Leave it to conductor Nicholas McGegan to showcase the "hidden" Beethoven: the composer's Piano Concerto No. 4 and Symphony No. 4. These masterpieces from his middle, or Heroic, period -- understated and underplayed, respectively -- have an "anti-Heroic" cast, grabbing the listener with sheer poetry and magical craft. Go hear them. McGegan is showcasing them through the weekend with the Philharmonia Baroque Orchestra and pianist Emanuel Ax.

Thursday at the Center for Performing Arts at Menlo-Atherton High School, this nifty program had its first Bay Area performance -- and McGegan didn't save his star-power for the end of the show. At 8 p.m., out walked Ax, settling himself at the keyboard of a fortepiano built in Vienna in 1840 by the firm of one Maximilian Schott. Then began this historically informed performance, with clean chords and timbres, letting the audience hear Beethoven's inner voices and harmonies clearly and directly.

It's a different sonic world, as articulated here by McGegan and his 40 or so players on their period instruments.

The opening Allegro was vigorous yet lyrical, with springing rhythms, punchy and intimate. The conversation between soloist and orchestra was -- well, right there, in your ears. The sound of the fortepiano (predecessor to the steel-framed modern concert grand) was woody, subtly resonant, at times harplike. Ax made adjustments, getting around some thinness in this particular instrument's upper treble range. But his passagework was crisp and elegant as a figure skater. Clearly, he loves this piece.

William Meredith, the Beethoven scholar from San Jose State, had invited the audience in a pre-concert talk to "use your poetical imagination" while listening to the concerto's famous second movement. Going back to the 1840s, more than one commentator has suggested that Beethoven intended it to musically depict the myth of Orpheus taming the wild beasts.

Thursday, Ax's poetic utterances gradually subdued the orchestra's outbursts -- though not quite so dramatically as Beethoven wished. In the score, the composer instructs the soloist to apply the soft pedal, going from the standard three strings down to a single string, perhaps representing Orpheus's lyre. Ax couldn't do that; this instrument only goes down to two strings.

Interesting back story: With Beethoven at the piano, this concerto had its first performance in 1807 in Vienna. To make this weekend's program as time-specific as possible, Philharmonia Baroque had arranged for Ax to play a fortepiano dating to 1805-10, built by one Johann Fritz. However, there were technical problems with that instrument, which Ax played in Friday's performance at the Mondavi Center in Davis. Luckily, the owner of the Fritz fortepiano (pianist Belle Bulwinkle, who teaches at Mills College) also owns the Schott fortepiano. So, on the way from Davis to Atherton, the piano movers stopped in the East Bay and switched instruments.

In any event, one still felt the myth. And the Haydnesque finale was a charmer. There was a gorgeous chorale, led by plummy winds. And Beethoven's lush colorings were vivid: the warmth of the violas and the pungency of the cellos, surrounding Ax's surging figurations and cadenza.

The program's second half began with more charmers: Beethoven's Twelve Contredanses for Orchestra, works of somewhat misty origin, shining a light on the composer's love of rustic dance and street bands. The theme of No. 7 points straight at the finale of the "Eroica" Symphony, by the way. These dozen dances also illuminate some of the swagger and lilt of Symphony No. 4 (especially its Scherzo).

Wedged between the "Eroica" and Symphony No. 5, No. 4 doesn't get the respect it deserves. But it is a marvel, a sort of absolute composition, in which every single line is a valid theme in itself. Taken as a teeming whole, the fabric of the symphony becomes a paradigm for Beethoven's imagination and resourcefulness.

Interpreted by McGegan and the orchestra, it was a joy: the motoric burblings of bassoon; the rumblings of kettledrum; the thrumming rhythms of the Allegro vivace; the spotless pizzicato of the Adagio; the swagger of the Scherzo; the wind-swept strings of the finale. In other words: clear, direct, intimate Beethoven.

Contact Richard Scheinin at 408-920-5069, read his stories and reviews at www.mercurynews.com/richard-scheinin and follow him on Twitter at www.twitter.com/richardscheinin

Philharmonia Baroque Orchestra

Nicholas McGegan, conductor; Emanuel Ax, piano

When: 8 p.m. Friday
Where: Herbst Theatre, 401 Van Ness Ave., San Francisco
Also: 8 p.m. Saturday and 7:30 p.m. Sunday, First Congregational Church, 2345 Channing Way, Berkeley

Tickets: $30-$103, 415-392-4400, wwww.philharmonia.org