Lately I have been buying a lot of Blu-ray movies as well as some DVD’s so that I could watch movies on the train on the way to and from work. Some of the Blu-rays come with a Digital HD copy that I can download to my tablet, otherwise I can rip DVD’s to watch. The problem is that the volume over headphones on the tablet isn’t that loud. I have to turn the volume all the way up and I still can’t always make everything out when there is a lot of noise on the train. One possible solution would be to use a battery-powered headphone amplifier. Typically these are used for driving bigger headphones that require more power than earbuds, but it should work in my situation as well when the source just needs to be louder. Ten years ago I wrote about a type of headphone amplifier called a CMoy amplifier after its inventor, Chu Moy, who apparently passed away last year. I don’t hear as much about these anymore, but you can still find homemade ones here and there. One guy stopped making them, but now sells the parts as a do-it-yourself kit. His design is powered by two 9-volt batteries and gets pretty good reviews. At $20 for the kit, it is cheaper than ones that are already made and in a mint tin for $35. I have a few empty Altoids tins that I was saving for something like this, which I think is key because it seems likely I will mess at least one of them up trying to get the four holes in exactly the right places and the right size. So I went ahead and ordered a kit, picked up some batteries at Walgreens, and got the kit today.
I am not particularly good at soldering and have a very basic soldering iron. But this is all through-hole soldering which is easier than some of the stuff I have to solder on flashlights. I felt like I did pretty well making the Mintyboost years ago and that thing still works. I was glad that instructions were included and they were well thought out with pictures, but it would have been better to have been able to download the instructions since the black and white pictures weren’t that clear. Fortunately, I was able to get some good pictures from the Etsy and eBay pages, but even so they didn’t show everything perfectly. I have a hard time getting a nice solder joint on the first try, and multiple tries tend to end up overheating the components. Ideally you heat the work and the wire and touch the solder to those and get a perfect joint. But its hard to have the soldering iron touch the board and wire at the same time and then bring the solder in with the other hand. I have a magnifying lens that helps. There are maybe 50 solder joints to do. Three of them have to be done very quickly to avoid burning out a transistor. But I eventually got everything on there and just had to attach the leads from the battery connectors. There were three holes and four leads for the two connectors. The outer ones were positive and negative, but the larger center hole was not labeled. I should have put the positive from one connector in the positive hole and the negative from the other connector, but instead I put the two black leads together in the middle hole, thinking there should be a common ground. When I attached the batteries and turn the knob to On, nothing happened. I tried attaching the headphones and line in, but still nothing. So I looked over my soldering more closely and found a couple of spots that weren’t very good and fixed them up. Still nothing. So I looked at the instructions and decided to desolder the two black battery leads so that I could hook the batteries up in series, like I should have. Once I connected the batteries and turned the knob to On, the LED lit up. Yay! Then I hooked up the line in and headphones and it seemed to be working. But there was no sound in my left ear. Nothing wrong with the headphones, so I started putting pressure on the headphone jack and every now and then I could get stereo. So I figured maybe I needed to make sure the jack had a good solder connection and fixed up those three points. But I was still getting nothing. I decided to try to desolder it and then maybe reseat it, but I broke one of the prongs trying to pry it up. So game over. I thought maybe I could get the seller to send me a new jack that I could solder in place, which should be pretty cheap. I also found some jacks on eBay that look the same, 10 for $1 with free shipping from Hong Kong, but will take a month to get here (or more since Chinese New Year is coming).
After that I played around with getting it to fit in an Altoids tin and, true to form, managed to screw that up pretty good, but learned some things. After I got some stuff figured out, I tried again on the other end of that tin and came a lot closer, but it is incredibly difficult to get 3 holes in a perfectly straight line with everything spaced exactly right. I think I can do it though. The hole for the LED on the side should be a little easier and I left the leads longer so it would have some play, but that will have to wait.