Korpa 
The Bass Envelope Filter delivers classic analog envelope filter sounds with separate Dry and Effect level controls to keep your low end intact.
This pedal can be powered by a 9-volt battery, a Dunlop ECB003 9-volt adapter, or the DC Brick™, Iso-Brick™, and Mini Iso-Brick™ power supplies (not included).
The Long Story
The MXR Bass Envelope Filter delivers classic analog envelope filter sounds in an easy-to-use and compact pedal designed specifically for bass. Separate Dry and Effect controls let you dial in the perfect mix of effected and direct signals, so you can bring the funk without losing the low end. Its wide range of filter tones can be shaped with the Decay and Q controls, and a simple twist of the Sensitivity knob can tailor the pedal for your attack—as well as adjust for passive or active basses. The Bass Envelope Filter's power supply circuitry is designed to withstand over-voltage or polarity mishaps, and its true-bypass switching preserves your tone in bypass mode. It's all packaged in a durable, lightweight, Phase 90-size aluminum housing for the ultimate in portability and pedalboard space-saving dimensions.