So, my fish tank needed an amp.
The sound levels in the iMac speakers are too low when driven by the Raspberry Pi that powers the thing, and since I want it to be an internet jukebox for my office, that’s not going to work very well. After much searching for the right amp to drive the speakers, I decided to build one instead. Why? That’s not really a question one asks around here.
It’s been a long time since I did any significant electronics work, and my skills probably weren’t up to designing one from scratch in the first place. Fortunately, Sparkfun makes a kit, the STA540 Audio Amplifier. Two channels, 38W, and according to theory (and a calculator I found on the Internet) more than enough to meet my needs without overdriving the speakers. Plus, it fits in the available space under the fish tank.
I was pretty pleased with the kit itself. Reasonably packaged – aside, maybe, from the 7 loose resistors, 5 of which are coded Black-Brown-Orange and two Black-Brown-Yellow – and nicely designed. Plenty of room for all the components on the PCB without crowding, and excellent instructions. A confident novice with a little soldering experience should have no trouble building the kit. That’s a good thing, because it’s been long enough “novice with a little soldering experience” is probably a reasonable assessment of my current skill level.
The kit walks through the process in a reasonable fashion, smaller components first. Despite the years of rust I quickly remembered the little tricks and details – soldering iron goes in your non-dominant hand, use the dominant hand for the solder. Various ways to clamp components in place with the little third-hand stand. How to burn the crap out of your hand with a soldering iron. (Skip that last one.)
I expected the kit to take a couple of evenings to assemble, but once I got into a rhythm it went together very quickly. I didn’t get started until after 8, and I finished by 10:30. Then of course it took another 40 minutes to find and eliminate all the bad solder joints and other issues, but even so it’s still one evening’s project.
I did make a couple of minor adjustments –
There on the left, in front of the large capacitor, is a box labeled “STBY”. That’s intended to be the standby/on switch for the amp (it’s partially powered whenever it’s connected to power, so it’s not really “off”.) But I want to mount this inside the base of the fish tank, and it won’t be easily accessible. So instead I soldered in a three-pin header and wired the switch using jumpers so I can put the switch in the old IO port panel. I originally intended to wire the LEDS and potentiometers the same way, but I changed my mind. First, the LEDs will be visible in the amp’s mounting location, so running leads to them seemed like overkill. Second, the potentiometers (“volume knobs”) would be a pain to wire that way, and more than likely never adjusted after the first round of testing. So although it goes against my nature, I followed the directions for the rest of the kit.
Want to know what the slowest process in the world is? Soldering a heat sink.
The kit instructions give a set of instructions for testing the board, which in my case revealed a minor short where solder bridged a couple connections. Once fixed, I showed rare maturity in deciding not to wire it up to a sound source and the speakers and seeing what I get from it. We’ll try that tonight – there’s nothing good on TV anyway. 😉