Stars of Opera Philadelphia's O17 festival have mostly been composers, librettists, directors, and designers — all elucidated by fine singers. But how about a grand, old-fashioned, big-voiced, sumptuously dressed diva?

That was Sondra Radvanovsky, who committed a while ago to an O17 recital Sunday at the Perelman Theater — and then was cast to open the Metropolitan Opera season Sept. 25 in a new production of Norma — one of the summits of the opera repertoire. But she hung on to her Philadelphia engagement, while endearingly excusing herself for using a music stand for most of her two-hour concert with pianist Anthony V. Manoli, since Norma, with its Medea-like drama, has to be distracting.

But there was no coasting. The generous program included Bellini, Strauss, Liszt, Barber, plus three encores and a great generosity of spirit — not to mention grandeur of wardrobe and an inviting, confiding stage manner — that made the afternoon memorable, even though the recital medium doesn't show her at her best.

Not yet 50, the Chicago-born, Indiana-raised, Canada-based Radvanovsky specializes in infrequently sung bel canto opera roles written for the great vocal personalities of the 19th century. Though such operas don't hold up in modern times unless sung by the likes of Maria Callas and Beverly Sills, Radvanovsky obviously has withstood comparisons to them, having sung several of these roles at the Metropolitan Opera in recent seasons. Yet she has been curiously lacking a high-profile recording career.

The voice itself isn't innately beautiful and can be unruly. Though she has some lovely pianissimos, she isn't one of those golden-throat beings who could seduce an audience by singing the phone book. She goes to the very heart of whatever language she is singing, but her actual diction is only intermittently clear. The high notes are there, though one hears the vocal machinery at work while she's achieving them.

Whether or not she's singing those long-spun lyrical vocal lines of Bellini, the music comes out sounding like the Italian bel canto composer of Norma. These are not exactly criticisms, but observations on what she has to offer, which is a lot, since the core experience of operatic singing is visceral. And she does not spare herself on that front.

Thus, the three Bellini songs felt much more substantial than one had a right to hope for. Richard Strauss is usually heard with more vocal sheen, and the tempos she favored were questionably slow. In terms of diction, the most successful set was a trio of Liszt songs with French-language texts by Victor Hugo that were not only more clear but effectively altered the color of her voice.

She clearly adores Samuel Barber's 1953 Hermit Songs, in which the West Chester-born composer set to music poems by medieval Irish monks that contemplate religious mysteries but also enshrine their pets (such as a white cat named Pangur). But her affection was more felt than articulated. Among her encores, Dvorak's "Song of the Moon" from Rusalka had a heart-stopping moment two-thirds of the way through: When the words speak of the human soul, you heard Radvanovsky's inner being.

After her 2:30 p.m. Monday master class at the Perelman Theater, she's back into Norma rehearsals, and no doubt the better for having a dose of Philadelphia love plus the detail-demanding "cross training" benefits of recital singing. So, if the broad strokes of Norma come out more precisely than usual, we can say we were kind of in on the process.