Ok, finally i gained some motivation back to search for a better STE-audiostage. Just for reference I first made a connection to the YM-2149 and fumbled a little decoupling-stage together. And what a surprise it was! Such a clear and refined sound I haven't heard from the STE till now. As we know the overall problem is the far to loud and noisy sound from the original phono-jackets. So I drilled my own and laid a second line from the LM1992 to it. For decoupling I tried some things, everything has made a difference. It was not easy to decide. When it comes to verification I had to compare the waveforms I recorded in Audacity, as I have no oscillator of my own.
I used "Arkanoid" as a reference at first and some other MAD MAX and TAO-Tunes for verification. It came out, that the decoupling for the YM2149 had to be different compared to the LM1992 summarized audio. As you may have experienced and read her in the forum the STE-Output is too loud. And when I say loud, I mean LOOOUUD!!! It' s so noisy that every amplifier gets voltage overload. Some have experienced distortion, especially if you want to record something you cannot adjust it to a normal level - it's just too LOUD!
So if you want to get a normal level, you have to reduce the sound level with a voltage divider. I arranged something and compared it to the Chip-output. I looked into the original schematics, but that didn't helped me, because there is an complicated OPA-stage combined with the decoupling. I' am honest: I didn't understand it. After a while I got a clue: "Arkanoid" sounded more and more the same via my two stages. But something was still missing: The big bassy (a bit boomy) sound in the beginning was kind of thin with the original YM2149-Output, while the LM1992 really blasted some bass out. I first thought the YM-sound is the way to go. But as I compared the sndh-file I realized that I was wrong.
Things are goin' better and sound was becoming better. I added some resistors and capacitors, far less than the wild original LM1992-Output, and made some more tests. Now I think I have got it. Both outputs are singing now and this is the important thing: without any annoying sounds. May you experienced that the high notes on the STE are really annoying and nearly distorted? That is not how the YM sounds, it have to be nice and smooth, also with a deep bass and punchy dynamics.
I also use another external PSU for less noise on the 5V-rails. With the original psu you get a darker and less refined sound.
I will show some waveforms in comparison, so you can imagine the difference.
Anyway, hope you are interested!??