What did the Federalists believe the most apparent source of tyranny was?

Government is not what the federalists feared to be the most apparent source of tyranny. The federalists supported the idea of government. They actually believed that the most apparent source of tyranny was the Majority Rule - that is to say they thought that the popular majority would be able to unite and "trample on the rules of justice". It was the anti-federalists who believed that government was naturally tyrannical.