The acoustic quality of madd alif follows the heaviness or lightness of the preceding letter. After a correctly emphatic letter, the long vowel also has a fuller sound; after a light letter, it remains light. Alif does not establish an independent new articulation point for emphasis.
In «قَالَ», the long vowel follows an emphatic qaf.
In «طَالَ», it follows emphatic ta.
In «كَانَ», it should not be made emphatic without cause after kaf.
The lam of Allah’s name, the emphatic letters, and certain forms of ra also affect the quality of a following alif. The traditional discussion distinguishes degrees of emphasis: the heavy lam in Allah’s name, then ta, sad and dad, za, qaf, ghayn and kha, and ra; the letters must not all be forced into one identical level of heaviness.
Simply stretching a vowel loudly is not tafkhim, and exaggerating alif after a light consonant is incorrect. Produce the preceding consonant properly, then allow the vowel to follow its established quality.