Dual-YM2612 hardware synthesizer

For anything related to sound (YM2612, PSG, Z80, PCM...)

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Aly James
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Post by Aly James » Fri Apr 18, 2014 7:42 am

great! sent you some infos 8)

foobat
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Post by foobat » Wed Apr 23, 2014 3:28 am

Today I got my YM3438 chips in, which will require a hardware change to support. It's definitely much louder than the YM2612 and might require a voltage divider at the coupling capacitor. It's definitely clipping. I should be able to figure out how to fix it with a scope and I remember reading somewhere that TmEE (I think) already fixed this problem with the 'crystal clear sound mod'. For now I can just ugly the connection with airwires but for the final design we'll want to decide whether we should have a different board for the YM3438, a selector switch or detector of some kind, or one YM3438 and one YM2612.

I'm also a little interested in what HardWareMan said here:
Clock on the YM2612 is best served through a capacitor. Otherwise it starts to warm up.
My 2612s get a little warm but I wouldn't call them hot. Serving a clock signal through a capacitor confuses me though, I've never seen or heard of doing anything like that before. Anyone know what that means?

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Aly James
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Post by Aly James » Wed Apr 23, 2014 8:44 am

That might help for both question... :)
It is an YM3438 output stage + clock (btw: its called load capacitors, google it for precise infos)
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Post by foobat » Wed Apr 23, 2014 10:01 am

That shouldn't be a thing because I'm using an oscillator IC instead of just a crystal.

That schematic is... interesting. There's no load on MOL! I thought this thing would burst into flames with no pulldown on MOL and MOR?

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Aly James
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Post by Aly James » Wed Apr 23, 2014 10:37 am

Well, as you experienced it, the output on MOL & MOR is significantly louder than 2612 so the output impedance should not be the same, however I think that adding pull-down should reduce the volume in this case...

Somebody actually kind of piggy backed an YM3438 on top of a YM2612 in a stock Genesis Model 1 console,
he latter found that problems he had with sound were related to Timers differences.
so he kept the YM2612 just for the timers and used the 3438 for the rest..
here is how it is simply wired.
Image

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TmEE co.(TM)
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Post by TmEE co.(TM) » Wed Apr 23, 2014 3:37 pm

YM3438 needs no loading resistors on its outputs, it has push-pull output (or should, as its CMOS).
Mida sa loed ? Nagunii aru ei saa ;)
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foobat
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Post by foobat » Wed Apr 23, 2014 8:16 pm

Sweet! Thanks, guys. I'll come up with something.

@Aly, I'll send you a device with one YM2612 and one YM3438 when I get it ready, and I'll update the board layout to have selectable voltage dividers. Automatically detecting the YM3438 will probably be too much for me, so we're gonna have to use a switch or a #define in the program.

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Post by foobat » Sat Apr 26, 2014 3:01 am

Haha, well, I took a scope to it today and found out that it was clipping the bottom half of the audio not the top half - the elimination of the 12k pulldown resistor that the YM2612 needs and the elimination of the 10uf cap are the only changes necessary to support the ym3438.

I did find out something else cool, though - the ym3438 is boss as hell and uses the full range peak-to-peak. It just straight up outputs the signal you want, no amplifier needed, just a voltage follower. That does mean that the whole right half of the board is a waste of space for the YM3438 and actually barely-perceptibly (I can see a little loss on a 100mhz scope) DEGRADES the audio quality. It almost makes me want to make a whole new board layout for the 3438 with just a voltage follower instead of an amplifier... but the YM3438 is kind of hard to come across anyway...

Maybe I'll just leave it be.

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TmEE co.(TM)
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Post by TmEE co.(TM) » Sat Apr 26, 2014 5:15 am

Your problem with the voltage follower is that you need power supply that goes below ground and above power supply of the YM. Opamps don't take rail to rail input, and most of them certainly don't have rail to rail output.
When I have tried to use a voltage divider or something on the output to reduce the range it will overload the chip and distort the output...
One thing you can do however is lift up the DAC ground level and reduce the DAC power supply. I'm not sure how well the chip likes it though...
Mida sa loed ? Nagunii aru ei saa ;)
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Post by foobat » Sat Apr 26, 2014 6:26 am

The chip doesn't mind a voltage divider if you use it *after* the coupling cap that removes the DC component, so you can use that technique to reduce the signal to something the amp can handle. I have to do this for the lm386es, you can see it at the 12k<->220k divider after each of C30 through C33. This works ok, but it's a little sad to put an lm386 in the way of a perfect audio signal. Compared to the YM3438, the audio signal coming out a YM2612 is garbage!

Here's the second prototype, fully functional with dual YM3438:

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Post by neologix » Sat Apr 26, 2014 5:07 pm

I...I think I want one now :shock:

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Post by foobat » Thu May 01, 2014 4:22 am

It needed some kind of case to survive the trip overseas, and there was a can of spraypaint nearby...

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can't let good spraypaint go to waste

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Post by yogi » Tue Jan 27, 2015 7:48 pm

Very cool project Foobat! I'm new around here but when I ran across this had to jump in. Aly James hinted about this project a bit over at Chipmusic.com a while ago. Glad that I wandered over here.
I guess this topic is kind of cold ATM but hope it's not dead :( Any news?
I was interested in doing a order at OSH park but don't need 3 boards. So if anyone is interested I would 'at cost' + shipping to your address. Works out to $42 per board+shipping.
IF there is more then 2 people interested I wouldn't want to go over 9 boards total. Looks like parts would be about $50 but depends on your junk box and where you shop. Anyone?
Yogi

foobat
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Post by foobat » Sun Feb 01, 2015 12:51 am

Good projects never die, they just fade away. Aly pinged me about it a couple times and I didn't answer because I feel bad for being so busy with work :(

The hardware is basically complete, there are minor issues but nothing major.

All that is really left to do is put some finishing touches on the software (finishing the MIDI implementation) and make some kind of case for it. I just haven't had the time :(

If you order boards make sure you can get a 7.68mhz oscillator in a dil/dip-14 package first.

The board will also handle YM3438 instead of YM2612. You can do one of each or two of either. For YM3438, leave the resistors from the outputs to ground and the capacitors for the lm386 depopulated.

My Twitter is @grybranix, feel free to holler if you need a hand with anything. I check spritesmind once every couple weeks too.

If left to my own devices I hope to finish the software for this by 2016

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Post by yogi » Sun Feb 01, 2015 4:46 am

Oh hi Foobat. Glad you are just 'on break' from it :) I know the feeling. I've talked a bit with AJ also; he's kind of in the same boat with his time too.
Thanks for the pointers, I would like to see what I can do with the code so will diffidently have some questions.
ATM there doesn't seem to be much interest with a board run, so for my needs I'll build a proto on perf board. Just as well for now, had a household emergency this last week so a little tighter with spending for a while.
I've wanted to do a similar project for a while so this is perfect.
Yogi

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