  In 2007 a prototype of a new analog Sum-Trigger was installed in the MAGIC I
telescope, which lowered the trigger threshold from 55 GeV to 25 GeV and led to
the detection of pulsed gamma radiation from the Crab pulsar. To eliminate the
need for manual tuning and maintenance demanded by that first prototype, a new
setup with fully automatic calibration was designed recently. The key element
of the new circuit is a novel, continuously variable analog delay line that
enables the temporal equalization of the signals from the camera photo sensors,
which is crucial for the efficient detection of low-energy showers. A further
improvement is the much larger trigger area consisting of a fully revised
configuration of overlapping summing patches. The new system will be installed
on both telescopes, MAGIC I and II, enabling stereo observation in Sum-Trigger
mode. This will significantly improve the sensitivity in the very low energy
regime of 20 to 100 GeV, which is essential in particular for detailed pulsar
studies, as well as the observation of high-redshift AGNs and distant GRB
events. Here we like to present the results of functionality tests of a fully
working prototype and the basic design of the final system.
