Question for EEs (Neo Geo Sound Circuitry):

[FBX]

So to give some background on this question I'm about to ask: The home console AES Neo Geo went through several revisions (NEO-AES, AES3-3, AES3-4, AES3-5, and AES3-6). In the 3-3 and 3-4 revisions, SNK left out some caps from the stereo audio circuitry, which caused buzzing in the audio output of the headphone jack. These missing caps were intact in the original revision, and by the time revision 3-5 was manufactured, they realized their mistake and added the missing caps back in. You can add these caps manually yourself to the 3-3 and 3-4 revisions and restore good quality sound output from the headphone jack.

I'm in the process of doing this now, and I just have one question concerning the JRC 2066D audio chip. In the MVS version of the audio circuit, pins 8 and 9 are connect to a 470uF can, while the original AES instead has a 0.1uF ceramic-dipped cap:



In revisions 3-3 and 3-4 of the AES, it has neither cap installed. The guide I found online suggests both caps should be added to those pins. So is this correct to add both? I'm guessing since the 0.1uF cap doesn't have polarity and the 470uF can does, that they don't interfere with each other?

Much appreciated for your expert insight on this!

-FBX

[L-Train]

I recommend adding both. The large 470 uF capacitor is for low frequency decoupling and reduces low frequency ripple (which may otherwise show up on the output depending on the IC's PSRR), and the small 0.1 uF capacitor is for high frequency decoupling. The 0.1 uF capacitor should be installed as close to the VCC pin as possible, while the 470 uF capacitor can be placed reasonably close by (within a few centimeters or so), but again closer is better.

[FBX]

I recommend adding both. The large 470 uF capacitor is for low frequency decoupling and reduces low frequency ripple (which may otherwise show up on the output depending on the IC's PSRR), and the small 0.1 uF capacitor is for high frequency decoupling. The 0.1 uF capacitor should be installed as close to the VCC pin as possible, while the 470 uF capacitor should be placed reasonably close by (within a few centimeters or so).

Cool ty for the info on that.

What I did for the 0.1 uF caps is added them directly to the pins on the underside of the board:




The blue caps are 1000 pF 100V X7R (which are also missing from this revision). A couple mods I've seen photos online of this part actually scraped out spots on the ground plane in order to solder the caps to them, but I prefer this 100% reversible method I came up with by soldering to the ground pin of the nearby 22 uF caps.

The tan caps depicted are all 0.1 uF 50V X7R.

Edit: Just fired it up and ran an audio test. Buzzing and hiss are completely eliminated! Sound is crystal clear now. Joy!

-FBX

[FBX]

Just wanted to bump this with a link to a new music track test rip after this mod was finished. I realized I had stashed in my closet a top-of-the-line $500 Sony Mini-disc deck with full DAC and ADC converter functions. While the mini-disc is now an obsolete format, the deck is still quite useful for recording analog sources and converting them to 44.1Khz digital sound output. I opened it up and was quite impressed with the caps in this thing. Some are literally the size of D-cell batteries! Even more impressive was the recording quality of the ADC. It outputs quite simply the cleanest analog-to-digital recordings I've ever heard! I didn't even have to use any sort of noise cancellation filters! Below is the test recording from my AES3-3 post-mod:

http://www.firebrandx.com/downloads/mus ... onyADC.zip

As stated in that song: Pretty damn cool! (or is it "We be damn cool"?)

-FBX