passing chord preceding 4th in minor scale?

Question

I will demonstrate the topic in Am Scale but it could be in another scale.

in a lot of songs,music there is a chord progression that includes these to chords in this order : Am ---> Dm and musicians especially jazz musicians use pulling chords/passing chords in between these two so it sounds more effective.

In my musical life and thanks to experience I have discovered two :

1.using A7 :

Theoretically it is good because it's the fifth of Dm - this one sounds good and beautiful and I guess the scale becomes D harmonic minor in this pulling chord (it sounds like this).

2.using Gdim7 (diminished 7th):

This one sounds heavenly touching and more emotional and I don't understand why it sounds great (it would be great if someone can explain me why it is working).

But sometimes I hear even a more harmonically touching pulling/passing chord that I wish I could get to know/discover but I'm not able to.

So,can you tell me what pulling chords you know that can be used in this harmonic progression? It would be great if someone can explain me the why the Gdim works well.

Answer 1

The Go can be seen as an A7b9, which also would lead to D/Dm, but without the A, obviously. An F7 does the job, too, but needs particular melody note/s over it to sound convincing. Using a tritone substitution works as well, going from Am through Eb7 to D.

Answer 2

I'll explain why why Gdim7 works so well. The notes contained in Gdim7 are G, Bb, Db(C#), and Fb(E). One of the properties of a fully diminished 7th chord is because it is symmetric chord, it can function as a Gdim7, Bbdim7, Dbdim7 (C#dim7), or Fbdim7 (Edim7). Since C# is the leading tone of D major/D minor this chord acts as a dominant chord to the Dm chord leading to it quite nicely.

In general dominants tent to work best for convincingly moving from one chord to another. In this case there are a lot of choices you can make. For example, I could see either a Bb/D or an F working well. Bb isn't in the key of A minor, but is a typically substitution is referred to as a a Neapolitan chord. It typically goes to a dominant instead of a predominant, but I personally like the sound of it.

Neither of the above are dominant chords, but its add a little extra to the progression as passing chords should. It's really up to you and what you like substitution wise.