In the Rule entitled “Stats and Equipment”, change the text

There exist Armors, which are a type of item.

to

There exist Armors, which are a type of item.  An Armor has an integer amount of AC, which cannot be negative.

Append the following text to the Rule entitled “Wielding, Wearing, and Enchantment”:

Each type of Armor has a Base AC.  The AC of an Armor is its Base AC plus its enchantment, or 0 if it would be negative.

In the Rule entitled “Combat”, change the text

subtract from that the enchantment

to

subtract from that the AC

In the Rule entitled “Armor List”, change the text

The items listed in this table are armors

to

The items listed in this table are armors, with Base AC as specified.

Add a “Base AC” column to the list in that rule.  Give Ring Mail a Base AC of 2, Robe and Leather Armor a Base AC of 1, and everything else a Base AC of 0.