Jam Session, a “new” collection of live performances featuring fiddle maestro Mark O’Connor alongside some of progressive bluegrass’s most accomplished players, will be highly desirable to any fans of virtuoso bluegrass picking and mind-bending improvisational musicianship. That said, any O’Connor fan worth his or her salt already owns much of this new disc, since half of the material here is previously released. Four of these songs were taped at a Vanderbilt University concert in Nashville back in 2002 and were included on O’Connor’s Thirty-Year Retrospective, a live compilation from back in 2003.

Those performances, on which O’Connor is joined by Chris Thile on mandolin, Bryan Sutton on guitar, and Byron House on bass, are, as would be expected with such a lineup, simply amazing. There are certain pickers in the world of neo-bluegrass who are geniuses of their instruments. They produce gorgeous tones, solo with clean precision, can play at breakneck speed, can improvise with lyrical expressiveness and creativity, have instinctive mastery of chording and scales and modes, and have innate reflexes in spontaneous musical communication with other players. No one, however, does these things better than O’Connor, Thile, and Sutton.

One of the reasons we have to go back nearly a decade to find these recordings of O’Connor playing with similarly talented bluegrass pickers is that O’Connor is a somewhat reluctant bluegrass hero, having long ago moved on from being a Nashville cat to exploring jazz and classical music, associating himself with the likes of Yo-Yo Ma and Edgar Meyer. After all, a virtuoso like O’Connor can hardly be pinned down to a particular genre, and O’Connor could no more limit himself to “The Orange Blossom Special” than Béla Fleck could be confined to “Foggy Mountain Breakdown”.

Unlike O’Connor, though, Fleck has kept a firmer foot in bluegrass circles, planning his summer around the Telluride Bluegrass Festival, for instance, the annual mecca of acoustic jamming. Thile and Sutton also play Telluride every year, joining perennial mainstays like Sam Bush and Jerry Douglas, pushing the boundaries of newgrass, certainly, but returning more frequently to traditional bluegrass than O’Connor has. O’Connor plays Telluride only sporadically, just once in the last ten years, instead letting players like Stuart Duncan and Luke Bulla step in as the reigning fiddle champs.

O’Connor’s musical journeys away from bluegrass, then, make Jam Session a noteworthy release to anyone interested in hearing the world’s best players cut loose. That said, there’s nothing very traditional about this set of tunes, which also includes never-before-released songs recoded at the RockyGrass festival—Telluride’s more-traditional sister fest—and three songs recorded with O’Connor’s Hot Swing Trio, which includes Django-style guitarist Frank Vignola and bassist Jon Burr. Instead, Jam Session features a variety of world-hopping jazz, swing, and classical styles, all of them consistently thrilling.

Jam Session plenty of action from Sutton, who has redefined what can be done with flat-top picking, and from Thile, the boy wonder who possesses a brain-to-fingers continuum unmatched in the annals of history. Fair warning, though: some of the jamming here involves the kind of abstruse skronking and plinking that might send some people heading out for clearer air after awhile. To keep themselves from getting bored, these masters sometimes risk boring their listeners with interminable outward improvisation, as a couple these songs extend past the 12-minute mark. Still if you love listening to O’Connor, Thile, and Sutton show off—and nobody does it better—then Jam Session is not a disc that you should be expected to live without hearing.