SNES High Quality Analog Audio mod

Includes but not limited to: SNES, Genesis, Sega CD, PlayStation 1, Nintendo 64, Dreamcast, Game Gear and I guess the Virtual Boy.

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lontas
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SNES High Quality Analog Audio mod

Post by lontas » Wed Oct 17, 2012 11:31 pm

Greetings! I am Lontas, and I'm new here. I'm a huge fan of the SNES and its music, and a pretty big audio geek as well, so I decided to poke around and see if there was a way to somehow improve the SNES's audio quality. I came up with a mod that does exactly that, and it does it in a way that I happen to like better than any other similar mod. What follows is a set of instructions for how to do the mod, packed with far more information and opinions than you ever wanted to know about it. I will be referencing the original revision SNES; later revisions will probably have differences, but the concept of the mod is so simple that it shouldn't be that hard to make it work for any SNES.

The Problem

The SNES audio signal begins inside the APU, where the DA converter takes the digital bitstream and converts it to a stereo analog signal. The signal gets amplified by the 2904 opamp and then exits the APU through pins 21 and 22. It then enters the SNES mainboard, where it gets mixed together with any other audio signals that might be coming from the extra cartridge pins and/or the expansion slot pins. The whole mix then goes through a discrete amplification stage, and finally, a pair of surface-mount electrolytic capacitors to remove the 1.5V DC offset right before it gets sent to pins 11 and 12 of the multi-out port.

So, what's the problem then? Simply put, it's the capacitors. If you've done audio electronics before, you've probably been taught that you can put an electrolytic capacitor in series with the signal to remove DC voltage while still passing the AC audio signal. This is theoretically correct. However, the problem in practice is that electrolytic capacitors suck for audio quality. Why is this? It is because in practice, no capacitor has perfect electrical characteristics. Electrolytic caps have a high ESR (equivalent series resistance), and what the datasheets don't tell you is that the ESR increases as frequency increases, which means that the caps will resist higher frequencies more than lower frequencies, artificially cutting out the high end and giving audio a muffled/muddy quality.

The Solution

Because there is DC offset in the SNES's audio signal, we can't just remove the caps entirely (unless you don't mind potentially damaging your TV/receiver that you connect your SNES to). Luckily, there is a viable solution. What we need to do is somehow compensate for the bad effects of ESR that kills the higher end of the audio frequencies. The answer is to install small-value film capacitors, which have much lower ESR than electrolytics, in parallel with the existing on-board electrolytic caps. Why does this work? Because electricity always prefers the path of least resistance. By placing a film cap in parallel with an electrolytic cap, the film cap will automatically "take over" for those high frequencies which the electrolytic fails to pass very well. The end result is that you get clearer, more linear audio reproduction!

The Directions

The mod is simple in concept, but it involves some difficult and creative soldering.

1. Take apart your SNES, and remove the shield from the mainboard that covers the large power capacitor.

2. Locate the two surface-mount audio capacitors. They look like metal cans mounted to the board via two small solder pads, and the first number on the top of each cap should be "10". On my SNES, they are near the multi-out port. If your SNES has a different layout, there's an easy way to find which caps are the two correct ones using a multimeter. Simply set it to check for continuity, place one lead on pin 11 of the multi-out port, then place the other lead on the negative end (black stripe) of all the capacitors until you get a beep. That is an audio cap. Repeat for pin 12 of the multi-out port, and that is your other audio cap.

3. Solder a .01uF film cap in parallel with each of the two audio caps you just located. ("In parallel" just means you attach one lead of the film cap to one lead of the existing electrolytic, and the other lead to the other lead.) Here you will notice that you don't get much room to solder, so you will probably need to do some creative lead bending and/or taping/clamping in order to make it work. Remember that you will be reattaching that metal shield, so make sure the new caps fit under there.

For the .01uF film caps, I recommend either a polypropylene type or a PPS (polyphenylene sulfide) type of 50V or higher voltage rating. It's not that you need it to handle high voltage, it's that the higher-voltage film caps happen to pass high frequencies better than lower-voltage ones. You can also use polystyrene (if you can find them), or lots of other exotic types, but any of these is more than sufficient. It is possible to use C0G ceramics as well, but in my experience those don't work as well as film caps in this application. Avoid polyester/mylar.

To go off on a tangent - What I ended up doing, just to be extreme, is I soldered not one but TWO new caps in parallel with each audio cap. I took a 22uF 50V Panasonic FR electrolytic cap, attached a surface-mount Panasonic PPS cap to its leads in parallel, and then soldered that cap-cocktail onto the existing cap to make three caps in parallel. Adding more caps in parallel further reduces ESR, and also raises the capacitance value which can potentially benefit the low frequencies. But you can get an improvement with just a film cap too.

4. Put the SNES back together, pop in your favorite games, and see if you can hear an improvement! I found the improvement to be subtle but noticeable, especially worthwhile if you connect your SNES to a decent sound system. The higher frequencies come through more clearly. The audio sounds more balanced overall to my ear. If you run your SNES audio through regular TV speakers, then this mod probably isn't for you.

5. Optional - Install RCA jacks. I chose to add RCA jacks to the back of my SNES because that gives me the option of using my own choice of high-quality RCA cables. Those multi-out cables tend to be built very cheaply, and this gets around that problem. To do this, grab some copper hook-up wire: Connect pin 11 of the multi-out port (you can solder to it on the underside of the mainboard) to the left (white) RCA jack's center pin. Connect pin 12 of the multi-out port to the right (red) RCA jack's center pin. Connect either pin 5 or 6 of the multi-out port to both RCA jacks' ground terminals. (Or just run two separate ground wires from pin 5 and 6 to each.) Drill some holes in the case and install the jacks where you think they will fit, and solder them up.

Other comments

At first, I tried connecting a pair of RCA jacks directly to the output pins on the APU (pins 21 and 22) through high-quality capacitors and resistors for protection. But this lead to some non-obvious problems. For one, the level was too quiet because at that point the sound hasn't gone through the discrete amplification stage on the mainboard yet. Also, you won't hear any sound that may be generated by a game cartridge itself that uses the extra audio pins (which the Super Gameboy does, for one) or from the audio pins of the expansion port (if anything actually uses those?). I ditched this idea when I learned of these problems.

Why not just use the existing digital audio mod and avoid the analog stages entirely? I chose not to go that route because the analog electronics are part of what defines the SNES's sound to me. I was afraid that the digital audio might sound too different from that SNES sound that I am used to. Plus, that 2904 opamp has such a small bandwidth and low slew rate that I think it may have been chosen specifically to filter out the digital aliasing that may otherwise be heard. That filtering effect would be totally absent with the digital audio mod (I think?? I've never actually tried it, correct me if I'm wrong).

Another thing to try that may improve the quality even further is to create an entirely new mixing/amplification stage: Tap the APU, the cartridge audio pins, and the expansion port audio pins, sum them together on a separate PCB where you can choose the highest-quality caps and resistors, then amplify through a dual opamp to a pair of RCA jacks. But that may be a bit beyond my ability.

If you try this mod, I would love to know whether you can hear an improvement or not, and also whether you actually like the change or prefer the stock audio. I would also love to hear your questions, opinions, counter arguments, and corrections. Have fun!
SL Audio Enhancements - Electronics upgrades for your audio gear

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evilteddy
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Re: SNES High Quality Analog Audio mod

Post by evilteddy » Mon Oct 22, 2012 6:45 pm

Awesome guide. I might give it a go sometime.

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