m00dawg
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Texas

I finally started looking at my NES Audio amp again. The first two major designs were largely based on the CMoy Headphone Amp. The first design was a single-board, through-hole solution; the second used SMD parts for size and lower voltage requirements (both the NES and NESA can be powered via a 9VDC power adapter).

The biggest feature I want to add is a passive mixer for the 4th audio channel (mixing the 2 NES channels with cartridge input). I am also debating about switching to unity gain buffers for the discrete outputs as I found them to be rather hot and it's likely not necessary to amplify the audio as it is already near line-level (I need to verify that).

I want to try and position the new board so it sits underneath the 2A03. I measured 65x44mm worth of room there which should be plenty (I just may be height limited which is important for the power caps). That way the mod can be installed without having to cut anything inside the case.

Finally, I think going back to a through-hole design makes the most sense. Easier to assemble and many through-hole OPAs can be powered by as little as +/- 4.5V. That leaves no headroom if using a 9VDC supply, though, so I'll have to test that out.

If anyone would like to follow the project, the Eagle schematics and board layouts are available on Github:

https://github.com/m00dawg/NESA

Likewise if you have any suggestions in what you might want to see in this design, do let me know!

Theta_Frost
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Milwaukee, WI

Super interested!  If it has that 4th channel passive mixer I would make one in an instant.  I think through-hole would be great.  I do plan on powering a PS1 LCD from the NES though, so I may run into power limitations.  Can't comment on the unity gain buffers, but as long as I get 4 high quality level matched outputs I'm happy. smile  Excited about this!

Jazzmarazz
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Michigan
m00dawg wrote:

I am also debating about switching to unity gain buffers for the discrete outputs as I found them to be rather hot and it's likely not necessary to amplify the audio as it is already near line-level (I need to verify that).

I am mulling over a similar conundrum with one of my own designs. Think about what you are making; a noise filtered line-out? An amplifier? A mixer? All three? Anyone composing with their NES should be responsible enough to have an external mixer and amp, so just by going with a clean buffered connection sounds good to me. smile

Isolate it, filter it, mix them, buffer it and you'll have a sale.

yogi
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Jelly Stone park, MD USA

I like the idea of a unity gain buffer; with just filter caps I have found the output of the 2A03 to be hot. I've never messed with expansion audio so I don't know what kind of levels you get from it but it would be nice to be able to balance the levels across the channels.
  I'm kind of both minds as to mixing; on the one hand an external mixer offers the best production control but there is a lot of convenience to a built-in mixer.
  A+ on through hole smile
Yogi

m00dawg
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Texas

Woo lots of interest it seems! So, I'll try to address things in one go. The primary goal of the NESA is to protect the CPU from naughty things. I don't like the idea of just hooking up an RCA jacks directly to my previous 2A03 so the OPA is there to sacrifice itself. It has similar benefits to the standard RCA mod in that it bypasses the original audio pipeline, so is a great deal quieter.

I hadn't thought of any sort of filtering beyond the input caps I originally used as part of the CMoy design. I was thinking of actually getting rid of those, but that runs under the assumption that the output sources being used have their own DC correction (if there is any from the NES to begin with, which I need to test). As an aside, I AM looking at modifying my StageBoy (Arduino + Audio Balanced box for my GameBoy) to use a low pass filter to try and tame some of the aliasing.

Anyways, the major things I am going to look at is to see if I can balance out the audio from the NES and cartridge in (and at least estimate how close to line-level those really are) and then see if I really can just buffer, or if I need to do any sort of amplification.

But the ultimate idea is as described - clean, safe, discrete line-level output with an addition mono mix out. The nice thing is if some of my current assumptions are correct, the resulting design will be simpler than my previous designs. Meaning it can be smaller, or there is room for more fun stuff.

DSC
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If you are serious about noise, you need to consider a better option than your standard top loader.  low_budget has reworked the original pcb and redesigned it for much better noise separation.  Here is the link to the forum post with more info:
http://atariage.com/forums/topic/203257 … 8-project/

The other benefit low_budget tackled was making it easier to use native RGB PPU chips.  Overall you get pristine video and pristine audio (all three channels)

I have worked with Nick and helped him with his Polycase cases.  I can personally attest to the amount of work he has put into this project.  He has models available for people who are serious about overcoming the issues the original NES has.  Of course all components have been replaced other than the original PPU and CPU chips.  Extreme maybe, but very thorough big_smile
Here is an example:

m00dawg
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Texas

Oh wow that's awesome! Looks like the thing is basically available (or almost available)? Indeed, that would be a solid solution - I was griping today after Mega Man 2 froze on me (serves me right for not getting the password when I had the chance). I disabled the lockout chip and have a new 72-pin connector but it's still a bit sketchy to get working well.

In terms of audio, we both appear to be using an op-amp based solution so I suspect the audio performance would be similar as I am totally bypassing the original audio pipeline for NESA. NESA is still a good reasonably inexpensive option for those that can't get a Super 8 (or a CPU or PPU to use) and it's not a particularly invasive mod (save for adding the audio jacks).

That said, the Super 8 looks glorious and I hope to get my hands on one for sure! I even have a busted NES mainboard whoose CPU and PPU is just begging to be liberated.

Someone was working on a similar solution for the GameBoy...wonder how that's progressing...

m00dawg
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Texas

I hooked up an o-scope to my NES yesterday and, actually, it seems that the current amp design may be accurate. Off the NES I was getting around .1V and after the amp, around 1V. That's about right for line-level, but I swear I'm hearing distortion in some games on the square-wave channel that make me think something is off. Either way, the audio off the PowerPak is hot - 4V after amplification in the current designs - that's well above line-level.

This brings up a bit of a problem though. I didn't test it, but I don't recall the Famicom audio being nearly that hot, so I need to research that more. Ideally it would be nice to have a solution that worked for both situations without having to replace any resistors and other such nonsense, but I'm not sure of the best way to tackle that. I never have found a good standard way of handling cartridge in (off of the PowerPak or a Famicom cart).

Apart form that issue, the other bits of the circuit I may end up leaving as is - with the discrete outputs as well as the mixed output being amplified up to line-level.

m00dawg
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Texas

Well, one other potential gotcha is that the cartridge pin tends to be noisy when not in use. In an ideal world the mix out would only use the cartridge input when it actually has audio on that channel. I don't have a solution for that at the moment.

herr_prof
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Unsubscribe

"low_budget" indeed smile

m00dawg
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Texas

Hah, referring to the Super 8? Yeah the price is currently a bit steep. I'm curious if an un-assembled kit version is going to come out for a lesser price perhaps. But, either way, I want one!!!

But I'm still going to keep my toaster NES regardless. Fixing up a toaster (new 72-pin connector, disable lock out chip, caps, optionally NESA or some other audio mod) gives pretty good results for less. The current NESA designs are cheap (my first design is a single layer board you can even etch yourself) and the new one I don't expect to be expensive to build.

That's taking nothing away from the Super 8 mind you as it's a thing of beauty. Simply pointing out the toasters aren't going away since the Super 8 isn't for everyone (largely due to cost).

kineticturtle
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Matthew Joseph Payne

I think I saw somewhere that he is specifically disinterested in selling a kit version of the Super 8 because he doesn't want to provide assembly support.

m00dawg
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Texas

Awe that would be a shame, but it understandable.