This is not a direct answer to your question but let me explain about difference between voicing/devoicing vowels and prolonging vowels.
There are several ways to pronounce or the likes.
1. des (1 syllable ~~non-moraic~~ 3 morae, /de/ is longer than /s/, sounds ~~chopped~~ foreign)
2. des: (1 syllable 2 morae, /de/ and /s:/ are even, sounds eastern-dialect-ish)
3. desu (2 syllable **s** 2 morae, /de/ and /su/ are even, sounds western-dialect-ish)
4. desu: (2 syllable **s** 3 morae, /su:/ is longer than /de/, sounds feminine)
Japanese ears perceive no.1 as , no.2 and 3 as the same thing and normal pronunciation of while no.4 as , which is different from the rest and perhaps feminine.
However, English speakers perceive no.1 and no.2 as the same word "des" while no.3 and 4 as another same word "desu". If they fail to differentiate no.3 and 4, they may mix features unique to each of them.