Getting right to the point, in English, the T (usually aspirated at the beginning of a stressed syllable) is unaspirated after an S in the same syllable. This is true among nearly all speakers, except for RP (so-called standard British — actually, relatively few people talk that way). If your native language distinguishes more between aspirated and unaspirated than between voiced and unvoiced (true in many Asian languages — I don’t know about Thai), then you may hear this T as a sort of D sound. Note that if it were phonemically a D, it would be unvoiced in English after an (unvoiced) S, thus neutralizing the difference. For example “disgust” sounds exactly the same as “discussed”. (The rule applies to all the occlusives after S.) None of this is shown in dictionaries; it's allophonic and native speakers do not consciously pay attention.

A following R combines with the T in place and manner of articulation, but that's another question.

This is a great thread - really informative. Quality stuff!

You're lucky to have a couple of expert phonology teachers helping you, Benny.

My pleasure, Benny! You seem to have a good ear.

@Phil and @Chris

Thanks so much for useful information :)

Benny, in answer to your follow-up question <em>“Let say we call T after S the "Unaspirated T", do you usually produce the same sound in -ted form (past tense)?</em>”:

(This answer refers to American English, and may also apply to British accents other than RP.)

Yes, the T is usually not aspirated in an unstressed syllable: “lifted”. Unstressed and between vowels, it is the voiced alveolar flap /ɾ/ (sounds like the “r” of many languages): “waited”. Unstressed and after /n/ and before a vowel, it is often dropped in American English: “wanted”. Even though the T is unpronounced, it will still shorten the preceding vowel (as all “unvoiced” consonants do in English).