From Trap Door Electronics:

This is the sound of of the original Prophet 5 filter in a pedal. Active Acid, capable of transforming your instrument like few pedals can, is a powerful four-pole low-pass analog filter with an incredible amount of control available to you, the user. Equally excellent for acid house/techno, psych, and otherworldly ambient, it features envelope control of the cutoff with sidechain capability, LFO with 8 wave shapes and tap tempo, fully self-oscillating resonance that can shake your guts and shatter your teeth, external expression control, and a pre-filter overdrive to give it all some gnarl. Importantly, and often insanely, multiple things can control the filter at once. For example, the envelope follower and the LFO can both be manipulating the cutoff differently simultaneously, all while you use an external expression pedal too. That’s a lot of possibilities! Basically infinite really. 

Its heart is the beloved SSM filter found in the original Prophet-5 synth, and more than anything, Active Acid is simply a gorgeous-sounding filter pedal. With all the additional controls, it is effectively a synthesizer with your instrument as the oscillator. And yet! It’s intuitive and easy to use. 

It’s incredible on guitars, bass, synth, percussion, and vocals, and playing it is addictive. Gentle, gooey filter sweeps, psychedelic chorused textures, tremolo, radical combinations of LFO and envelope response, sample-and-hold computer talk, sidechained madness and more all await.