Richard Fox, 39, entered a plea of not guilty Thursday in the explosion at his mobile home in Potrero. Jeannette Ogara, 38, was killed in Tuesday's blast.
Prosecutors say both Fox and Ogara were drinking inside the trailer when Fox stepped outside to ignite the cannon.
"I went out there and I lit this stupid homemade cannon with this firework in there and it went off and blew a hole through the side of the house, and oh my God, oh my God, I don't think she's alive anymore. Oh my God," Fox is heard saying on the 911 tape.
Fox was arrested for investigation of exploding a device resulting in death. The judge ordered bail to be set at $3.5 million.