Front AUX Input The 3.5 mm stereo input jack found on the front of the receiver allows the connection of any audio device with a headphone jack or other type of AUX output. Music from the connected device can be heard through the vehicle's speaker system when AUX is selected as the source. It's most commonly used to connect MP3 players and other portable audio devices. Single Preout With Low-Pass Filter A low-pass filter allows the outgoing signal to a connected amplifier to filter out midrange and high frequencies. This is useful for amplifiers powering subwoofers as it keeps damaging mid and high frequencies from being amplified and passed through to a speaker designed to handle only low frequencies.