Fred Hersch plays his jazz music for today

To the uninitiated, Fred Hersch might seem like a hack. How else could this jazz composer-pianist appear on six CDs out this spring-including The Fred Hersch Trio Plays, the follow-up to the group's Grammy-nominated '93 debut (Dancing in the Dark), and Last Night When We Were Young, the Hersch- produced all-star jazz release to benefit AIDS? The reason is, unfortunately, bittersweet. ''I am HIV-positive, and it's made a world of difference on my work-ironically, for the better,'' says Hersch, 38, who talks like he plays-with muted strength and disarming warmth. ''My music is more focused now, because I'm realistic about when I ought to do it-right now.'' He adds: ''What has always impressed me about jazz is how people of different ages and races, with every kind of problem, work in a trusting and supportive way. For who and what I am, this is a good place to be.''