Modding the Resmed S8 CPAP Machine

Finally, after several overnight studies in two different sleep labs, my doctors have prescribed me a C-PAP machine. Basically, it's a bed snorkel. The mask provides pressurized air to your airways while you sleep, reducing the number of times your airways collapse and try to choke you. Now, if you just died in the middle of the night, this wouldn't be so bad. I mean, it's just death, but what actually happens is that your brain, for some odd reason, realizes you are choking, and wakes you up, so you semi-consciously start breathing again. If this happens frequently enough, instead of dying peacefully in your sleep, you wake up feeling like sleeping is very hard work. In other words, barely rested.

So, enter the CPAP machine. Keep your airways full of air, and you don't wake up before choking to death. In fact, you don't choke much at all. No dying, no waking. Just nice even sleep.

The one major problem I see with almost all CPAP machines being sold today, and the Resmed in particular, is that they really are not designed to be all that quiet, but more importantly for allergy sufferers, the filtration really stinks. They fit this tiny bit of polyester fluff in at the entrance of the machine, which basically won't filter out anything but big tufts of cat hair. Everything else in your room, you will breathe, at high pressure. Not a great way to spend the night. They do make in-line filters, but these have a chance of tricking the pressure meters into thinkign your air pressure is higher than it is. It really would have been oh so very easy for Resmed to put in a HEPA filter on the inake.

I should have taken a picture of the intake before I cleaned it for you to see. It was really dirty. Also, instead of providing you with a place to put an intake filter, or at least a place to attach one to, Remsed instead has an opening that makes it almost impossible to attach anything useful to the intake, like for instance, a muffler, or a HEPA filter. Very frustrating. And on top of everything else, my MD prescribed this particular model. Mind you, it works very well, and I understand it's very reliable, and small and portable and the integrated humidifier is pretty sweet, but they are like morticians. They really don't want you shopping around for the best casket, they want you to buy the one they tell you to buy.

So, what is a hacker like me to do? Well, it turns out that in the woodworking and industrial worlds they make these huge dust collection/air cleaning systems. Often a professional mill work or woodworking shop, and sometimes advanced hobbyists, will have large dust collectors with 4-6" ducts running around their shop, sucking up all the wood chips and keeping the shop clean and the air breathable as the craftspeople work. The air gets expelled in a variety of ways, including through these long polyester felt tubes, at the bottom of which is usually a trash bin of some sort. Anyway, the point is, the polyester felt used in these bags normally filters materials down to 1 micron, and after getting nice and packed with wood dust, they get even better, filtering down to 0.3 microns, or about. Not quite HEPA standards (0.3 microns all the time) but MUCH better than what Resmed provides.

So, my solution? Purchase a bag from Oneida Air , cut out just enough filter media to cover the intake, duct tape it in place, and voila, suddenly I can breathe easier at night. :) I've tried it at least one night, and it worked fine, with no apparent change in performance of the CPAP machine. If I was really clever, what I would do is measure the amperage while it runs to see if there's a difference in strain on the air pump with and without the felt, bur right now, my work and school are taking up too much of my time. If I had more time I would have created a felt "pocket" so that I could increase the surface area of the filter. In the next incarnation this is probably what I will do, though honestly it's hard to tell it needs it.

The Things We Do for Love

When you are looking at a mid-80s piece of gear the first thing you have to look at are the power supply filter caps, like the one's I pulled out of the Yammie yesterday.

They are in pretty good shape, and will probably last a while longer, but let's face it, these caps are 20 years old. They aren't fresh anymore, and if they go, you risk loosing everything else in the component. As it turns out, the Yamaha, Sumo Polaris and other amplifiers from this era shared the same filter caps.

Today, unfortunately, these caps are extremely difficult to come by. What makes them so special is the 80V rating. You can buy 15,000uF 100V caps right off the shelf, without any problem however they tend to be 120mm long. About 30 mm longer than the originals. The only manufacturer I could find that still makes them is Nippon Chemi-Con, so as a result of this it looks like I'm going to be forced to purchase a small lot of them, and then try to resell any I don't use. Not exactly how I planned on making extra cash this year.

However, if you find yourself in the same boat as I am, and need these caps, send me a yell, I'll be happy to sell you what I don't use. :)

The Yamaha P2100 Amplifier

The P2100 was introduced in 1977, about 10 years before the P2075, and it's interesting to look at the differences that there are in the circuits of these two amplifiers.

The 2075 is about a 50 W/Channel stereo amplifier. It includes fully balanced inputs thanks to a pair of JRC op-amps, as well as monolithic chip driver and amplifier stages. While you may sniff at the chip amp design, they did a lot of things better in the 2075 than in the 2100. The inputs are truly balanced, many of the audio signal resistors are metal film, copious amounts of bypass capacitors are used throughout the design, and the output stage is heavily protected. Also, the 2075 relies on a lot of ceramic capacitors used as bypass caps.

On the other hand the P2100, while it has a completely discrete audio path, has a single ended input, and almost no bypass capacitors to speak of. The power supply rails go through three different boards with no local capacitor storage at all. While the resistors are mostly cheap carbon composite the capacitors are all polystyrene or tantalum. There's also almost no output protection to speak of. A couple of thermal cut off switches attached to the heat sinks are all you get.

Both amps use several cemented resistors especially near the output stages.

So, the question is what to do, what to do. First, the 15,000uF/86V screw terminal storage caps in the power supply are completely unavailable today. I'm going to have to find suitable substitutes. This probably means designing a new power supply board. If I'm going to do that, I might as well throw in some discrete, zero-recovery diodes, and large value bypass caps.

Next, the cement and carbon comp. resistors will have to go. So will every single electrolytic in the unit.

Last, if I get really ambitious, I'll design a new input driver board to turn the input into a truly balanced design. I'll probably use the design from the 2075 to stay true to the Yamaha spirit.

More when I start ordering parts. :)