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PostPosted: Sun Jul 01, 2018 12:50 am 
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The Twin Famicom has a low-pass filter that is responsible for its muffled sound. What are its exact characteristics? I have read that the C205 capacitor responsible has a 0.022uF value, but I have not found a value for the respective resistor with which it forms the low-pass filter, and am unable to determine it from inspecting pictures.


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PostPosted: Sun Jul 01, 2018 10:48 am 
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To answer this question basically involves reverse-engineering the audio path.

But if I were going to take a stab in the dark ... we know that the bare FDS has an extreme lowpass with a corner frequency of 1.75kHz (first order). I would hazard that maybe they just mixed the 2A03 audio in and subjected it to the same lowpass filter.


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PostPosted: Fri Jan 11, 2019 6:11 pm 
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@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.htm
The 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


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PostPosted: Sat Jan 12, 2019 5:09 am 
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Why replace the LPF capacitor with a different one? I just removed it, and it works and sounds nice.


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PostPosted: Wed Jan 16, 2019 10:58 pm 
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To raise the corner frequency of the LPF to something more reasonable and still allow for smoothing of undesirable high-frequency noise.

I haven't calculated the new -3dB corner; I took it for granted that it was raised enough. Calculating it should be extremely straightforward if you retrace the circuit, but I haven't yet.

EDIT: In any case, if it sounds nice to you now, then I'd say that's a job well done :D


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PostPosted: Thu Jan 17, 2019 1:56 am 
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If someone can read the label on C205 I could make an educated guess as to what the original corner frequency is. Maybe. (Maybe additionally a picture of the solder side)

The only possible problem with omitting the lowpass filter altogether is some audio hardware does bad things (e.g. get problematically hot) when fed ultrasonics.


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PostPosted: Sun Feb 17, 2019 5:33 pm 
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forum post here claims 0.022uF as the c205 capacitance


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PostPosted: Sun Feb 17, 2019 6:07 pm 
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Well, if the original lowpass was C205 · R201 = 22nF · 8.2kΩ → 880Hz
Switching to 8.2kΩ · 270pF would then be 72kHz

3.3kΩ · 820pF would be 59kHz, and 890Ω(1k//8.2k) · 2.7nF → 66kHz

If these were chosen as the first point at which audio stopped sounding muddy, I'm probably making a bad assumption, because that normally be a corner frequency closer to 10kHz.


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