Proposal: Standard modification for audio on toaster NES

[l_oliveira]

After a lot of tinkering on my NES I found an simple method for making the real VRC6 cart sound right on it (sound the same as it does on the Famicom).

First I analyzed the original NES audio circuitry and made this blueprint:



That is a pretty decent audio circuit, but Famicom's circuitry is quite different: (And simpler)



With this in mind (and the fact that the audio is sent to the cartridge for mixing) simply putting an resistor on the output for the NES might not cut it, right ?

The circuit for the PowerPAK requires that an resistor of 47K is added to the NES expansion connector pins 3 and 7 of the said connector. That will add up to an total series resistance of 67K at the input pin of the mixer inside the NES. I don't happen to own a PowerPAK to know if the output level is correct with this resistance, but let's assume that it results on correct audio level.

When using a real chip in this configuration, the VRCVI sound output is way louder than that of the NES internal sound resulting in an unpleasant (for those who are aware it's wrong) or not faithful (for those who never had a chance to see it working on the real Famicom) experience.

Also I happen to be using an T89 cartridge converter (people sometimes talk bad stuff about this little fella but it's really the best adapter I ever had...) and it does not have pin 54 (The pin PowerPAK uses for audio output) available at the edge connector. At first I found that annoying/unlucky but the T-89 actually have pins 51 and 30 available and unused. I choose pin 51 which goes to pin 6 of the expansion connector.

At that point with too loud VRCVI, I decided to tinker with the resistor value. I went up to 100K and never got anything satisfying ouf of it.

Then I decided to analyze the VRCVI output circuitry:



To pin 1 of the 053334 resistor array comes audio signal from the Famicom.

Pin 2 is the output.

Pins 3 to 8 are the R2R DAC ladder and Pin 9 goes to GND.

After seeing this, I thought that it's possible that an "parasitic load" from the output at the Famicom circuit is necessary to "balance" the level of the output on the VRCVI.

I added an 10K resistor going from pin 45 of the 60 pin connector at the T89 to GND and changed the resistor from pin 3 to 6 of the NES to 82K (This sums up to 102K at the mixer input). This resulted on perfect audio level for the VRCVI cartridge. I kept the 47K resistor at pins 9 & 3 for PowerPAK compatibility so that remains unchanged.

Opinions, suggestions, criticizing is welcome.

[Bregalad]

I think the FC and NES audio circuits are very similar, but the NES lacks mic (obviously) and have more gain.

I don't have a FC but I have a normal PAL NES (which is identical to the NTSC frontloader - only the CPU, PPU, CIC and RF modulator are different but otherwise the mainboard is the same), and an imported USA toploader where I removed the RF block and put AV jacks instead (cause TV heres doesn't decode American standard RF correctly).

I can say the sound is way louder on the fronloader than on the toploader. I have looked carefully and on the toploader there is apparently no amplification stage or anything like that - the sound goes out of the CPU to a bunch of resistors and capacitors and goes directly to RF.
So on my TV I have to put the volume to arround "30" when playing the toloader but I have to put it around "14" to play the frontloader, or the Super NES, else it will be way too loud.
On a side note, on my Playstation consoles I also have to put the volume arround "30" for a normal volume level.
So yeah I have to play with the volume button a lot when I switch among various consoles.