Amsterdam Baroque Orchestra / Yo-Yo Ma, Barbican, London

Yo-Yo Ma is becoming such a tease. From tango to Silk Road via bluegrass, one never knows what guise he's going to pop up in. The latest incarnation is that of fully fledged baroque cellist. Gone are the metal strings, the "modern" bridge and bow, and the end-pin. With Ton Koopman and his Amsterdam Baroque Orchestra, Ma went through his paces, exuberantly throwing down the gauntlet to narrow-cast cellists of our day. And the result? A bit of a curate's egg.

In the two concerti he played, Ma was determined to be primus inter pares. Yes, he was sitting in the soloist's spot, but not a tutti went by without him joining in. In Haydn's D major concerto, he began playing at once, discreetly with the orchestra and, as any self-respecting continuo player might have done, setting his own improvised line. The result was fascinating, but it meant that when he entered as soloist, the volume needed turning up.

Ma plays the Stradivarius cello once owned by Jacqueline du Pré. We remember from her performances (and of course Ma's) that this cello can produce a lot of sound. But Ma, stripped of his modern bow, becomes a completely different cellist with a completely different sound. In recent years Ma, with a heavier bow, has squeezed and pressed every drop of sound from this instrument, sometimes to its detriment. But with this bow, a light, airy, nasal sound emerges.

It was strange, however, to turn classical Haydn into almost sound-alike Vivaldi. Of course, Ma is a fabulous cellist, wearing his (or Haydn's) brilliance lightly, but in the third movement his tiny bow movements, amazingly co-ordinated with the left hand, risked aural oblivion.

In Vivaldi's C minor concerto, accompanied by only strings and harpsichord, Ma should easily have floated atop. But again, Koopman, encouraging the special sense of intimacy that Ma was clearly wanting, allowed the distinction between soloist and tutti to be blurred.

Just how Ma managed to sneak into the back of the cello section (there were only two for him to hide behind) without the excited audience noticing seemed yet another jack-in-the-box trick, but there he was, minding his own business, quiet- ly chugging away in Haydn's G minor symphony, No 83.

What a remarkable band this is. The brassy sounding natural horns utterly captivating (hogging the limelight delightfully in the frisky opener, a selection from Handel's "Water Music"); the strings silky and smooth; the winds - in particular the flute - mellow and rounded. But it was the encores - three of them - that addressed the real business: selling the assembled company's latest CD. Vivaldi's Cello is the new album and a lot of fun it must be. Ma gave us a few samples, not least a splendid snatch of a double concerto with the Amsterdam Baroque's principal cellist. Terrific stuff.