Making dc barrel usb cable and can't find the positive polarity
"Zip cord" or cable that has two conductors connected together via the insulating jacket, is frequently used for things like DC power supplies (wall warts), speaker cable, etc. Sometimes it will have a stripe, a texture, or printed characters on one side. The "- - -" should not be taken to mean negative. It's merely a marking. Always check which wire is connected to tip or ring using a continuity meter, and of course a volt meter to determine polarity provided by the supply.