"Unmixed, prepared" means fully prepared. For when the wrath of God is mixed with love for mankind and goodness, it is definitively prepared. The measure of wrath and goodness is not the same, for love for mankind greatly surpasses it.
If righteous wrath and goodness were equal, no living thing would stand. Understanding this, the prophet said: "If You, O Lord, should mark iniquities, O Lord, who shall stand?" (Ps 129:3).
The same prophet hinted that the goodness in God's cup greatly surpasses righteous wrath in the words: "The Lord is merciful and righteous, and our God has mercy" (Ps 114:5).
He placed His righteousness in the middle, so that it might be constrained by the mercy set beside it and could not act according to its own character. For it belongs to righteousness to render to each according to his deserts.
Having once mentioned wine and thereby spoken of wrath, John keeps to this image and calls a cup what was given to sinners from the hand of God.
The Septuagint reads this way. In the Synodal translation it is: "For a cup is in the hand of the Lord, and the wine foams in it, full of mixture, and He pours from it. Even its dregs all the wicked of the earth will drain and drink."