When playing Rolling Thunder for Famicom, one may notice that the audio is very quiet. The Namco 163 expansion audio is not very well mixed with the 2A03 system audio, and the expansion audio gets drowned out as a result. The later revisions of the Famicom have the 2A03 audio insufficiently amplified, causing the expansion audio to be too loud, but it is just right for games like Rolling Thunder that use the quiet N163 audio.
However, for other models of Famicom, like the original ones, the Namco 163 audio is indeed too quiet. My solution is to build a small pre-amplifier into the cartridge.
This was a while ago, and I do not recall which schematic I used. It is a simple NPN-based amplifier (I am using a general purpose 2222A here). By searching for these keywords probably you can find the one I used. I played around with resistors on the output for mixing until it sounded right. Now the 163 audio is loud enough to be heard clearly while not over-powering the internal 2A03 audio.
Here is the shot of the guts, the real work!
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