This year's Meltdown festival has spilled out of the South Bank's modernist blocks for one night only and expanded upriver to Wonderground, a pop-up spiegeltent amid a temporary pleasure garden. It's the kind of venue you might stumble into at Glastonbury late one night, and a suitably unorthodox one for Andy Butler's New York dance collective Hercules & Love Affair. Butler has a nose for detail and historical resonances, and he uses disco and house music in a way that foregrounds their gay, underground origins.

There are a lot of people on this small stage, including string and brass sections and three vocalists. It gets even more crowded when the guests arrive. First is caramel-voiced American singer John Grant, who introduces new song Talk to You with the startling revelation that he wrote it last year after finding out he was HIV positive. The tent falls silent as he explains further before smiling and saying: "It's supposed to be an uplifting song." It is, ultimately, and all the more powerful because even as the music promises transcendence, the vocal lays bare the trauma that needs to be transcended.

Next comes Meltdown curator Antony Hegarty, who prefaces his first-ever live performance of his band's big hit Blind by cheerfully reminiscing with Butler about how poor and unknown they were when they first recorded it. Even though the song is about pain and loneliness, he sings it with an expression of utter delight. The problem is that the sound is too muted, only really packing a punch during the set's more functional, house-oriented second half, after the guests and musicians have gone. First you get the drama, then the dancefloor impact, but never the two together. It's a frustratingly mundane reason for a great band to fall short, but on this occasion the idea of Hercules & Love Affair proves more potent than the delivery.