Pentatonix Losing a Member ... How Does TETRAtonix Sound?

5/12/2017 10:47 AM PDT

Pentatonix Losing a Member, Continuing Summer Tour

Breaking News

And then there were four ...

Five-person a capella group Pentatonix is about to lose Avi Kaplan ... who announced his departure Friday morning in video posted on the group's Facebook page. 

Avi says he'd been struggling to keep up with the fast pace of the band, and missed his friends and family as well as not being able "to escape into nature when I’m feeling overwhelmed or just need some time to myself."

Pentatonix won the third season of NBC's "The Sing-Off" ... and they pop up on almost every award show doing tribute performances. They're about to kick off a summer tour that includes gigs in Japan, and 3 nights at the Hollywood Bowl.

Avi says he'll still do all those shows, but after the tour ... he's done.

For what it's worth ... the other members did NOT seem thrilled during his taped announcement.