I believe that I can pronounce the words 'call' and 'balloon'
"a" in call sounds like "o". To me, there is no difference between "a" in balloon and "u" in lunch. How can we distinguish between them?
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(I provided links to the definitions of the words with graphical representation of the pronunciation and audio sample.)
Question to self: is "a" ever pronounced as "a" in English? :)
No one has mentioned the real difference between the vowel of lunch and that of balloon so far.
In most--if not all--accents of English, the first syllable of balloon is unstressed while lunch is stressed. The vowel in the first syllable of balloon is /ə/ (schwa) while that in lunch /ʌ/ (it varies from accent to accent, though).
/ə/ almost always occurs in unstressed syllables, while /ʌ/ can chiefly be found in stressed syllables. That's the main difference between /ʌ/ and /ə/.
The acoustic difference between [ʌ] and [ə] is that [ʌ] is a bit lower (meaning the tongue is in a bit lower position) and backer (meaning the tongue is positioned back in the mouth), but this distinction is almost imperceptible.
As you can see in the vowel chart above, [ʌ] is a bit lower and backer while [ə] is central.