Soprano Margaret Willig's recital at the National Gallery Sunday night was a fine example of well prepared and thoughtful singing. Her voice is large and smooth, well controlled and quite agile for its size. It was, indeed, the ideal vehicle for the collection of pieces she chose. All were, of course, American (for this, the second concert of this year's American Music Festival), and, although they represented almost the entire span of this century, all featured a lush and almost timeless lyricism and sentiment.

While Willig handled all this beautifully, there was, after a while, a sense of sameness about the proceedings that was relieved only with the singing of the lovely southern hymn she did as an encore.

One of Willig's assets is her power of concentration, and nowhere was this more in evidence than in her singing of "St. Ita's Vision" and "Promiscuity" from Barber's "Hermit Songs," in which she carried the intensity through to the end of every phrase.

Another major asset was the playing of pianist Margaret Singer, who accompanied with sensitivity and a useful sense of musical presence.

In its first Washington performance, Three Psalms by Ronald Roseman, scored for soprano, flute, clarinet, viola and cello, while pleasant enough, seemed to be striving for something that was never defined, experimenting, along the way, with an enormous number of instrumental devices.