ABOUT THIS ITEM 5.0: Digital power amplifier module features5.0 with a transmission range of up to 49 feet (unobstructed). Features a 3.5mm/0.1in AUX sound input jack. MORE LAYERED SOUND:power amplifier board by adjusting the high and low frequency knobs of for ZK MT21, you can make the sound more layered and the bass more shocking. CONVENIENT TO USE: Sound power amplifier board is easy to use, the DC adapter (plug size 5.5x2.1mm / 0.2x0.1in) on the market can basically meet the needs of for ZK MT21power amplifier board. MULTIPLE PROTECTION MECHANISMS: Amplifier board has various protection mechanisms to ensure its safe operation. It has short circuit protection, over current, overheat and anti reverse protection. THOUGHTFUL DESIGN: When overload occurs,digital power amplifier module will report the fault condition to the processor to preserve itself from damage. ERK 1018