What’s the correct transcription and pronunciation of the ‘un-‘ prefix in General American English?
Cambridge Online dictionary provides the following transcription: /ʌn/
It’s the same in words with this prefix: /ʌnˈdu/.
Why do they use the /ʌ/ and not the /ə/ (schwa)? It’s the /ə/ sound that typically occurs in unstressed syllables.
In a comment, John Lawler wrote:
Phonemically, there’s no difference between /ə/ and /ʌ/. They both name the same phoneme, at least in American English. [ʌ] — the phone, not the phoneme — is the allophone that occurs with primary stress, while [ə], shading off to [ɨ] or syllabic resonants, occurs most often as the allophone without primary stress. On the basic principle that the most common allophone should be the name of the phoneme, I use /ə/ instead of /ʌ/. Also, people vary considerably in their individual pronunciation of reduced vowels; individual variation usually swamps “official” pronunciations.