The Westminster Confession of Faith (the doctrinal standard for many Presbyterians), Chapter III, makes it clear that at least one well recognized portion of Calvinists firmly rejects the claim that God is the author of evil (emphasis added):
I. God from all eternity did by the most wise and holy counsel of his own will, freely and unchangeably ordain whatsoever comes to pass; yet so as thereby neither is God the author of sin; nor is violence offered to the will of the creatures, nor is the liberty or contingency of second causes taken away, but rather established."
Point VII might also be worth mentioning (emphasis added):
VIII. The doctrine of this high mystery of predestination is to be handled with special prudence and care, that men attending to the will of God revealed in his Word, and yielding obedience thereunto, may, from the certainty of their effectual vocation, be assured of their eternal election. So shall this doctrine afford matter of praise, reverence, and admiration of God; and of humility, diligence, and abundant consolation to all that sincerely obey the gospel.
This clearly shows that these Calvinists recognized such as a "high mystery" and vulnerable to careless interpretation and application.
On the "please elaborate" front, I think Calvinists are trying to embrace God's omnipotence, omniscience, and Goodness (and pancracy--all-ruling, "Almighty" in Revelation) while still recognizing the presence of evil in the world--all of which are strongly witnessed in Scripture. Accepting God's absolute sovereignty and goodness (so could not be the author of evil) and the presence of evil results in a logical problem that is resolved by assuming that the understanding of the terms is inadequate (i.e., mystery). This, of course, does not mean that one should reject one's partial understanding of the terms even when one does not yet know how they are reconciled.