Little Gem part 2: Combo

The Little Gem is very simple. It uses a tiny chip called “LM386-N” that does most of the work. A few capacitors help shape the tone, a potentiometer controls gain, and a rheostat for volume. With just a 9 volt battery, it’ll run an 8-Ohm cabinet– folks have bragged online that they’ve powered a 2×12 cab– that’s 2 twelve-inch speakers. I wouldn’t have thought it possible, until I built my own.

My first version of the Little Gem is a combo. Rather than use a crackerbox like Make, I picked up a little unfinished wooden box at the local craft store. Parts I picked up at a local electronics retailer, and Radio Shack (Rheostats are hard to find these days, the only place in my town to carry them was Radio Shack). The box is 6″ wide, 6″ deep, and 3.75″ tall. I bought a 4-inch 8-Ohm speaker that would fit nicely. I have no idea what the intended purpose of the speaker is, but I found it amongst other equipment for security systems. So it’s probably only supposed to be used to make an alarm wail. Fortunately, that pretty much is the range of my playing ability, so I didn’t expect any trouble.

As I mentioned in the previous post, the Make approach to the Little Gem uses a “prototype board” as the platform for the components. A prototype board is a little piece of perfboard that has traces glued to it in a useful pattern: 2 long columns down the center, and small rows connecting 2 perf holes at a time running perpendicular. One of the columns is wired to the positive terminal of the power source, the other to the negative. This way, any component that needs to go to ground (which is most of them) just needs to connect anywhere along the negative column. And anything needing power connects anywhere on the power column. Simple and logical. I never would have thought of it.

Another bit mentioned in the Make article was the use of a socket for the amp chip. Taken for granted by all the folks at Runoff Groove, I’m sure, but not something I would have thought of on my own. The socket is soldered to the board, and then the chip is just inserted– this way you don’t run the risk of cooking the chip while trying to solder connections to it. I also found the socket useful for another reason on my second build. More on that later.

Using the circuit diagrams and schematics provided by Make and Runoff Groove, and this perfboard layout drawing on flickr, I soldered all the connections, and gave it a quick test. Success! I had noise! Time to mount everything in the box. After initially screwing up the nice finish I had going on the box, I went for a more distressed look. It’s far easier to make something look like a piece of crap than it is to make something nice neat and shiny. Go figure. I drilled holes for the switch, input jack, and controls, and cut a ragged square out of the bottom for the speaker. I decided to mount the speaker to the bottom, and just turn the box over. At the time I was thinking that the larger mass of the box bottom might anchor the speaker better, and not create extra noise, but that’s probably just magical thinking.

The pots are designed to mount to a sheet of metal or plastic, no more than 1/8″ thick. The box’s sides are 1/4″ inch thick. I used files and patience to ‘shallow out’ the box walls around the mounting holes, so that the pot and rheostat would stick out far enough for their threads to accept the nuts that anchor them down. Next time, I’ll make a control plate or something. This part was a pain in the ass.

I stole a scrap of fabric from my wife’s collection scraps-for-quilts to mask the speaker through the hole. Pretty, isn’t it? I hot-glued plastic washers to the inside of the box to act as ‘standoffs’ to keep the perfboard and speaker from pressing directly against the walls. Then hotglued the perfboard and speaker down.

I had forgotten to buy a clip to secure the battery. So I taped it down. Wouldn’t want it rattling around in there, I’m a beginner at soldering, and some of those joints in there will probably snap easily. Chickenhead knobs and rubber feet added a nice finishing touch. Then it was time to play!

How did it sound? Pretty good, considering who built it, the cost of the components (~$25), and the fact it was powered by a 9 volt, of all things. But not great. For one thing, this design does not do clean. The only way I could get a clean tone was to turn the volume way down, where it could be drowned out by normal conversation. But distorted? Fuhgetaboutit! It sounded almost like my brother’s Pignose. I had accidentally dripped hot glue on the speaker cone as I was mounting it to the box– and thought I was out $4. But then I remembered how Link Wray used to claim he ‘d poke holes in his amp’s speakers with a pencil(Fender amps! the vandal!) to get that awesome distortion of his, so I figured hot glue couldn’t hurt too much. Maybe it colors the sound, who knows? I haven’t heard this particular speaker without hot glue on it, so… maybe it’s an improvement.

Here’s two .ogg files that will give you an idea of how it sounds (and an even better idea of how badly I play).

LGcomboWCM.ogg – this is as clean as I can get it. Not very.
LGcomboBB.ogg – this is with the gain all the way up. Rock out!

I decided that I’d never know how good or bad this amp design was without hearing it through proper speakers… and that led to my next build, coming in the next post.