In the Phonetics and Phonology part of the course "Introduction to General Linguistics" I had a few years ago, the teacher said the usual German /a/ was somewhere between a front vowel [a] (IPA: [a]) and a central [6] (IPA: [ɐ]). I think that corresponds pretty much to the way I pronounce /a/ and /6/. I think I make a difference between /a/ and /6/, but it's only very slight.
During the last week, I was confronted with two dialects that pronounce those two phonemes in a very different way:
Hessian (as spoken in Frankfurt/Main) pronounces /a/ as [a] (further in front than the /a/ I pronounce) and /6/ as [E]. In diphthongs, /6/ is dropped, for example the first name "Herbert" is pronounced ["hEbEt] there.
Basel German (an Alemannic/Swiss German dialect) pronounces /a/ as [a] (somewhere between Hessian and High German pronunciation I'd say) and /6/ as [@4]. Their /r/ (or at least the /r/ articulated by one speaker at a colloquium yesterday) seems to be an alveolar tap. In diphthongs, /6/ is pronounced [4], and adjacent words are bound in speech. For example "Wortart" is pronounced ["vO4.tart] instead of ["vO6t.?a_"6t]
When I tried to imitate that Hessian /a/, I realized it's more difficult in pure vowels than in diphthongs ([aI] and [aU]). When I imitate Swabian, my /a/ is more frontal, too, but it's different from the Hessian one. In some dialects of my region (particularly in Neunkirchen), there is a distinction between [a] and [6] where the [a] doesn't sound like any of the previously mentioned ones.
I know the phonetic alphabet is just a convention, and it's quite to describe the exact sound of a language's vowels. In fact, even someone who speaks a language very well can be exposed by the vowels s/he articulates...