This was more like it. Nonesuch Records is celebrating its 50th anniversary with a month-long series of London concerts, and needed a suitably brave reminder that it is one of the world's leading folk and roots labels. There had not been enough innovation in the opening concert featuring Devendra Banhart and Rokia Traoré, but this was different. San Francisco's celebrated Kronos Quartet, themselves celebrating their 40th anniversary, acted as celebrity backing band for a set in which they performed "14 new songs in one day". The result was a very classy package show in which Kronos provided sensitive and thoughtful violin, viola and cello accompaniment for four of their more folky Nonesuch companions.

First up was Rhiannon Giddens, of Carolina Chocolate Drops fame, who is known for her gutsy reminders of the role that black Americans played in the development of string-band music, and here showed her potential as a thoughtful and versatile soloist. Her opening two songs were a banjo-backed story of slavery and the lament Factory Girl; she later returned with an unlikely but rousing treatment of rapid-fire Scottish Gaelic "mouth music".

The impressive young English singer Olivia Chaney showed off her delicate, intense style and range, with a treatment of Henry Purcell's There's Not a Swain, followed by songs from France and Ireland, in which she switched from guitar and piano to harmonium. Then came two more US singers. The London-based Sam Amidon played banjo, guitar and fiddle, and included the American shape-note hymn Weeping Mary, while the finest songs of the session came from Natalie Merchant, once with 10,000 Maniacs, with a quietly pained Butcher's Boy, followed by a delicate, personal treatment of Johnny Has Gone for a Soldier. She was the only performer to offer only two songs, and it wasn't enough.

• Explorations: The Sound of Nonesuch Records continues is until 30 May. Box office: 020-7638 8891. Venue: Barbican, London