The Portland Chamber Music Festival, a 10-day explosion of chamber music in the summer, occasionally stages surprise concerts at other times of the year. The festival offered one of those, “Music From the Heart,” with works by Dvorák, Bach, Vivaldi and 20th-century Swiss composer Frank Martin on Saturday evening at Congregation Bet Ha’am in South Portland.

The concert was, at least informally, a collaboration with Electric Earth Concerts in Peterborough, New Hampshire, an organization that presents chamber concerts all year round. Both of its artistic directors – flutist Laura Gilbert and violist Jonathan Bagg – were in the ensemble, and the program was scheduled to be repeated, as part of Electric Earth’s own series, at First Church, Jaffrey Center, New Hampshire, on Sunday afternoon.

Bagg was joined by violinists Gabriela Diaz and Jennifer Elowitch, the Portland festival’s artistic director, for the program’s opening work, Dvorák’s Terzetto. As string trios go, the Terzetto is strangely balanced, as if it meant to be a string quartet, but lost the cello line along the way. Its scoring sounds top-heavy and slightly shrill at first. You get used to it as the work unfolds, and you focus more on the content than the timbre. But that top-heavy sound is why other string trios – Mozart’s E-flat major Divertimento (K. 563), for example – keep the cello and drop one of the violins.

What can’t be denied is that the same melodic gift that enlivens so many other Dvorák scores is heard in full force here, not only in the appealingly decorative opening movement, but also throughout the thoughtful Larghetto, the more vivacious Scherzo, with its brief, surprising sul ponticello effects (playing near the bridge, to yield a bright, metallic sound), and the almost operatically contoured finale.

Diaz, Elowitch and Bagg gave the piece a suitably relaxed performance, and once the ingenuity of Dvorák’s writing came into focus, the stridency of the opening melted away.

The rest of the program was devoted to vocal music. The trio that played the Dvorák was joined by soprano Ilana Davidson, Gilbert on flute, cellist Robert Burkhart, bassist Jered Egan and keyboardist Emely Phelps (who played an electronic keyboard, using a harpsichord sound) for Bach’s Cantata No. 209, “Non sa che sia dolore.”

A secular cantata that pays tribute to a departing friend – a scholar who was about to become a sailor and whose qualities are enumerated in one of the recitatives – the piece is cast in five concise movements. Davidson brought a sweet, light timbre to the recitatives and arias, and conveyed the text with an admirable clarity and directness.

The ensemble writing here, as in much of Bach’s vocal music, is not simply a supporting backdrop, but an active participant in the conversation, with flute and violin figures winding around the vocal line, echoing and answering it, and supplying added emotional weight. The ensemble here kept the piece sounding vital.

There were more Baroque works at the end of the program – the “Laudamus Te” from Vivaldi’s “Gloria,” and a Bach cantata movement, “Wir Eilen,” from Cantata No. 78, “Jesu, der du meine Seele” – for which Davidson was joined by Jazimina MacNeil, a mezzo-soprano with a beautifully burnished tone and a lyrical phrasing style. The two singers meshed and contrasted effectively, again with the thoughtfully rendered support of pared-down versions of the ensemble.

But the real treat on the program came at the start of the second half, when MacNeil was joined by Gilbert, Bagg and Burkhart for “Quatre Sonnets à Cassandre,” by the Swiss composer Frank Martin. A setting of four, lovelorn poems, in French, by the 16th-century poet Pierre de Ronsard, Martin’s writing for both voice and instruments is urbane and expressive, and MacNeil and her colleagues did a fine job of balancing the pain, passion, frustration and barely suppressed anger that runs through Ronsard’s texts.

Allan Kozinn is a former music critic and culture writer for The New York Times who lives in Portland. He can be contacted at:

allankozinn@gmail.com

Twitter: kozinn