Amid the dazzle of woodwinds, brass and percussion, it is easy to forget that strings are the defining foundation of an orchestra. In recent weeks, the Blaisdell stage has been packed with the huge, colorful orchestras used in the 19th and early 20th centuries.

Last week, for its "Exploration for Strings" concert, the stage looked different — comparatively empty, with only about 25 musicians.

The Honolulu Symphony's program presented a refreshing change of pace, refocusing attention on the strings with works from the 18th century, returning to the orchestra's origins as a smallish string ensemble, consisting almost exclusively of violins, violas, cellos, string bass, and harpsichord.

The change of pace was planned, but the change in program and artists was not.

Illness and logistics brought in two new guests, Daniel and Todd Phillips, best known in Hawai'i as the founders of the Orion String Quartet, which has performed a couple of times for the Honolulu Chamber Music Series.

Brothers born seven years apart, the Phillipses performed both as soloists, trading off first and second parts, and as conductors, conducting as they played. Unusual today, the double duty was typical in the 17th and 18th centuries.

"Especially in this kind of repertoire," Todd Phillips explained, "more often than not, (conductors) get in the way. There's such a rapport between the soloists and orchestra."

Both Phillipses delivered exciting performances, blending the intimacy of chamber music and the drama of orchestral music. Each dazzled in their Vivaldi solos, but the evening's highlights were their tandem work in the Bach and especially the Mozart, as they intertwined, echoed, challenged, soared with, among, in and around each other.

The brothers' connection to the orchestra was at times so close, it lent the impression that the orchestra's entire sound was emanating from their two instruments.

Thursday's repertoire included three of the best-loved works of the late Baroque and early Classical eras: Bach's Concerto for Two Violins, Mozart's Sinfonia Concertante for Violin and Viola, and the whole of Vivaldi's "Four Seasons" Concertos.

The Phillipses re-ordered the printed program, shifting its emphasis from individual works to the program as a whole centered around the soloists. Each half of the concert opened with two concertos from Vivaldi's "Four Seasons" and closed with a double concerto. The first half included "Spring" with Daniel Phillips, "Summer" with Todd Phillips, and Bach's Concerto for Two Violins featuring both; and the second half, "Autumn" with Todd, "Winter" with Daniel, and Mozart's Sinfonia Concertante with both, Daniel on violin and Todd on viola.

Although the announced change was received with murmurs, the sequence worked well, integrating what could well have been a blocky sequence of concerto movements. What was lost was the cyclical sequence of seasons, which would only have been problematic if the work were less well known.

More important, what was gained was a coherent, intimate whole, with a graceful pacing that flowed easily.

Audiences sometimes have the impression that chamber orchestras are not as exciting as the huge, multihued orchestras of the late 19th century, but it is precisely their lack of timbrel distractions that sensitizes the ear to the strings' nuances and that yields vivid, even transformative moments, of which there were many on Thursday.

Chamber orchestras are also much riskier than large orchestras, in that they expose every tiny little imperfection — every squeak, every slip in intonation.

Even such details as the disparity in attack between the harpsichord, which is plucked, and strings, which are bowed, show up. In the languid opening of "Summer," for example, the harpsichord and strings may well have played their notes at the same time, but the harpsichord sounded ahead of each string chord, injecting a subtle push to the beat.

Thursday's program was especially gratifying in its showcasing of the strings and their leaders: Concertmaster Ignace Jang, first violins; Hung Wu, second violins; Mark Butin, violas; Andrew Eckard, cellos; and the lone string bass, Kirby Nunez, all of whom presented stellar passages.

Pianist Thomas Yee, not listed in the program, expertly supplied the all-important harpsichord foundation for the Vivaldi and Bach pieces.