Systems music composer Philip Glass' taste regains as refreshingly catholic as ever, but the London performances of his latest portrait opera, Akhnaten, were disappointing.

Seat Q10 in the London Coliseum, while affording a perfect view of the sand-covered set of Philip Glass' third opera Akhnaten, would have held one drawback to anybody used to appreciating the amplified outpourings of the Philip Glass Ensemble. It was just too quiet. Even the climactic moments of the third act paled into insignificance compared to, say, Tangerine Dream's tuning-up procedure.

Never mind. We're here to listen to the music, and there's plenty of it - three acts, each around 40 minutes long and introducing a couple of musical innovations to the Glass repertoire. One of these comes in the first act, a hectic percussion passage that marks the ascent to the Egyptian throne of Amenhotep, who swiftly renames himself Akhnaten to signify his belief in a single God: Aten, the sun. Akhnaten, played by Christopher Robson and bizarrely made-up with bald wig and false breasts (apparently, the man had some hormonal problems which Ancient Akkadian isn't equipped to describe), delivers Glass' repeated arpeggios in English, Akkadian and Hebrew at a surprisingly high pitch. Higher, in fact, than that of the female players (Sally Burgess and Marie Angel) who take the parts of spouse Nefertiti and mother Queen Tye respectively.

Backstage before the show's commencement, the composer is talking background into the Walkman. Akhnaten is his third 'portrait opera' in less than a decade, but as the world's best-known writer of systems music, Glass is a lot more prolific than that statement might suggest. In addition to his operatic activities, Glass' 1985 will witness an album of songs written in collaboration with Laurie Anderson, Paul Simon and David Byrne among others, a soundtrack for a Paul Shrader film by the name of Mishima, and a sprinkling of other works in the theatre and ballet fields. Obviously, each of these projects has its own set of artistic and commercial raisons d'etre, but why Akhnaten?

'The Stuttgart Opera had commissioned a piece, and I'd decided while writing Satyagraha (which portrays Mahatma Gandhi's early life in South Africa) to write a third portrait opera. Now they've decided to do the three portrait operas in the Spring of '87, which is very ambitious. They're doing a new version of Einstein on the Beach, which deals with science, while Satyagraha is about politics and Akhnaten is about religion. I don't intend to do any more portrait operas, though.'

In fact, Akhnaten is the first large-scale Glass piece to be put on in London, though his Ensemble has played at The Dominion and at Sadler's Wells previously. Why the delay?

'Well, it's all money, isn't it? And having an opera company committed enough to put it on. It's good, if not vital, to have a company with a real commitment to playing the piece. I first spoke to Lord Harwood (English National Opera bigwig) two or three years ago. He was interested in Satyagraha, but then he came to Houston and saw Akhnaten. Some of the singers were European, and it was the latest work, so it seemed the best piece to do. Also, quite a lot of it is in English whereas Satyagraha is entirely in classical Sanskrit, because the spoken text of Akhnaten is always delivered in the language of the country in which it's performed.'

So is Akhnaten a straight classical piece?

'There's a DX7 in the orchestra which I've written a short solo for, but from the point of view of the music I've done in the past - working with the synthesiser ensemble - it's very trad, because it's written for a repertory opera company. But it appears very different to the people playing it, partly because it requires so much endurance and stamina. In fact, it's an unusual orchestra - Akhnaten is a counter-tenor and the orchestra has no violins, just violas, cellos, basses, a lot of percussion and the synth. So all the high voices and low strings make it a very dark piece, which I like.

'Because Akhnaten doesn't have a large orchestra, the synthesiser helps to smooth out some places where the wind and brass parts don't get much time to breathe. I wrote the piece in '83 and I've found different ways to do that now, so I don't use it any more. But in the studio we usually have a synthesiser doubling the brass parts, or some Emulator strings underneath the real ones. That's what I call extended instrumentation - synthesisers doubling acoustic parts to give them more power. '

Time to spoil the plot, Michael. After the enthronement of the first act and a ritual bathing scene accompanied by the most lyrical solo of the opera (a beautifully repetitive oboe 'loop'), the second act chronicles Akhnaten's destruction of the old pantheon of gods, represented by giant mummies standing at the centre of the stage. Taking their place is a huge sun-symbol which hangs at the back of the stage, and Akhnaten calls out to the sun as he ascends a huge ladder to try to reach it. The act closes on this note, and in the final act, Akhnaten's downfall begins.

Isolated in his city of the sun, Akhnaten sings to his six daughters and ignores pleas from his advisors for guidance on the problems of the country. Gradually, the chorus from outside the city builds up and the people of Egypt appear, to kick down the walls of sand which have been built to represent the city of the sun. Texts spoken in English emphasise how far Akhnaten has lost touch with his society, and the final scenes take us forward to the present day, where we're presented with a group of tourists littering the remains of his abandoned city. After their departure, the ghosts of Akhnaten, Nefertiti and Tye reappear to play for one last time among the ruins.

At this point the action comes to an abrupt halt, and looking through the libretto, it becomes clear that all the complex pieces of stageplay that characterised earlier productions have been removed. There's no final stately funeral procession, no royal balcony appearance, no heavenly land of Ra. In fact, the lack of variation in the set lets the music down badly, and makes it difficult to follow the course of the action - itself intentionally episodic.

So we're left with little but the music which, despite the percussion and some synthesiser brass effects, is not Glass' most powerful. It isn't just the lack of volume. Most of the material is listenable, sure enough, but only at two or three points do we encounter huge, shifting chord patterns such as those in Koyaanisqatsi (Glass' soundtrack for Godfey Reggio's film of the same name), or compelling solo passages like those of Glassworks or, indeed, Satyagraha. Perhaps a front-row seat would have made a difference, but the fact remains that the staging is disappointing, emphasising what the music lacks, rather than what it achieves.

The performance is a sell-out, as all the others at the Coliseum are, and those that miss Akhnaten miss the chance to hear Glass in a lyrical, rather than overtly technological, frame of mind. For this writer, it's Glass' excursions into the latter territory that make more rewarding listening.

That said, if anybody wanted to pay my air fare to Stuttgart in Spring '87, you'd have trouble holding me back.