San Francisco Performances presents an early, not frequently heard work by Philip Glass in concert on Tuesday evening: Music with Changing Parts. And the parts have been changed a bit since their original orchestration of the 1970s. Lisa Bielawa, who has sung with the Philip Glass Ensemble since the early 1990s explains that the San Francisco Girls Chorus and musicians from the San Francisco Conservatory of Music will be joining the PGE.

There’s more information about the concert at the San Francisco Performances website. The Girls Chorus sing another work by Glass on this week’s Bay Area Mix show, on KDFC at 8 pm on Sunday.

“People often say, ‘Oh, what’s it like for you up there, you know, are you sort of in a trance?’ And I say, ‘No, man. It is the opposite of a trance,’ I mean, it is one of the most exciting practices of extreme focus and concentration.” The work changes gradually over the span of more than an hour. “It was written originally in 1970, so it’s one of those very early minimalist works that predate Einstein on the Beach. This is very early stuff, when Philip was working with his ensemble and playing in warehouses in Lower Manhattan. So it’s a pretty early piece, but of course we’ve given it a whole new set of clothing.” Bielawa herself would usually have sung the ‘long tones’ that arc over the repeating keyboard patterns. “I managed to fire myself, by bringing in the San Francisco Girls Chorus, which is great. They don’t really need me anymore. I don’t need to sing long tones, so I’m actually playing the third keyboard part… This new arrangement really maximizes the potential of this long tone as a way to bring texture and to create a shape whereby the expansiveness of this iconic sound just keeps deepening and broadening throughout the piece.” The Girls Chorus is in New York this weekend, singing the program with the Philip Glass Ensemble at Carnegie Hall before returning for Tuesday night’s performance here in San Francisco.