the 12th fret on my d string buzzes like none other. it buzzes so much i can't even hear the note. also, my 12th and 7th frets on the a string buzz a bit. every other single note, open or fretted, is perfectly fine.

what could be causing this? i just recently put on my new bridge and adjusted the action, yet these 3 frets still buzz.

any help would be much appreciated.
the minimalist σƒ τλε τρπ βπστλεπλσσδ
Did you make sure you set it up according to the gauge of the new strings? I'd fool around with the saddle untill it goes away, or maybe boost the nut?
Same deal as guitars. Make sure your string gauge is appropriate for the tuning your in. Check the action and check the neck relief.

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-Ibanez BTB 400 Bass

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Go read the FAQ section on bass maintenance. Ben goes into a good deal of detail on how to set intonation and set the truss rod (neck relief) correctly. As always, if it doesn't make sense, I'd take it to a shop and get them to show you how to do it properly. Good Luck!
I had the same problem with my 3rd fret on my acoustic bassd.

I raised the nut and it was fine.
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