One of the first builds of anybody getting into the DIY world is definitely the MXR Envelope Filter. This 100% analog circuit is an envelope filter. This kind of effect works like a wah, but instead of being controlled by the position of the pedal, the filter is controlled by how loud your playing is. The MXR Envelope Filter works great with almost any instrument you can think of (bass, guitar, keyboard…), and is a great effect if you want a funkier sound!
In this version of the MXR Envelope Filter, we’ve added a Reverse Switch so you can make the wah work the other way around. Usually, the filter would start highlighting the low frequencies and progressively go to higher frequencies as louder notes are played. Well, when turning on the Reverse Switch, the MXR Envelope Filter will initially highlight the highs and walk his way to the lower frequencies. It’s a cool mod that allows the MXR Envelope Filter have a complete different sound.
Download here the manual for the MXR Envelope Filter Kit: