suffixes - Why does the preterite of verbs such as "deal", "feel" and "dream" have a devoiced dental suffix?

I am trying to explain the morphology of some irregular weak verbs. I could explain "leave-left" as the result of assimilation with v being originally intervocalic f, but I can't see the reason for the verbs as mentioned in the title. The only thing I notice is that they all have their stems ending in a liquid or a nasal. Any suggestions for this?