The un-aspirated "d"

This question pertains to American English only. I would appreciate some comments from native-born AmE speakers.

On the sound "d" it looks like some people aspirate it and exaggerate it a lot (almost to the point of a "j") while others keep it flat against the palate and just make it a regular "d".

Let's say we have phrases like

- Going down, shut it down
- This decision
- Doors closing
- Every day

I've noticed that some people will really give the "d" a "pop" while others will pronounce it without a pop, in a flat way, kind of like the non-aspirated "t" in "star"/"stop". In particular, females seem to be giving it more of a pop, while males are pronouncing it flatly.

My accent tutor, an American woman, told me I was pronouncing the word "door" wrong when I was just pressing the tongue against the palate without the pop. She asked me to make a pop sound, almost like a "j", and when I did that, she said that now I was doing it correctly.

But I could have sworn lots of Americans don't make that pop sound. What's more, sometimes there's not enough time to make it, you can only brush the tongue against the palate for so long.

So the question is: Is it correct to always pronounce the "d" as the non-aspirated "t" in "stop"/"star" (the "t" is a little different and that one does have to be aspirated, but not the "d")?

Well, aspirating a voiced stop = breathy voice. I think thats not what one want. What you are doing: release /d/ slowly. Releasing a stop sound slowly is another trick when you want to stress a vowel that follows it

I think the average D is a little softer than an unaspirated T. I haven't noticed any gender differences in pronunciation myself.

It may be that your teacher is simply trying to get you to change whatever your natural D sounds like, and this is the solution she has come up with. (For instance, some Spanish D's sound very much like the hard TH in "that" to American ears.)