Overrepresentation of select groups doesn't prevent tyranny of the majority, it makes it more likely (and tilts which majority is likely to enjoy it), and even makes tyranny of the minority possible.

Tyranny of the majority can only be prevented by establishing norms that limit what government can do even with support of majorities in representative institutions either as absolute norms or with some things requiring a higher bar than a one-time majority (bicameralism with both houses not being elected fully simultaneously does this, requiring concurrence between branches does this, supermajority requirements for certain actions does this, Constitutional limitations with amendments requiring supermajorities of states even after legislative passage do this.)