Greetings. I'm in the middle of working on an NES project and wanted to add that "stereo" mod to it. (You know, where you isolate the two channels coming off the CPU, mix in a little of the mono signal with a couple POTs, yadda yadda. It's all over the internet and even somewhere on these forums I think). Don't get me wrong, what I have so far sounds great. The left channel has about 25% of the mono signal mixed in, and the same goes for the right channel. There's a nice meaty bass boost in there as well that sounds incredible. (Almost TOO bassy at times. It's kinda funny actually). Anyhow.
The thing that bugs the crap out of me is that it's QUIET! According to my tests, the stereo mod ultimately produces sound somewhere around only 35-40% of the volume that the mono signal spits out, straight off the mainboard. (Ripped the RF modulator off. No need for it).
I've tried a couple different methods of amplifying the "stereo mixed" audio using some transistors and that little LM386 IC, but both of those cranked the audio up so high it sounded horrible and distorted, like your landlord SCREAMING into the telephone about rent or something, I dunno I wasn't listening. (Barfed up a bunch of interference on the video signal too). Color me unsatisfied!
So I've been looking up some schematics online and manually following traces around on the mainboard with my eyeballs (yeah and a multi-meter too) trying to map out the audio signal's journey from it's birth to the point where it would hit the RF modulator.
Apparently, the magic is happening on a little 14-legged chip labeled "74HCUO4P" (or sometimes TC74HCUO4P, depends). I've looked it up online and it's some kind of hex inverter.
...I'm not going to pretend to know what the hell a hex inverter is, but I'll assume it's primary function in life is to invert hex. (Did I mention I need some formal training in electronics? Cause I do. On my to-do list). I probed around the chip a little bit and discovered that the audio signal goes into the pin 11 on the chip VERY low, and comes right back out of pin 10 nice and loud. Thought I'd experiment a little. Grabbed one of the pre-squished "stereo" signals and touched it to pin 11 on the 74HCUO4P, sure as shart, it came out loud and clear.
Jumping Jesus on a pogo stick! That IS where the magic happens! So... what now? How might I go about amplifying the two separate audio channels like this? I'd probably need summore of these here hex inverter dealies. I've got a couple dead NES board laying around, might try experimenting a bit.

Has anyone else already been down this path? Am I wasting my time? Is there an easier way to do it? Suggestions? How's my driving?

PS: PLEASE don't say "just turn your TV volume up higher". Looking for some useful input if there's any to be had.

Okay! So, I think I'm beginning to figure something out. With the NES board running, I pulled one of the "pre-mixed" channels off the cpu and piped it into the hex inverter dealy on a SECOND nes board (something else on it is already fried, just using it for parts mainly), hooked up the +5v line and ground to the chip, and wouldn't ya know it... nice blaring audio coming out of that same chip, loud and clear.
Next I'm going to try removing the chip from the second (fried) NES and try to breadboard something together. I wonder if the signal volume is being controlled by other components on the NES board, that'd kinda suck. We'll see. I'm curious. I'll update with any significant findings.
This could become a very useful topic in time, since I have yet to see any kind of "definitive" stereo mod for the NES. Mostly just a bunch of "tweak this, tweak that, the volume is low but oh well". The stereo mod, when done right, sounds freaking amazing. Especially when heard through headphones. (Hint hint super ultimate burrito supreme porta-NES hint).

More to come.

Alrighty, it appears as though I've got something working on breadboard. Kinda sorta, a little too early to tell. Using the 74HCU04P chip off a dead NES board, a couple resistors and capacitors. Looks like I'll be able to amplify each of the channels now.

I'll be working on something a little more concrete as I find time. If it works, this will be wonderful.

Basically, I've been looking over the NES audio path from chip to the RF modulator (Kevin Horton has a nice schematic on his site) and the specs of this "74HCU04P" fella.

Here's the audio path:
http://www.tripoint.org/kevtris/nes/images/NESAudio.gif

Regarding the audio path, I'm mainly concerning myself with the area between the 12K/20K resistors and the little 39uH... green... thing. That little doodad labeled "FC1". Actually, I don't even know what that's for, doesn't seem to make much of a difference and everything beyond that point in the path is the useless RF modulator.

I apologize, this is starting to sound like some kind of personal blog. I swear it'll be useful though.

that's cool! I'm working on something similar right now. Thinking of adding pro audio connections to nes for use as a stage/studio instrument. Instead of a stereo mod it will be more like a dual mono mod for use with a midines cartridge. maybe add a couple of line drivers so I can plug it into a mixing console or a guitar or bass amplifier.

In case you're still wondering the hex inverter is actually six inverters on one chip (that's why it's hex). An inverter normally changes a logic low (0 V) to a logic high (5 V) or vice versa. Nintendo cleverly decided to re-purpose one of these inverters as an amplifier/mixer for the 2 audio channels. Surely saved them a couple of dollars per unit in manufacturing costs...

Well, did you finish your design adding the 2nd inverter chip to drive the 2nd channel?

Topic revival LOL.

I am just now finishing soldering up my first circuit which is using the whole of the components in the above schematic with the 74HCU04, and am going to try it on my NTSC revision 4 toaster NES. If it works I'll post more.

EDIT: YAHEY!! It works... only done 1 circuit so far... Now the "white noise & triangle & sample channel" is AS LOUD as the NES mono sound, I can tell by putting mono from my NES's red audio socket through 1 channel on my HHB CDR 800 CD recorder analog input (sound goes to -6dB peak max when input is at max., meter is 2 segments down from 0dB), and the above single CPU sound channel comes to the same level on it's own when it's being put into the other input on the CD recorder, from the audio out of my new PCB with the 74HCU04N on it.

Now to do the other one.

BTW my NES has the RGB mod already done, see ;-

[url]nfggames.com/forum2/index.php?topic=1592.0[/url]

where I show my mod and how I did it.

Here is the other topic that shows the audio circuit that I used ;-

http://forums.benheck.com/viewtopic.php?f=57&t=28534&p=467312#p467312

I am using the exact schematic here (originally from Kevtris i.e. Kevin Horton) ;-

viewtopic.php?p=340162&sid=f950113efd2c12115d966fffc0a7be18#p340162

I copied all the components onto a perf. board and used the same ones as the schematic including the Philips 74HCU04N that I got from http://www.quarndon.co.uk with a precision IC socket. The Transistor that I used was the 2SC1740 in TO92 package, NPN, hFE about 550, labelled "C1740 E N" (E= Emitter, N=NPN) that I got from http://www.littlediode.com via Ebay.

Best of all I get NO video noise or picture interference or jail bars at all on my 14" TV or IN76 projector, even with the new audio perf. board working. So now audio and video are perfect as they can be made.

BTW there is no overmodulation distortion with this circuit, the audio sounds great through headphones.

Regards,

Alistair G.