*This is an open box model and has minor markings on the back. Still brand-new and works perfectly.

The MXR M103 Blue Box is a famously weird octave-fuzz guitar pedal. It combines a heavily distorted fuzz tone with a two-octaves-down effect, producing a glitchy, synth-like sound that can range from sputtering and gated to surprisingly musical.

What it sounds like

• Octave fuzz: Generates a note roughly two octaves below the guitar signal.
• Aggressive fuzz: The underlying tone is heavily saturated and compressed.
• Glitchy tracking: Unlike modern octave pedals, the Blue Box doesn't track cleanly. The octave effect can splutter, jump around, and behave differently depending on what and how you play.
• Volume control: Sets the overall output level.
• Output/Blend-style control: The other control determines the character and intensity of the fuzz/octave interaction, giving you everything from recognizable notes to complete sonic chaos.

It's particularly famous for single-note riffs, sustained notes, and feedback, where the octave-down circuit can produce bizarre, almost synthesizer-like textures. Chords generally become much more chaotic.

The Blue Box has also become something of a cult classic because of its distinctive, unpredictable sound. It's the sort of pedal that can sound terrible if you're looking for a conventional octave fuzz—and exactly right if you're trying to make a guitar sound broken, alien, or electronic.