Generally if you’re reading romaji and you see an M followed by a consonant (especially if said consonant is an M, B or P) then it’s actually an N that’s being pronounced as M. And N on its own is ん. That is to say, it’s not AM+PU, but A+M+PU, which is A+N+PU. Which is あんぷ.
That said, this particular word is an English loanword (i.e. from “amp”) so it should be written in katakana. アンプ.
What are you looking them up in? Any decent Japanese dictionary should be giving you the word in kana anyway, even if it’s giving romaji as well.