Fangrui Product Details Features: Just Gently Rotate The Button Of The Controller. Different Frequencies Can Make The Song Sound Different. It Feels Very Good And Is Very Convenient. Connect The Rca From The Output Of This Controller To The Amplifier, And Wire The Car Stereo Output To The Rca Input Of This Unit. Great For A Sub Amp Volume Control. Used To Adjust An Amplifiers Power Level, Stereo, Equalizer Or Crossover Volume Form The Dashboard Or Other Remote Locations. Adds Sub-woofer Level Controls Or Balances A Multiple Amplifier Installation. Compact Housing Allows The Unit To Be Mounted Virtually Anywhere So Control Is At The Users Fingertips. Fine Tunes The Volume Of Any Rca Line Level Signal You To Control. Simple In And Out Connections, No Additional Wiring Needed. Connect The Rca From The Output Of This Controller To The Amplifier, And Wire The Car Stereo Output To The Rca Input Of This Unit. Specification: 4 Rca: 2 Rca Input 2 Rca Output Level Control Knob Frequency Range: 150hz - 40hz Volume Gain Range: 0db To -12