I can't take credit for the idea--a quick web search for "coconut amplifier" will bring up a couple of matches. I stumbled upon the idea while looking for a cool ukulele amplifier. The version presented here is based on the venerable LM386 amplifier. It's a project ideally suited for the electronics beginner--it's a really satisfying product that can be put together for under $20 (or less if you have some parts and junk laying around), with parts commonly available from Radio Shack. I have presented a stripped down, very basic circuit diagram for the LM386 amplifier. There are several Instructables for this chip--look around and pick one that works. The LM386 is not known for it's high-fidelity sound. Even at moderate gain they tend to clip and distort. For an amusing project like this, I think it's kind of pleasing. It gives a nice, overdriven amplifier sound, without the ear splitting volumes (the chip, at most puts out under a Watt of power).
To build the amplifier you will need to use some basic hand tools--a saw, drill, soldering iron, etc. Please be careful and use all precautions.
Step 1: Assemble the Supplies
The amplifier is built inside of a coconut shell which is a small, irregular housing for it. I started with locating a speaker. The little radio was purchased at a local thrift store and provided 2 2", 1W speakers. I lucked out, and they were 8ohm (which is ideal for this amp). 4ohm would work. Obviously--such a small, cheap speaker is not the ultimate for sound reproduction. You can purchase better drivers for cheap if you feel the need. The radio also provided several other capacitors. This amp also has a volume knob made out of a segment of bamboo. There is a stand behind my apartment and I found this pile left over from last summer. Find a piece of the right diameter. The other part needed, of course, is a coconut (grocery store), and a handful of other parts from Radio Shack, or more cheaply, from one of the online distributors. Other supplies used in this project are 2-part epoxy and super glue
Step 2: Cutting Open the Shell and Preparing the Bamboo
The shell should be cut so that it can sit on a flat base. You will notice the coconut has 3 "eyes". One of them is much softer than the other. I used a phillip's head screw driver to punch through it and drain out the milk. Then the shell was placed in a small coffee can with the 3 eyes pointing upwards, and a line was drawn around to define a plane. The shell was cut by carefully following the line with a coping saw. The shell is amazingly hard, but cutting through it goes quickly with a new saw blade.
I read that the meat can be removed from the shell easily by microwaving it. My first attempt resulted in a shell that was destroyed and full of large cracks. I removed the meat from this one by sawing into it with a heavy steak knife, and prying it out in chunks. Be careful with this--don't slip with the knife and stab yourself. If anyone has an easier solution to removing the meat. It's really stuck in there.
Bamboo has a structure of sections of tube with joints between them--places where the tube is closed off. I cut the piece here by going around the tube with the edge of a file and cracking it. This bamboo was kind of punky and soft. I stabilized/hardened it by going around the end grain with super glue. The spongy material soaked it up. Finally, I filled the base of it with 2-part epoxy which was drilled with a 1/4" bit so it could accept the shaft of the volume knob.
Step 3: The Circuit
Here is a circuit for the LM386 amplifier. Pins 1 and 8 set the gain of the amplifier. Leaving them open results in an internally set gain of 20. Bridging them with a 10uF capacitor, as shown here, results in the maximum gain of 200. At gain=200, the chip produces a sound that is heavily distorted (in a cool, musical way). I'm building this one for guitar, and decided to permanently set the gain on the board. A 5k potentiometer can be used instead, for an adjustable gain (as in this circuit http://makezine.com/projects/make-09/the-5-cracke... ) , or by adding a resistor in series with the 10uF capacitor to lower the gain. If you require a cleaner sound, the LM386 is not a great choice. A TDA052 amp (like a "Punch" amp, or one like this http://makezine.com/projects/make-09/the-5-cracke... ) is louder and cleaner.
Also, if the amplifier is to be used with a piezo pickup (a very high impedance pickup) a buffer stage should go in front of the amp (this one works great http://makezine.com/2011/12/20/collins-lab-diy-co... ).
