YM2149 filter section in the ST

[sandord]

As part of an experiment, I want to see if I can emulate the ST's filter circuit that sits between the YM2149 and the audio out pin of the monitor connector in software.

I've looked up the schematic and I've drawn it in KiCad (which I have very little experience with tbh).

Schematic.png

Screenshot 2026-05-14 185331.png

Since the schematics isn't very legible, I've guessed some of the values. I'm sure I made some mistakes.

Also, I don't understand what this L part is (all the way to the right). I took the assumption that it is some kind of coil because there's a coil component started with L in KiCAD but honestly I have no clue.

I've checked my actual ST but I can't seem to find the exact circuit or maybe my ST is a different version.

Can anyone help me get them right?

[chronicthehedgehog]

I think it's a 1mH axial ferrite bead

[redpixel]

I think your values are correct but the transistor is an NPN 2N3904. The L is a ferrite bead for filtering power supply noise and is probably not relevant for the sound filtering. Related thread at Atari-Forum: https://www.atari-forum.com/viewtopic.php?p=466004