This is an interesting progression, in which the chord changes, but the seventh stays the same. The Abmaj7 chords is supermodal: in the dorian mode of A minor: D dorian, I borrowed from D locrian. (D,Eb,F,G,Ab,Bb,C) Abmajor is the V chord in the D dorian scale.
The F#maj7 is supermodal too; in the Lydian mode of A minor: F Lydian, I borrowed from F phrygian. (F,Gb,Ab,Bb,C,Db,Eb) F#major is the II chord in the F phrygian scale.
I think this is interesting because the theory is hard to understand (I just tried all the possibilities with the supermodal tool, and after I found the right chord, I analysed it and tried to understand how it works… It’s not easy to find our yourself) but what’s even more interesting, is that it sounds so good! Even though Abmaj7 and F#maj7 are not in the A minor scale at all, the fact that the 7th note stays the same, makes it pleasant to listen to :).
This is how it sounds:
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What you describe is most likely just an instance of constant structure (or harmonic planing),
I didn’t know the name, but it’s like that indeed. http://www.hooktheory.com/hookpad/view/SJLGndFuXLP6
I just checked out the example. Since the polyphony update, Theorytabs without melody (like this one) throw an error on playback. Our developer in charge of this has been on vacation the last 10 days so we have not yet addressed it. A patch will be released soon. In the meantime, the OP can add one note to the melody and we will be able to play it.