If poor construction practices are used, the amp presented here can actually pick up noise from the surroundings. In fact, when I first built one on the bread board, I could hear some radio reception! Make connections as short as possible and direct. Long wires act like an antenna. Also, as shown here, the ground wire is run under the chip. This helps to partly shield the chip from RF noise.
Assemble the circuit, and plug in just to make sure it's working. If it does not work--calm down, take a break, and see if you can fix it. This amp was built after a long week. I was tired and in a hurry. Of course it did not function. Then I realized that while i hooked up the indicator LED, I neglected to actually run power to the chip! This is often how it goes--errors like reversing a capacitor, not grounding or powering something... Clean wiring makes it much easier to trouble shoot. Another common problem is installing the chip upside down (the notch in the chip indicates pin 1). There is little that can go seriously wrong here. Here are a few common problems. If the volume pot sweeps the wrong way (ie, turning it clockwise turns the volume DOWN), switch leads 1 and 3. If the circuit works but the LED does not light, switch the leads (electrolytic capacitors and diodes are both DIRECTIONAL, polarized devices). If everything works, I would stress relieve the wires for the battery connector. These tend to get flexed, and can not handle a lot of bending before the joint fails. I stress relieved it here by applying a small blob of epoxy glue on the joint.
Step 4: Putting It Together
I drilled out the "eyes" with a 3/8" drill. This acts as a kind of grill for the speaker, and it looks kind of cool. The actual purpose of the device is not immediately evident. I drilled holes for the switch and LED next to each other, and the phono jack and volume knob next to each other (out the back). I mocked it up, making sure everything would fit. The speaker was installed by gluing it (with 3 blobs of epoxy glue) inside, behind the 3 eyes. The LED, switch, pot, and jack were installed, and finally the circuit board was epoxy glued in to the side. It's not easy to clamp the pieces into place for curing, so I ended up holding them with my hands for a good 10 minutes until the glue got solid enough to hold it.
Step 5: The Finished Package
The knob was installed using a drop of super glue (it could be improved by installing a set screw). The only thing remaining is a method to hold the 9V battery. I'm considering holding it in place with a small neodymium magnet, or maybe purchasing a 9V battery clip from Radio shack. The bottom could (should?) be closed off with a cap, maybe thin plywood.
Here is a sound clip of the device ("Nearly Lost You" from Screaming Trees)
Forgive the musicianship. Dr. K is actually a chemist :-) The grungy, distorted sound comes from having the gain set to 200 with the 10uF capacitor. Removing this cap would largely clean up the sound. The distortion level can be controlled a little bit by turning down the guitar.
This is one of those projects I built once just for the heck of it, and every person who sees it wants one! It's just too silly, with this ridiculous Gilligan's Island vibe to it. Be safe, have fun--and I would love to hear any improvements on the construction. Removing the meat is one hard part, mounting the 9V, it turns out, is something I don't have a good solution for. Anyway, enjoy. With high quality components this could actually be a nice little practice amp.
Step 6: Final Thoughts, Last Minute Changes
The customer, aka my brother, listened to the sound clip and immediately asked "can it go clean?" The answer is, in the original circuit, no it cannot. The gain is set at the maximum of 200, but the 10uF capacitor between pins 1 and 8.
I just modified the circuit by de-soldering one leg of the 10uF capacitor between 1 and 8, and placing it on a SPST switch. This lets you toggle between gain 20 (clean) and gain 200 (filthy). A more elegant solution would be to replace the power switch with a center-off DPDT switch. One side would run power to the circuit in either switch position. The other would have the 10uF capacitor in one setting, and nothing in the other. I didn't have a center off DPDT, so instead put in a second switch. Or, as I mentioned in the text, the Make Magazine amp has a gain pot--you could replace the capacitor between 1 and 8 with a 5k potentiometer.
To clean up the controls, you can also install a switching 10k potentiometer, which would serve both as the volume control and the on/off switch. They are normally available at radio shack.
Have fun--I'm amazed at how many views this thing has gotten already! The circuit is amusing, and cheap to make, and very hard to destroy the chip. They can be built into any weird container you want.