Let me present my long-term project - "At Last: Silence".
The goal of the project was to understand whether the infamous noise on Avance Logic ALS120 audio chips could be fixed or improved.
And, although it did not end up hi-fi (around mid-tier, i would say), I dug out quite a lot of very promising things in the process and would like to share with you.
My main research is generally completed. But the process of creating a prototype and a finished card is very slow, for many reasons (including lack of time and finances).
I fear that my development at this pace may disappear in a dusty closet, so I would like to update and post all the information here, as I progress.
So, here we go.

In short:
1) I reverse-engineered the schematic, since original is missing in datasheet.
2) Recreated the card with bare minimum for output and completely cut out input stages (no one will ever use it for recording, anyway).
3) Determined main flaws of the IC, which are responsible for the main portion of the noise. (VREF and FM DAC)
4) Made two prototype versions in the process. V1 is a PC104 format card with a solid ground plane. V2 has analog and digital planes separated.
5) Developed a circuit for supressing VREF noise
6) Dug out some performance boosting chip registers (including, surprisingly, even one fixing DOOM and other games messing up micer volume).
Made some utilities and scripts along the way and discovered at least two revisions of the chip.

Here is what I got:
1) Idle noise on the uninitialized audio card has practically disappeared (VREF).
Noise from the PC is also gone.
2) The initialized card hisses, but much less. The main culprit here is FM.
Part of the noise was eliminated through filtering, while the SNR itself was boosted via registers.
By combining attenuation, gain, and registers, I managed to achieve a good balance where the sound is quite loud, but the hiss is virtually inaudible.
However, it didn't become hi-fi: at volumes above 80, that very same hiss creeps back in.

Things left to do:
1) Noise spectrums of V1 vs V2 and find out which design works best (ground planes, filtering, 7805 vs TPS7A etc)
2) Gameport (I have no means to test it ATM)
3) Best ratio of attenuation and gain or "gain staging" (those two ugly SpINNERS on top) and transfer values to final schematic.
4) Rework dual-rail opamp power into single-rail (over-engineered dual-rail vs two big caps and compatibility).
5) Test out A0-A1 IOW-IOR signal buffering (I encountered some note-skipping and other FM bugs on fast PCs)
6) Finally decide on the type of crystal\oscillator
7) Test the best LP filter configuration for DAC and FM
8) Possibly, make a utility program instead of scripts.
9) Make that project finished already, dang it!

This post for now is somewhat empty in details.
A lot of info on V1 of the project is mentioned here, including some sound tests.
Re: ALS120, ALS100 Plus audio fix ?

Over time, I'll pull and gather all the info here and organize it.
Since V2 is basically a continuation of V1, this topic will focus on V2.

This topic will have first 3 pages reserved for general info, schematics and sound tests.