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Just had a serious facepalm moment. For some reason I was under the impression that the expansion port sound mod used a 4.7K resistor. You can see "4.7K" on the ENIO EXP board silkscreen.

I'm getting 47K resistors on my next Digikey order, but I have one question for the audiophiles. As far as quality is concerned would it be better to go with 1/8w carbon film or 1/4w metal film? Digikey doesn't have any reasonably priced 47K 1/8w metal film, so these are the two I'm looking at...

http://www.digikey.com/product-detail/e ... ND/2022782

http://www.digikey.com/product-detail/e ... ND/1975140

Would it actually make a difference or am I splitting hairs?

Actually I think a 100k pot is more useful for the audio mod; or at least, it's what I did, and it helps a lot. The balance of the various expansions isn't really that great in the current powerpak mappers so it's good to be able to adjust. I also use famicom carts in my NES with an adapter sometimes, and I've really appreciated having the pot.

I can think of 4 possible sources of distortion:

• parasitic inductance and capacitance of either of these kinds of resistors is such that you really shouldn't be able to hear any distortion from the substrate material at all
• Since neither material is a semiconductor, I don't think you would encounter burst noise
• The resistance is too small for shot noise, and
• neither is foam-like so flicker noise seems unlikely too.

rainwarrior wrote:Actually I think a 100k pot is more useful for the audio mod; or at least, it's what I did, and it helps a lot. The balance of the various expansions isn't really that great in the current powerpak mappers so it's good to be able to adjust. I also use famicom carts in my NES with an adapter sometimes, and I've really appreciated having the pot.

Thanks for the suggestion. I would certainly be open to putting 100k pots on the board if I order another batch, but I already have the first 40 PCBs in hand. The existing footprint can take up to a 1/4w through hole resistor so I just need to figure out which one. If there's no real difference between those two for this purpose, I'd just as soon go with the 1/8w carbon film.

Given that it's mixing the output into the 2A03's output, even if the source signal was super clean, it's still getting mixed into a fairly dirty sounding source to begin with. I don't think some sort of high fidelity resistor is needed, as long as the one used doesn't significantly distort the sound somehow (but why would it?).

It wouldn't be super difficult to route some wiring to allow for a pot as you have it right now, so I wouldn't be too concerned about it. That was kinda what I was already planning to do. Then I could just have a pot off to the side for easy access. Then you wouldn't have to pick up and reach under the NES to adjust some tiny pot. Not sure if I'll have issues with distortion though...

For what it's worth I've realized that it doesn't take much to be better than the famicom. The sound level on Lagrange Point (VRC7) is super low and you have to crank the volume up to hear anything, only to have the static blare you when you shut the unit off.

The resistor choice won't make any audible difference, the sound is noisy to begin with and the extra you'd only see on a scope.
Things would be different if the audio was buffered right at the CPU and traces would go around the board not through right in the middle where they pick up fair bit of interference, although nothing too bad until video chip is reached.