After beating the last notes of The Nutcracker on Friday night, Pennsylvania Ballet music director Beatrice Jona Affron got up on stage to take a bow with the dancers, gestured to the orchestra pit, and directed applause to the musicians. But many were already packing up, and few, if any, looked up to acknowledge what everyone else in the room seemed to know: that Tchaikovsky's score is perhaps this work's primary source of magic.

"Music," Balanchine said, "is the floor we dance on." And yet in the case of this Pennsylvania Ballet production, running through Dec. 31, it's the weak spot. I know: These are times for feeling grateful for what we have, not aspiring to improvement. Other Nutcrackers have been threatened with recorded music, and we still have a live orchestra accompanying this annual production. Wonderful.

But Friday night, you had to believe that this city, known for orchestral achievement, can do better.

Part of the lack of sumptuousness is surely the Academy of Music, which, after a decade and a half of major renovations, still needs an expanded pit to fit more musicians. The orchestra has about four dozen players, often sounding anemic and unbalanced. With a string section this small, it's difficult for the brasses to do their job without overpowering the ensemble.

The orchestra's overall deficiency, however, is a consistent lack of ability to conjure the kind of emotion that makes The Nutcracker a transporting experience. When the Christmas tree grows, it's all very nice, and impressive to be surprised that something we took for granted as stable was somehow, mysteriously, transforming. But the real power of this scene comes by way of the music - a repeated, ascending, almost dangerous-sounding orchestral figure whose proportions expand in such a way that we can't guess where it will end. End it does, and when it does, with a great burst, it can send chills up our spines. But that kind of orchestral experience didn't happen here, and the size of the group was just one reason.

Affron, we all know, is often bound in her tempos by the dance she is accompanying. But what could have been the reason for her dull and steady pacing here? She failed to build the section as the ballet's key moment - the emotional occasion for our suspension of disbelief.

The playing itself was also to blame. You might think quality suffers in this ensemble because it's a group of freelancers. But in fact, membership has been remarkably stable in the last two decades. And that in itself might be an indication that job security is working at odds with quality.

There are many fine musicians to be heard - the double basses, a two-person percussion section as adept at castanets as at timpani. But in a town that yearly turns out top orchestra players from the Curtis Institute of Music, is it possible there is no more velvety and secure English horn playing to be had? Or trombones and a tuba with more presence and greater sensitivity?

The Snowflakes and Mother Ginger deserve a more reliable floor to dance upon.

George Balanchine's "The Nutcracker"

Through Dec. 31, at the Academy of Music, Broad and Locust Streets. Tickets: $24-$129. 215-893-1999 or www.paballet.org.

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