Famicom & NES - Simple Tweaks to Restore Audio Balance Levels

Audio balance levels in the NES and Famicom can cause some consternation and official hardware is not always consistent.  Here I will discuss some simple modifications you can do to your console to restore the balance between internal audio channels and internal audio channels and external cartridge audio.




Famicom PCB revisions HVC-CPU-01 to HVC-CPU-08

Congratulations, no audio mods required here!  This is the canonical circuit.  The CPU pin carrying the pulse waves is attenuated through a 20K resistor and the CPU pin carrying the triangle wave, noise and DPCM is attenuated through a 12K resistor.  Both pins are also connected to ground via 100Ohm resistors.  Then the combined signal is sent to a 1uF capacitor and then into an inverter chip.  The inverter chip and a resistor of 100K acting as a negative feedback comprise a simple amplifier.  This circuit appears on all Famicoms, NES Front Loaders and AV Famicoms.  Given that these systems comprise 90% of all official Nintendo 8-bit systems that were ever made, I would suggest that the sheer numbers speak for themselves when it comes to the canonical mix.

Famicom PCB revisions HVC-GPM-01 & HVC-GPM-02 (b-j)

A simple mod, remove the 43K resistor R7 and replace with a 100K resistor and your system will be identical to the older pre-GPM PCB revisions.  This may seem counter-intuitive, but increasing the resistance in this instance results in higher gain in the amplifier, thereby increasing the internal volume levels back to the level they were in the older, pre-GPM revision Famicoms.  Amplifiers usually have inputs for positive and negative voltage, but there is no source for negative voltage in Nintendo's systems, so it used increased resistance as a substitute for the negative input.

Why should we deem 100K and not 43K as the canonical resistance?  Boards with the old HVC-CPU revisions were being manufactured from 1983-1987.  The HVC-CPU-05 and especially the HVC-CPU-07 are very commonly found in Famicoms these days.  GPM boards did not appear until 1988 with the uncommon -01 revision and 1989 with the much more common -02.  Expansion audio began with the FDS, which was released in 1986 and had its best year in 1987.  After 1988, there were very few new FDS releases, with or without expansion audio.  Similarly, by the end of 1989, 15 out of the 27 cartridge expansion audio games had been released.  Given the time it would take to get new systems into developers hands, especially given that most of the developers who used expansion audio were among the earliest third-party licensees, I would suggest that few, if any expansion audio games took the lower volume levels of the GPM Famicoms into consideration.

Nonetheless, an argument could be made that Gimmick! is best suited for GPM and AV Famicoms.  After Gimmick, only two more games (Just Breed, Namco Classic II) would be released with expansion audio.  I can't speak for the Namco game, but Just Breed sounds appropriately loud while Gimmick sounds a bit quiet.

One caveat is that this mod may not sound very good when the original RF modulator is being used.  Because the 43K lowers the gain on the amplifier, the RF modulator amplifies the audio too much.  A standard audio mod using pin 46 and a 220uF cap is best.

Twin Famicom AN-500 & 505

Ace, the creator of the Sega Genesis MegaAmp, found that in order to disable the overdone low-pass filter, you must remove the .022uF at C205 (in between the audio and video RCA jacks).  This is applicable for the Twin Famicom Turbo models, although the designation may not be the same.  Something similar should be done for the Famicom Titler, but the rarity of that systems means that there are fewer people who are willing to look into modding it.

There are some of the older Twin Famicoms which have a pair of resistors switched.  The switch leads to distortion when using the internal FDS audio.  Tim Worthington, the creator of the NESRGB mod, identified them here : https://etim.net.au/nesrgb/installation-famicomtwin/  To fix this, they needs to be switched back. R103 is 1.2M and R104 is 2.2M on these boards, but if you change them so R103 is 2.2M and R104 is 1.2M, you will get rid of the distortion. He also advises replacing the 8.2K resistor at R201 to 2.2K to increase volume level output to compete with other Famicoms.

Finally, I would suggest a few resistor replacements to get the internal balance closer to the original Famicom and Famicom Disk System.  Replace the 22K at R125 with a 20K and the 2.2M at R103 with a 2M. This will increase the audio level of the square waves relative to the triangle, noise and DPCM and the audio level of the FDS expansion audio relative to the internal audio.  Note that Nintendo typically used resistors with a 5% tolerance in its mixing circuits, so a resistor designated 20K may have an actual resistance of anywhere between 19K and 21K.  The volume difference between a system using a 20K and a system using a 22K resistor is not going to be readily apparent.

AV Famicom HVCN-CPU-01 & 02

Unfortunately, this is not a simple mod, and the reason for it is the JIO chip that replaced the three discrete components on the older Famicoms, a 74LS139 and two 40HC368s.  While you may think that replacing the 68K resistor at  R14 with a 100K resistor would give a proper balance to the audio levels, it does not.  It is likely that there is some extra resistance added to the input inside the chip that plays havoc with the level.  Drakon suggested 200K, but Ace indicated that putting the value too high would overdrive the internal audio for games that do not use expansion audio.  No games with expansion audio were released after 1992 and the AV Famicom's -01 revision has a 1993 copyright date.  While no games would have been developed with its internal volume balance in mind, but its internal volume balance is only slightly louder than the GPM Famicoms.

I have been working on a solution where you remove the JIO chip and replace it with a daughterboard that has the discrete components and a resistor pack needed to pull up the controller port I/O lines and an extra inverter IC.  The daughterboard would also restore microphone functionality to Controller Port 2 (using a Famicom Controller II with a NES plug on the other end) and allow Zapper light guns and NES arkanoid controllers to work in Controller Port 1 like they do on the NES (if supported by the individual game).  It may mean, however, that ordinary Zapper usage would have to be done via an adapter for the Famicom Expansion Port.  If I can make this solution it would make the AV Famicom indisputably the best official 8-bit Nintendo system bar none.

NES Top Loader NESN-101

The NES Top Loader uses 100Ohm and 200Ohm mixing resistors whereas other Nintendo consoles used 12K and 20K.  The 100Ohm should be replaced with a 120Ohm resistor to restore balance levels with other Nintendo consoles (thanks to Ace for this).  The Top Loader uses 1/8W sized resistors whereas the older systems used 1/4W sized resistors.  This information should be good for the rare revised RF PCB and the PAL Australian Top Loader.  The rare official AV Top Loader does not need this because it uses 20K and 12K mixing resistors and is based off of the AV Famicom's design.

AV mods for the Top Loader are very common, but the Top Loader does not have an amplifier onboard, so the audio comes out rather quiet in comparison to other Nintendo consoles except through the RF modulator.  The Top Loader does not route its audio to its inverter chips even though there should be a free gate available to it.

NES Systems and Famicom Expansion Audio

It was the received wisdom for a long time that if you inserted expansion audio into the NES audio path using a particular mixer value, 47K for the Front Loader and 1.2K for the Top Loader, you would be all set for Famicom Expansion Audio.  But this one-size fits all approach will not give you accurate audio levels with every real cartridge that uses expansion audio.  Each type of cartridge used different mixing methods, and the Namco 163 carts used varying mixing resistors.  You should really use a potentiometer that can encompass those ranges and more and adjust based off samples taken from a real Famicom. See here for some samples :

http://www.mediafire.com/file/307kxf37qxy9h60/Famicom_Expansion_Audio_Reference_Samples.7z  (125MB download)