The differences between Wooden Shjips and Moon Duo, San Francisco-based singer-guitarist Ripley Johnson’s two best-known projects, aren’t usually all that pronounced. The former group excavates deep, fuzz-satiated grooves for as many as seven or eight or, what the hell, ten minutes at a time, cresting the point where repetition becomes more absorbing than boring. Moon Duo, meanwhile, often strikes as pretty much the same thing, except with a propulsive drum machine instead of a real kit, and keyboardist Sanae Yamada backing Johnson instead of his three fellow Shjips.

But on Circles, Moon Duo’s nine-song, 42-minute second album, the line between two acts becomes thicker. For the most part, the record brims with that signature fuzz, groove, and drone, but it’s not monochromatic like last year’s Mazes – hence the pluralized title. “Trails” burgeons as something like early-‘60s pop as imagined by Alan Vega – call it spectral Spector. The title track has one fleeting solo whose twanged-out tone is essentially the one heard in rockabilly classics like Gene Vincent’s “Rip It Up”. On “Sleepwalker”, Yamada proffers her own breathy vocals to create something borderline lush. These things certainly make Circles’ songs different, but they also make them better, supplying the heavy rhythms with contrast that brightens the plod without distracting from the inherent virtues of it.

As expected, Johnson’s ghostly mumble here is wedged firmly into the rest of the mix, making it impossible to identify lyrics without serious effort and/or really good headphones. But such opaqueness is part of the whole Moon Duo M.O. Mystery breeds curiosity, after all, and that’s something that works as well here as it ever has for Johnson, regardless of project.

Essential Tracks: “Circles”, “Trails”