@NRS this doesn't address your question directly, but I'm putting the info on here for future reference
Here's a great Japanese resource for twin famicom audio fixes:
http://vaot.mydns.jp/fc/tfaudio.htmThe end of this page also provides info:
https://etim.net.au/nesrgb/installation-famicomtwin/Here's a short guide for improving twin famicom audio output.
First off:
Make sure you have the correct values for R103 and R104. There was a mistake in component placement for some consoles.
The correct values are
R104: 1.2 MOhms
R103: 2.2 MOhms
Next off, to increase the volume, the Japanese site mentions two options:
1- Swap R201 and R220
or
2- put a 1kOhm resistor in parallel with R201.
This sets a higher gain for the line audio output
Now, for the LPF involving C205, you have three options:
1- if you did not alter the r201 and r220 configuration, replace C205 with a 270pF cap
2- if you swapped r201 and r220, replace C205 with a 820pF cap
3- if you put a 1kOhm resistor in parallel with R201, replace C205 with a 2700pF cap
Lastly, you can replace C113 on the main board with a 10uF cap for even more relaxed filtering.
The side effect is that some games that play a sound immediately after power-on will be muffled as the capacitor is still charging. This is a minor detail in comparison to the long-run benefits of having better sound during actual gameplay.
I haven't traced the twin famicom audio, but this should be enough information for anyone looking to get good sound out of their twin famicom