I agree that the "d" sounds in "buddy" and "bad" are different, but I don't agree that the "d" sound in "bad" is voiceless -- rather, it is unaspirated (there's not a puff of air when you pronounce it), while the "d" is "buddy" is relatively aspirated. (Though not as strongly aspirated as the "d" in "door" or "doctor".)

Note that voiced-ness and voiceless-ness relate to the vocal cords in the throat, while aspirated-ness relates to the amount of air released through the lips when a consonant is pronounced.