Dali lang. Cut the + wire. Then get a 7809 voltage regulator from Deeco. The configuration of this is In-Ground-Out. So yung pinutol mong wire, connect the In of the 7809 to the wire that goes to the adaptor. Then connect the Out pin to the crown jack side. Yung gitna, yung ground, connect that to the - wire of the adapter. Fancy putting a heatsink or not, and that's it.
Make sure the adapter you're converting is at least 9VDC unregulated or better, 12VDC unregulated. The 7809 needs about 2.5 volts of "burn" voltage. 9VDC unregulated will spit out 12 volts or so at no-load condition, but if you start off with a 12VDC unregulated (around 16 volts DC out at no-load) , your output will be cleaner and quieter, although the pitfall is mas mabilis mag-init ang regulator mo and you might really need a heatsink. Me nabibili namang slip-on types.