"WHERE are we?" asks the heroine at the start of Tristan and Isolde, and in David Alden's new production for ENO it's a pertinent question. Wherever we are, it isn't the boat to Cornwall Wagner envisaged: more like a forlornly empty railway station - the Eurostar terminus at Waterloo, perhaps - where Isolde sits with her suitcases waiting for the next delayed departure to nowhere. In fact we're in a devastated theatre; and it soon becomes clear that Alden's big idea of Tristan is as a Jungian dreamscape, in which audience and singers step through the great proscenium of life to an interior existence where disowned and buried feelings can be acted out.

So far as it goes this is a perfectly reasonable take on an opera that grasped the basics of Freud and Jung before Freud and Jung got there themselves; and for a director of Alden's iconoclastic reputation it's almost conventionally done. Apart from a surprise appearance by Kurwenal looking like Biggles in a skirt (the costumes aren't the strong point of this staging) there is nothing to upset anyone who knows the ropes at Bayreuth. It even looks attractive, thanks to lighting (Wolfgang Gobbel) that defines the open spaces of the dream in striking ways.

The problem is that Alden's idea never allows the protagonists to get their act together, as it were. They hardly touch, which is ironic in a piece that celebrates (like nothing else) erotic love. Alden would argue that anticipation can be more erotic than fulfilment, and it's true that Wagner's score is saturated with a musical figure - the famous rising 6th and falling semitone of the opening bars - which is the very image of yearning. But what follows is not all held in check. When Virgil Thomson claimed to hear seven moments of simultaneous orgasm in Act II alone it was wishful thinking, but there are undeniable implications of climactic fulfilment in the writing. What's more, Alden's physical separation of the lovers denies the logic of the piece as he himself presents it. If this is all an enactment of unspoken desires, and if the magic potion in Act I is, as Wagner suggests, an emotional laxative that merely releases feelings present in the lovers all along, then they must make contact. Wagner's prescience embraces many things, but phone sex isn't one of them.

That said, ENO's Tristan is a glorious achievement - principally because the music is magnificently done. The orchestra hasn't sounded so good since the days of Mark Elder; and that's because it is Mark Elder back in the pit, making a triumphant return to his old domain. The texture, balance, phrasing and articulation are all beautifully taken care of, nurtured into an ideal Wagnerian synthesis of radiance and clarity. And as for singing, it's one of those occasions when ENO upgrades from a production house to a purveyor of fine voices.

Jonathan Summers's Kurwenal, Gwynne Howell's Marke and Susan Parry's Brangaene are all outstanding. The imported American heldentenor George Gray plays Tristan like Peter Grimes - too much the Byronic bruiser for some tastes but exciting, strong, and rallying with a vengeance in Act III after what threatened to be an Act II vocal shutdown on the opening night. And then there is Elizabeth Connell's Isolde - another triumphant return, from an artist who vanished from the English opera scene after a promising start at ENO in the late Seventies. Then, she was a mezzo. Now she's a soprano, and the voice is fresh, pure, true: too oriented to the top, perhaps, with not much muscle down below, but singularly lovely and disarming in its girl-like brightness. Her Isolde is a thing of youthful innocence as well as power. It brought the audience to their feet after the curtain; and I'm sure, in due course, we'll be cheering her Sieglindes and Brunnhildes.

At Covent Garden the Elijah Moshinsky Aida is back again and not in such good shape as before, when Sharon Sweet brought an imperious, don't- mess-with-me-boy armour plating to the role (a bit much for an Ethiopian slave girl, but impressive none the less). This time the Aida is American soprano Andrea Gruber, making her Garden debut (the advertised Julia Varady cancelled, again) and doing it more sensitively than Sweet. The voice is ripe and full, not beautiful, but capable and with a good top. Supporting her is the same, deep-throated slave Amneris as before, Nina Terentieva, a commendable new Priestess in Sidonie Winter, and a fine Amonasro from Gregory Yurisich who supplies the best voice on the stage. But Sidwill Hartman, a South African tenor also en debut, makes a rotten Radames: narrow, tight, with no bloom to the sound and raw when the vibrato cover thins. And the energy level of the show was not high on the first night, spiked by technical problems backstage. The conductor Jan Latham-Koenig did well in the circumstances, coaxing cultivated playing from the orchestra; but it needed more pace and attack. Maybe as the run progresses it will come.

From Aida to Zaide, Mozart's rarely staged dry-run for Die Entfuhrung (not to say Die Zauberflote) and a frustrating piece in that it squanders some of the most beguiling vocal numbers in the Mozart canon on a dramatic confection that amounts to very little. The problem, of course, is that it was never finished. All that sur- vives is a truncated singspiel of 15 numbers, without even the connecting dialogue to hold them together. Any performance has to plug the holes and invent an ending; and I've never yet seen one that worked. Luciano Berio made an elaborate realisation for last year's Maggio in Florence. It was dire.

But this week I went to Antwerp to see a more modest solution by the Belgian chamber opera, Music Theatre Transparant, and in its simple way it was very engaging. Transparant are an enterprising and hearteningly Anglophile company. Two years ago they did a ground-breaking Maxwell Davies double-bill in Brussels; they use a lot of British talent; and the Zaide (directed by Ian Burton, with singers Quentin Hayes, Declan Kelly, John Bowen) will be coming soon on tour to Ireland (this month), Scotland (March) and the Covent Garden Festival (May/June).

Transparant's solution is to stay close to Mozart. Necessarily there's new dialogue, and they reallocate the jolly little slave chorus from the beginning to the end, which a) rounds things off with a Don Giovanni-like coda, and b) leaves an opening melodrama for the imprisoned hero that sets a tone of high moral seriousness for what follows. Zaide becomes less a romp through the seraglio, more a Beethovenian liberation drama. But otherwise Mozart comes out of it all rather well. With a pretty, light-voiced soprano - Anne Cambier - in the title role, good musical direction from Etienne Siebens, and effective designs, it's certainly worth seeing.

'Tristan and Isolde': Coliseum, WC1 (0171 632 8300), continues Tues & Sat. 'Aida': ROH, WC2 (0171 304 4000), continues Wed & Sat.