Double Jack is a power supply unit, which transforms the mains voltage to a low voltage - 9 and 12V DC. It is specially designed for use with guitar/bass effect pedals, but can be used as a power supply for any device which requires 9 or 12V DC.
In year 2004 a new version of the Double Jack was released for sale. The two 12V outlets were originally made especially for BOSS pedals of the ACA type manufactured before 1997 and could give a maximum current of 100mA.
The new reengineered version of the Double Jack is also able to supply Line 6 modeling pedals which draw 110-150mA when powered with 12V DC. It has two LED status indicators for checking your power situation and yields 500mA on the 12V outputs. The old version had 3 LED status indicators.