Sound Chip Replacement

Ok, so you all know I love my Sega Genesis, but it's sound chips flaws are starting to irk me.

It uses a YM2612 FM chip. As for it's datasheet, no one knows.

But Yamaha did design a drop in replacement, a YM3438. It has less quantization noise, and generally sounds cleaner. All pinouts are the same, and the same software can run on it just fine. It's also a CMOS version of the above.

However, when dropped directly into my Genesis, all it is producing is a loud staticy mess (but you can faintly here the correct music sounds, so I know the chip is working.

So here's the problem. Wikipedia states: "The chip also had higher output impedance, requiring heavier external noise filtering circuitry but outputting louder sound."

This is very probably the problem. I don't want to modify the Genesis' motherboard, and instead want to make a socket adapter.

Any idea how I can adapt the 3438 to work in place of a 2612? Someone told me I'd need an op-amp and caps, but I need a schematic on how it would work.

Wikipedia has the pinouts:
http://en.wikipedia.org/wiki/Yamaha_YM2612

Maybe this? I don't know how to wire some of the pins on the op amp though.

https://www.google.ca/search?q=noninverting+amplifier

Now question: If the Sega Genesis already has an op amp normally connected to the original YM2612, does it really need "another" op amp like I was recommended? Two op amps sound like a bad idea.

Here's the original YM2612 (though in a socket that I have installed), it looks like the op amp is literally directly after it already: http://img.photobucket.com/albums/v395/Evi...on/IMG_1513.jpg

It looks like it's just wired as a unity-gain buffer (I don't see any resistors) - so all it is doing is adding driving power, no (voltage) amplification... so it should be very low distortion, throw on another sure.

or mod this one to have gain. (unless VCC is too low to really do that...). Could make a little socketed daughter board that fits in where the opamp is.

http://www7.plala.or.jp/kikekike/fm/YM3438_APL.pdf

Page 46 of this data sheet. Does it look good? It looks like I'd simply use the second op amp in that example schematic as the op amp already in the genesis.

I guess I'm just not understanding japanese schematics.

Ground is yeah, ground. JP likes to draw them like that.

The 9V is powering the opamp, both of them presumably.

I'm guessing the box coming out of 'MOR' says 'ditto' in japanese, but no idea.

presumably yes for the 4.5. In reality it's usually from a voltage divider from +9 and gnd, say 2x 100k, as a ref for the opamp.


The 10k, .00047, and 220p are for making the response something other than flat. They both happen to be low pass, to get rid of IC noise probably, etc.

Hmm this system only has 5v inside. Going to need to find a way to get more power options.

MOR and MOL,are,I'm guessing,the Right and Left audio outputs. So yea,it's probably a box that says "Same as above".

ED:Only have 5V available? That's kinda low,but I'm sure there are op-amps that will work at 5V (+/-2.5V).. Can't think of any off the top of my head,that I know for sure work that low.

Seems like there should be a 9V supply,or something in there.I dunno.

Nope, power from the 9v PSU goes directly to dual 7805's.

I just tried for shits and giggles, just dropping the YM3438 directly in the socket for a YM2612, and it does play, but the audio is so heavily distorted for the original reasons mentioned.

http://www.youtube.com/watch?v=sSWMr4rjfd0

well,if you can tap the 9V to power an op-amp,that gives you +/-4.5V,and most common op-amps can work with that. It will have to be AC coupled,so you'll need caps on the in/outputs.

Kinda/Sorta similar to the C.MOY headphone amp..
http://www.headwize.com/projects/cmoy2_prj.htm

Wow, well I kinda figured this out without ANY op amp at all.

http://www.youtube.com/watch?v=lu9OgXuzA6E

Single cap, and bam! It worked.