THERE'S not much difference between classic hard rock and heavy metal these days. In both worlds, men will be men and women will be objects of men's fantasies, and about the only growing that gets done is on the ego front. Hard rock seems to be a bit more commercially acceptable (e.g., airplay), hence not as blatant. Several new albums point to the ways and means of the genre.

Rainbow is the band that guitarist Ritchie Blackmore evolved out of Deep Purple, one of the more successful sonic descendants of Led Zeppelin. Though the band suffers overall from a Tuetonic severity, it has served nobly as an adrenalin-enhancer of the first order for many years. The credit must go to Blackmore, who has Always managed to keep Rainbow about three decibels and a dozen chord changes away from heavy metal drudgery. "Difficult to Cure" (Polydor PD 1-6316) is not totally successful, but it does point to an increasingly commercial approach that doesn't obsure the traditional hard edge.

Despite personnel changes -- a new lead singer and a new drummer to replace veteran Cozy Powell -- Rainbow churns out a compelling mix of potential singles and showcase guitar pieces. Russ Ballard's "I Surrender" and "Magic" are both sprightly rockers that beg for airplay, while "Can't Happen Here," extending an old British theme into the rock arena, is vaguely reminiscent of another Rainbow hit "All Night Long." Blackmore shines on "No Release" and "Midtown Tunnel Vision," both of which conjure up images of Jimmy Page, with echoes of Jimi Hendrix and Robin Trower for loud measure.

"Vision" slows things down a bit, allowing Blackmore to accent the blues edge that also appears, somewhat morosely, in several other cuts. The album's major drawbacks, besides the band's career-long absence of interesting melodies, are some midsong expansions on "No Release" and "Spotlight Kid" that go nowhere, and the rather pretentious, wholly bombastic "Viellecht Das Nachster Zeit (Maybe Next Time)." This extends the routine of classical takeoffs that began with Deep Purple, with Beethoven being the victim this time around; Wagner is another good source for this type of gargantuan rock. It's little more than a showcase for Blackmore's unbridled audacity and seems curiously dated.

Like Blackmore, Nazareth has been around for more than a decade. Through close to a dozen albums and incessant touring, it has managed to build a solid following, but its most interesting work has come in its album titles: "Malice in Wonderland" and "Razamanaz." Lead singer Dan McCafferty may be one of the stronger singers in rock, but this is basically a no-thrills band. The most interesting cuts on "The Fool Circle" (A&M SP 4844, produced by ex-Doobie Brother Skunk Baxter) are an uncharacteristically tender ballad, "Moonlight Eyes" (for McCafferty's son), and several anti-nuke songs that keep the band more esthetically constrained than normal. There's also a raucous, herky-jerky live version of "Cocaine" which seems to be here purely as filler.

April Wine's "Nature of the Beast" (Capitol SOO-12125) and Ufo's "The Wild, the Willing and the Innocent" (Chrysalis CHE 1307 and definitely not to be confused with a somewhat similarly titled work by Bruce Springsteen) see like all fillerr. These guys just wanna rock n' roll, which is neither a new sentiment nor a well-realized one. This bacchanalian . . . no, let's make that prehistoric attitude allows for a basic artillery approach -- crash, boom, bang at supersonic levels. Forewarned is forearmed, and if yu can't adjust to the volume, well forget it, because the melodic vacuum leaves few survivors.