The National Symphony Orchestra is making its biggest pitch ever to an audience that might be interested in smaller, lower priced concerts -- something other than long symphonic compositions replete with crashing cymbals. This fall the NSO will add two chamber music series.

An imaginative series called the Prelude Concerts begins Oct. 9 at the Kennedy Center Concert Hall, at 7:15 on the evening of the NSO's regular Thursday concerts. The works performed in this series will all be tied musically to the main programs of the NSO later in the evening. In the first of this three-concert series, a Beethoven string trio will be played as a prelude to the full symphony's performance of Beethoven's Symphony No. 7.

The other series, of four chamber concerts similarly tied to that week's main NSO program, begins Oct. 6 at the Jewish Community Center in Rockville. The first concert of that series features piano trios by Weber and Beethoven and a string trio by Schubert.

The two series will "highlight the relationship between ensemble playing on a large scale" and that "on a small scale," says Jane Bowyer Stewart, violinist and NSO artistic advisory board chairman. "Rarely in a single evening would you be able to hear a Schubert string quartet and a Schubert symphony."

The new series are designed to gain some additional attention for the NSO, whose musicians often perform chamber music around town -- but with other local groups, like the Manchester String Quartet and the Folger Consort.

" NSO Conductor Mstislav Rostropovich felt that it is very important to showcase this talent . . . and that these people play under the NSO banner," says Stewart. "Orchestra musicians and Rostropovich have really wanted to display these talents in an intimate setting."

Not necessarily for the same reasons. It is a chance, says Stewart, to be "working with autonomy, without a conductor." And it is "musically invigorating" to explore these composers' other works, she says. There were so many requests from NSO musicians to play in the new series that quite a few performers were turned down, says Stewart.

Early American Portraits

Now that Chief Justice Warren Burger is stepping down to take charge of the bicentennial celebration of the Constitution, there's talk of art exhibits to be mounted in conjunction with that celebration. One such exhibition, "The Portrait in America From 1700-1776," will open in October 1987 at the National Portrait Gallery, which says it is the first such showing of colonial portraiture since the 1930s. The exhibit will include works by John Singleton Copley, John Wollaston and Henrietta Johnston, "the one known woman artist of the period."

Also in the works: a spring 1989 exhibition called "Portraits of Distinguished American Jurists." Expect to see Warren Burger's distinguished visage.

Care to Tango?

Though the popular Broadway dance revue "Tango Argentino" will be here Sept. 30 at the Warner Theatre courtesy of the Washington Performing Arts Society, the El Bodegon restaurant is trying to beat WPAS to the punch with tango performances through the month of August and the first week of September. No dancing on tables, though, and only on unoccupied chairs.

Mixing Math and Art

If you want to go where no art lover has gone before, venture into the realm of fractal art at the American Association for the Advancement of Science, which last month opened a new gallery in its building on H Street NW. The first show in that new space is "Frontiers of Chaos," running through Aug. 15. This dynamic field of mathematics explores boundaries, according to Virginia Stern, codirector of the project on science, technology and disability at AAAS. The computer-generated images "confirm the idea that coding information in a two-, and to certain degree, three-dimensional form, will show connections that cannot be imagined from the formulas alone," writes Herbert W. Franke in an article from the show's catalogue.

A Turn for the Verse

Senators can be as poetic off the Senate floor as on. Sen. William S. Cohen (R-Maine) is trying his hand at verse again, with "A Baker's Nickel," to be published next month by William Morrow. His last book of poetry was called "Of Sons and Seasons."

Odds and Ends

Architecture magazine and Architectural Technology magazine, both published by the American Institute of Architects, will merge in October, the AIA announced. The new magazine also will be called Architecture. Robert G. Kliesch of Philadelphia has been named its publisher . . .

The Shubert Foundation and the Shubert Organization granted $10,000 to the Corcoran Gallery of Art and $17,500 to the D.C. Cultural Alliance as part of $3.3 million in arts-related awards for 1986 . . .

The Library of Congress is marketing a 28-minute video interview with Gwendolyn Brooks, its consultant in poetry. The video sells for $25. Information can be obtained from the Library of Congress, Information Office, Box A, Washington, D.C., 20540.