I oppose this as unnecessary, costly and problematic to implement. There's no issue whatsoever with being able to vote in Germany. If we want to increase turnout, the voters must be given a reason to vote.
How about the Brazilian solution (vote or face a legal consequences)?
I also oppose mandatory voting. I understand the logic behind it, but I disagree that having lots of people casting votes that don't want to be there is a good thing. And fundamentally, freedom to vote should also be freedom from voting.
I don't find 70% turnout all that problematic, but if we're really interested in raising it again, there need to be policy incentives for those that are uninterested in voting. Just passing a law doesn't solve the fundamental issue.