is a good word,” summarizes Prince, and it’s Welton, his new muse, who takes much of the credit.

The album will be released on Labor Day exclusively on Jay Z’s Tidal streaming service, primarily because Welton and Prince are attracted to the idea of the high-quality audio option that Tidal provides to premium subscribers.

“That’s how we wanted this music to be heard — the way it sounds inside [the studio],” says Welton. “It’s peace of mind for us. People who know Prince and what he stands for will hopefully look at this and wonder why he’s doing it.”

In just three years, Welton has worked himself up to a prime position in the Paisley Park hierarchy, but he came in through the back door. The 25-year-old Illinois native is married to Hannah Welton, the drummer of Prince’s latest backing band, 3rdeyegirl.

Back in 2012, Hannah received the call to audition and Welton tagged along. Hannah impressed with her drumming ability, but Prince and Welton bonded over their shared spirituality.

“The first real conversation I ever had with Prince was a two-hour conversation about Jesus,” says Welton, who is Christian (Prince has been a Jehovah’s Witness since 2001).

“When Hannah first got the call to audition, we were living in Atlanta, I was working at a security firm, and we didn’t even have a place of our own. We went from that to being here. The way I look at it, that’s God’s work.”

It wasn’t all divine intervention. Once 3rdeyegirl began to talk about releasing an album, Welton’s name was thrown into the mix as a producer.

His own musical pedigree dates back to the tender age of 15, when he was part of hip-hop sextet Fatty Koo (who were briefly signed to Columbia Records). Prince gave him some ideas to work on, liked what he heard, and Welton found himself thrust into the producer’s chair.

Although his work as co-producer was also featured on the 2014 album “Art Official Age,” the new “HITNRUN” sees Welton and Prince on a more radical path. “It’s not like I was waiting for an opportunity to show Prince what I could do,” he adds. “At first, I was there purely as support — I would lend an opinion, or get coffee if that’s what everyone needed.”

He’s part of the inner circle now, but Welton can’t completely hide his sense of awe at Prince’s supernatural aura. “There have been times I’ve been working on a song, and he’ll slip into the studio like a ghost. I’ll turn around, and he’s just sitting there! He’s scared me so many times doing that. Now he tries to make a lot of noise coming in, so I have some warning!”

As Welton guides The Post around Paisley Park, his own fandom comes closer to the surface. He points out a small, framed black-and-white photo of Prince’s father, John L. Nelson, a jazz musician and a massive influence on the artist’s life.

In the live room adjoining Studio B, huge portraits of Prince, Hannah Welton and 3rdeyegirl’s other members — Ida Kristine Nielsen (bass) and Donna Grantis (guitar) — adorn the walls. In the middle of the room is a pingpong table. Forget the “Chappelle’s Show” skit about the blouse-wearing, basketball-playing Purple One, because table tennis is now the preferred pastime at Paisley Park.

“Prince can beat pretty much everyone at pingpong,” says Welton. “He’ll stand in the same spot the whole time — sometimes with one hand behind his back!” He points over at Prince’s paddle, which no one else is allowed to touch. It’s well-worn, the rubber frayed, “because it’s always on fire,” quips Welton. (Sadly, it’s green, not purple.)

““HITNRUN” is the most aggressive and progressive set of songs the musician has put his name to in many years.”

Another part of the complex hides a long hallway, which serves as an informal Prince museum. On one side is a picture timeline of his career dating back to the 1978 release of first album “For You.”

On the opposite wall are Grammys, American Music Awards, MTV gongs and more. At the end of the hallway, a door opens out into a foyer, in which stands the iconic motorcycle from the “Purple Rain” movie. “I’ve never seen [Prince] ride it,” says Welton, with a hint of disappointment.

Suddenly, Prince himself appears, wearing a knee-length gray sweater. He wants to clarify something he said on the phone earlier. “I’m not the kind of person who’s down on young people in music,” he explains.

“Joshua is one of the new generation who can do multiple things. Janelle Monáe is making her own music industry with her music and her label [Wondaland]. She’s not afraid of anybody. One day, she’s gonna be president and no one’s gonna even know what happened!”

To shed light on what they’ve been cooking up, Prince asks Welton to play a 3rdeyegirl work-in-progress, which unfurls like a Beach Boys song but suddenly snaps into vintage-disco territory.

It’s stunning, but neither Welton nor Prince knows when, or if, it might see the light of day. It might just be another addition to the Prince vault, a Holy Grail of thousands of unreleased and unfinished songs dating back decades.

“The trick is to make the music mean something in the world outside of here,” says Prince of his secret stash. He may not like to look back on the past, but hints that there are loose ends yet to be tied up. “I’ve vaulted so much stuff, going way back to the ’80s, because I didn’t want people to hear it — it wasn’t ready. One day I’ll go back and finish it, and it’ll feel like no time has passed. To me, time folds back on itself.”