"Ja" is also "yes" in my language (Slovenian) and it's pronounced the same as in German. After listening to the recording a couple of times I've also noticed a slight difference. It's like there's some air released while pronouncing the "j" sound, or at least that's what it seems like to me. :/
Here are more: http://el.forvo.com/search/ja%20tack/sv/ It has both: "JA", "kan JAG få notan tack". They are almost the same since JAG is pronounced without G, but the vowel sound A is shorter than in JA. You can of course also pronounce JAG with the G sound in the end. Then the vowel A is longer.
so, there's no difference in pronouncing "Ja" or "ah" ? can both of them be used in the same context? I had come to think that one says "ah" when asked a simple yes or no question, and "Ja" when the answer is affirmative but giving further information, but maybe that's just my idea. i.e.: -Are you OK? - ah -Are you hurt? - Ja, I hit my head.
oh thanks!! but i didn't know how to write the phonetics. this is regarding a past comment when they say sometimes JA is pronounced like YA and some others like AH in casual speech. Actually I hear it as "Ah?" with a questioning tone. But thanks a lot, all the ones in the link pronounce it the same so I guess there is no big difference.