Sets of musical variations make an alternative tradition to the grand sonatas of classical and romantic piano music, and Emanuel Ax's programmes at the Barbican explored the kaleidoscopic diversity of variations from Beethoven to Bernstein.

The essence of Beethoven's two sets of variations, Op 34 and Op 35, is improvisation, with simple melodies and basslines subjected to ever more extreme forms of embellishment and transformation. Beethoven throws out the rulebook of variation form in both pieces, as he distorts his themes beyond recognition.

In the Op 34 set, each variation moves to a new key centre and creates a different expressive effect, and Ax relished the crunching gear-changes from the gentle undulations of the first variation to the energy of the second. Instead of a set of embellishments of an ever-present theme, the effect was like a collage of different impressions of the same object, a constellation of juxtaposed fragments. But Ax made the final return of the theme, buried beneath a dense layer of ornamentation, a serene conclusion that connected the drama of the whole piece.

The Op 35 "Eroica" Variations are even more radical. The piece is founded in musical fragmentation; the themes contain loud, unpredictable explosions and turns of phrase. Ax tore into the volatile drama of the music, building towards the bizarre later variations, as the theme was boiled down to a series of dramatic gestures: a melody spread over the whole piano's compass, and a sequence of crunching dissonances. But Ax pulled off the same conjuring trick as he had in the earlier set, and the final fugue imposed a structural integrity on this collection of wildly contrasted miniatures.

Ax was equally impressive in his Tuesday night concert with André Previn and the London Symphony Orchestra, performing César Franck's Symphonic Variations. He found an expressive clarity in this dense work, revealing the subtlety of its structure, which mixes symphony, variation-form and fantasy.

It was the most convincing performance of Previn's programme, devoted to orchestral sets of variations by Brahms (on the St Anthony Chorale) and Elgar (the "Enigma" Variations). Previn mustered only a lukewarm response from the orchestra, robbing Elgar's work of its mystery in a bland and cliched performance.