In Andre Previn's first season as music director of the Pittsburgh Symphony Orchestra, he hired a talented young harpist named Gretchen Van Hoesen to be principal.

Recently, she has returned the favor, asking the PSO to commission Previn, music director from 1976-84, for a new harp concerto. Van Hoesen will premiere it this weekend at Heinz Hall.

"He is such an excellent composer," she says. "He likes the harp and has put it in all the pieces we have played, but hasn't ever written anything for solo harp. I thought since he is a master orchestrator and has really good ideas it would be wonderful."

But writing for the harp within a large orchestral score is a far cry from placing it out front as the featured instrument, and Previn was a bit daunted. "I thought, 'What the hell do I know about the harp?' " he says, with a gentle laugh. "Since I don't play the instrument I talked to her about what could be done and she gave me a book on the harp, which I referred to as I wrote it."

The two met a few times in New York City to work out details, and Van Hoesen says Previn has created a piece she thinks "will get into the repertoire and be played by other soloists." It is in three movements, which "escalate" in energy from an "introspective" first movement to a "lush" second to an "almost jazzy" finale, she says. "You will know it is Previn. It will sparkle with color and has two long harp cadenzas."

Her harp itself will be colorful. Van Hoesen will again perform on her special lavender harp. It is a Lyon & Healy instrument built in the 1920s for a harpist who toured with actress Sarah Bernhardt, who only performed in lavender gowns.

Previn has included some extended techniques, such as calling for sliding the harp's pedal while playing and tapping on the instrument's frame. But "It is not complicated, and is quite tonal," says Previn. He is not one to talk much about his music. In fact, he resisted providing a description of the concerto in the PSO's program.

These days, he would have much to talk about, if he wished. Known for most of his career primarily as a conductor and a performer, Previn lately has been busy as a composer. It's been the most activity he has experienced since completing his first opera "A Streetcar Named Desire" in the late '90s. The largest project is a new opera, "Brief Encounter," based on Noel Coward's one-act play, "Still Life." Houston Grand Opera premieres it in Spring of 2009.

Previn had to adapt a beloved work in "Streetcar," but setting Coward's play was even more difficult, he says. " 'Streetcar' is larger than life, it is very melodramatic and operatic. [Still Life] is a very small, intimate British theater piece and you really can't do much with it. I was more hampered, if you like, but I didn't consider it as that."

The plot centers on a suburban woman's temptation to cheat on her husband after a chance encounter with a doctor at a rail station. The twist is that "they do not wreck their marriages, they do not have a screaming, flaming affair, they just love each other and then it is over," says Previn, who suggests that if you watch one of the film adaptations, rent the 1945 British version, not the 1974 one. "Just hearing the cast alone [Sophia Loren and Richard Burton] will make you laugh," he says of the latter.

Other compositions on Previn's plate or recently completed include a piece for the Vienna Philharmonic, a clarinet sonata, a piano trio and new song cycles -- a genre he cherishes -- for sopranos Barbara Bonney and Renee Fleming. "I love words and poetry," he says. "Sometimes I just feel like setting words, whether it is commissioned or not. The idea of accompanied voice appeals to me."

The precise yet elegant playing of Van Hoesen also appeals to Previn. "She is a wonderful harpist."

He should know, he hired her.