Depending on how you set the crossover frequency, the return balance, dry blend, and phase, you can fluidly shape, blend, and move between sounds that are subtly different or radically deconstructed. On the surface, it might look and sound like a cumbersome process. In reality, it’s intuitive, fun, and full of surprises

 Fear Not the Frequency Shift

The XO’s control set will probably look alien to most guitarists. The largest knob controls the crossover frequency, which determines the point at which the full frequency band is divided and sent to the low and high send and return. The range button just to its right selects two frequency ranges: 50 to 600 Hz, or 300 Hz to 3.4 kHz. The first is recommended for use with bass, the second for guitar, but you can experiment with either setting for any instrument. The return balance knob sets the relative levels of the two effects returns and the dry blend knob performs its namesake task. The phase button can be used to either correct phase issues when the two bands are out of phase or applied creatively to fashion out-of-phase variations on a sound. A very useful send button, meanwhile, switches the high and low sends, enabling instantaneous selection of mirror-image frequency and effects mixes.

Mutating Tone Tangles

My first experiments with the XO were simple: sending the low band to a delay with long repeats and the high band to another delay with fast repeats, lurking just at the brink of oscillation. The ways I could blend these divided and reconstituted tone composites were often unexpected, surprising, and totally inspiring. I could set up signals that found trebly repeats hovering at the edge of feedback, while low and low-mid frequencies (which can overwhelm a self-oscillating signal) provided a fat foundation for the resonant, ringing top end—a totally cool sound that responded in really interesting ways to picking dynamics and different rhythmic patterns. In a modification of that formula, I routed an intensely throbbing Vox Repeat Percussion clone, slow-sweeping phaser, and long-repeat delay to the low band and assigned a clean, heavily compressed, slapback to the high frequencies. In this configuration, simple folk-rock chords and melodic lead lines took on complex, alien alter egos, sometimes sounding like two players—one handling a bubbling bass synth, and a guitarist carrying the tune via the clear detailed high end. When the pulsing low end got tiresome, it was easy to dial in more dry signal via the dry-blend knob or dial in a mix favoring the tighter, chiming high band.

“Simple folk-rock chords and melodic lead lines took on complex, alien alter egos, sometimes sounding like two players.”

The spins you can put on these recipes are endless. Situating an octave-down pedal amid the tremolo and phaser made the two bands even more distinctive and heightened the illusion of a guitarist and synth player working together. You can mix fuzzy, thumping low end with ringing and heavily chorused top-end output. Or you can blend two similar but distinct effects to create oddly chorused and powerful widescreen tonalities.

The cool part of all this potential is that it can be realized with a single amp and just a few pedals. Some of my most radical sounds came via just four or five pedals including the XO, which adds up to a very modest and portable array, all things considered. Players that work with pedalboards that count stomps in the double digits could disappear in labyrinths of sound that are as immersive as those afforded by synthesis. And while XO is, after some practice, easy to control, the new, chaotic molecular reactions provoked by unorthodox stimulation of your pedals all but guarantees unique results. You will definitely find new sounds and new ways to play and compose here.

The Verdict

The XO Variable Crossover is more likely to see service as a studio tool than become a staple of live setups, though plenty of courageous musicians will find it practical in that environment. Although the mechanics and principles behind its workings can seem complex at first, it can be used effectively and dramatically with just a few stompboxes. The sounds and voices it can extract from, say, a phaser and a delay are exponentially greater in number than what you’d get by simply using two such effects in series, even if some of them are subtle. And the ability to manipulate and warp these sounds on the fly with the XO’s elegant, simple control interface could bring out your inner Lee “Scratch” Perry or DJ Shadow—creating new moods, scenes, and tapestries that can turn a simple song or riff into a moving, mutable, and flowing tone story.