It was dark and wet, as I remember it, the wind whipping the rain in off the Mersey. But it was 38 years ago, so I'm not sure how reliable that memory is. It was certainly a November night in the Bluecoat arts centre in Liverpool, and it was certainly the Philip Glass Ensemble. I remember too that there weren't many of us in the audience, perhaps a dozen, perhaps fewer. Had I heard any of Glass's music before? I don't think so. I was there because going to contemporary music concerts was a part of my identity, something to distinguish me from the other music undergraduates at Liverpool University.

It was a gallery space, so there were no dramatic entries – the musicians walked on and sat in an arc: a singer, three wind players and two keyboard players. The music was loud – not rock-concert loud but much louder than anything I'd ever heard in a classical music setting – and it was Glass's Music in Twelve Parts. The ensemble played for about 20 minutes, stopped and then played some more. Then they stopped again and went off stage. I tried to persuade the two friends I'd dragged along that they ought to stay for the second half. They agreed, but when we took our seats for more of the Twelve Parts, we discovered that the original dozen of the audience had shrunk to a handful.

Some years later I met Annette Morreau, whose Contemporary Music Network had promoted the Glass concert in Liverpool as part of a tour which also went to Bristol, Carlisle, Leicester, Newcastle and London, and I told her the story of the Bluecoat Dozen. She said that by the end of the tour, the gap between the income from ticket sales and the cost of bringing the musicians to the UK was so wide that it would probably have been cheaper to fly the audience over to New York to hear the ensemble in one of their local venues.

Over the past four decades I've often told this story, partly because it's interesting to remember a time when Philip Glass was not famous. These days Glass's music is ubiquitous – no Newsnight footage of a desolate cityscape seems to be complete unless it's underscored by the haunting saxophone melody of Facades – and when the Glass group play the complete Music in Twelve Parts in the Royal Festival Hall on 9 November every seat will be taken.

But that Glass concert in 1975 also changed the way I thought about music. What I found so arresting and then beguiling was that the music managed to be both abrasive and smooth – abrasive because it was loud; smooth because the amplification blended the sounds of keyboards, voice and saxophone, and because the surface of the music was a fast-flowing stream of notes. It was the aural equivalent of standing at the top of a giant waterfall, mesmerised by the illusion that such a rushing body of water could also appear so stable, so even. At a time when most new music was neither even nor fast-flowing, it was important to be reminded of how powerful and effective these qualities could be.

What was also striking was that beneath that abrasive yet smooth surface there was always something going on. Each of the 12 parts lasts about 20 minutes, uses the same small group of instrumentalists, and has a structure based on the repetition and juxtaposition of blocks of music. It was a way of composing that Glass had been developing since 1966: something – alternating chords, a melodic figure, a rhythm – is repeated, and then some or all of those elements change. That's all that happens, but by 1971, when he began writing Music in Twelve Parts, Glass knew how to make each change sound like a revelation. With hindsight it makes sense to call Music in Twelve Parts a masterpiece – in the traditional sense of the word as a work in which the artist displays all the distinctiveness and diversity of his craft.

Less than a year after that Liverpool concert, Glass was famous. His opera Einstein on the Beach was premiered in July 1976, and although it was a financial disaster – for a while, Glass had to go back to his day job, driving a yellow taxi in New York – it was a critical and popular success. As a result, his career was irrevocably drawn towards theatre and then film. So Music in Twelve Parts was not only the culmination of the first phase of his compositional career but the last time he would write an extended piece of purely instrumental music for his own ensemble. In November 1975 none of us – neither Glass, nor his dwindling audience – knew any of this.

I should add that Glass's memories don't quite match mine. In an interview with Chris Heaton in 2001, he recalled that "in the cold north of England, Newcastle I think it was, about eight people turned up […] but there was a good audience in Liverpool". Well, northern English seaports, they're all much the same in the dark. What I remember with certainty is how exhilarating it was to hear Glass's music. Even better, it is music that sounds just as exhilarating today as it did then.