The first four songs on A Little Ain't Enough are driving, sassy tunes entirely in keeping with our image of David Lee Roth, the smart, self-consciously dissolute former lead singer of Van Halen. But the fifth, ''Tell the Truth,'' slows abruptly to mid-tempo, and features some of his most expressive singing ever. Getting serious and attempting to lure a wayward lover back home, Roth's croon is a solemn croak; he sounds like Tom Waits after a tracheotomy. Like all good pop stars, Roth makes his limitations work for him. His thin growl of a voice has always managed to convey lust, irony, and intelligence. Ain't Enough offers ample evidence that the preening Roth has his tender side. On ''Sensible Shoes,'' which cleverly uses a two-note guitar riff from Creedence Clearwater Revival's ''Born on the Bayou'' as its hook, the usually cocky singer admits that he's ''a heartache on parade'' since his baby left him; he implores her to return. Of course, range and courtliness aren't exactly qualities that help sell hard-rock records. No dummy, Roth makes sure to include some crass, insensitive music to please his longtime fans. Accordingly, two flagrant Van Halen rip-offs, ''Last Call'' and ''It's Showtime,'' feature guitarist Jason Becker doing his best to approximate the screwball arpeggios of Eddie Van Halen. Becker is no Eddie, but look at the bright side: David isn't a howler like Van Halen lead singer Sammy Hagar. Those two songs are, in other words, honorable rip-offs, and this is as admirable a record as a cynic like Roth is likely to release.