Can 大家 be pronounced like dà a ? I heard lots of Chinese'd pronounced just like that. Who can help me with this?
This phenomenon is called 懒音. And it can be very common in casual spoken language and it's really a personal habit varies for different people. In the case of 大家, the consonant of 家 is omitted for an easier pronunciation, since the vowel of 大 and 家 are both open vowel
/ʨ/ in the center is somewhat hard to articulate. Therefore, it becomes something like
No. Always da4 jia1 in mandarin.
In television programs people often speak very fast, then 大家 sounds like dà-yā or dà-ā.
Sometimes in informal speech, in Beijing anyway, a "j" is pronounced so subtly as to be inaudible to the foreign ear. In a restaurant yesterday the staff discussed my request to leave out soy sauce 酱油 (to keep down sodium). To me they seemed to say "iang you."
But they certainly did not pronounce it the way they would say 样油/yang you (if such a thing existed), or 羊油 （mutton fat, which does exist, but has yang with second tone).
So I think renfeng is right that to the native ear the j is being pronounced.
I suspect that, if these people who seem to drop the j in 大家 were to say 大鸭, then you would hear a more distinct i-onset to 鸭 than you are hearing to 家.
大家 is pronounced as dà jiā you can convert pinyin to chinese characters: http://www.hanbridgemandarin.com/article/chinese-characters-learning-tips/pinyin-to-chinese-characters and you can also convert chinese characters to pinyin http://www.hanbridgemandarin.com/article/chinese-learning-tips/how-to-convert-chinese-characters-into-pinyin-online
It should be pronounced like "dà-jia". I think you hear "da-a" word is because of fast speaking .