I heard about the dan bau on sites such as Thanh Cam musical instruments (plus, see this fantastic dan bau virtuoso video!) and decided it must be easy to make. My version has none of the grace or refinement of the original - but you can make bendy harmonic music with it, and that's all that I ever wanted.
The dan bau is basically a single string held taut, anchored to a stick of bamboo at one end which you can bend in order to vary the tension in the string. You play a whole range of notes on the dan bau, by: (1) bending the bamboo to change the tension in the string; (2) playing the harmonics of the string rather than just the fundamental.
Total cost: about £8.
The basic plan is shown in this cross-section:
The first thing you'll need to do is get your piece of wood (sawing down to size if necessary - the string should be about a metre long, so perhaps 130cm or 140cm is good) and drill two holes through it:
Put the machine head and the bamboo in their respective holes. Choose a good point on your bamboo to drill a little hole (~2mm) - it should be about 2 or 3 centimetres up when the bamboo is in place. Mark the point, take the bamboo out of the plank and drill this hole. The hole will go through both 'sides' of the bamboo - it's wise to widen the hole at one side to prevent damping of the string (see diagram below).
You will now be able to assemble the dan bau so far - put the bamboo back in place, and then thread the string and tighten it somewhat. The magnetic pickup is going to sit just by the bamboo, directly underneath the string. Mark exactly where the pickup should sit.
This is where soldering comes in. If you've bought a simple electric guitar pickup then it has two simple bits of wire poking out of it, and you need to connect each of these to one of the two connections on the jack socket. It doesn't matter which way round. You probably want only about 10cm-20cm of wire between the pickup and the socket, depending on the socket placement.
Soldering is pretty easy, especially like this when you're just attaching some bits of metal to some bits of wire. (It gets more difficult when you're trying to do stuff with microchips etc, because you have to be very careful not to make accidental cross-connections, and not to fry the circuits!)
Once you've got these connections made, you need to attach the pickup in place on the dan bau. I achieved this by drilling shallow holes in the wood to make a small pit for the pickup to sit in, and then sitting it in there and screwing it in place.
You'll also need to anchor the socket somewhere good on the wood. I didn't have a good way of doing this, really - I cut a chink out of the wood and pinned the socket down with a couple of screws. The only thing to be careful of is to make sure that the socket (and more importantly, the guitar lead which you're going to plug into it) isn't going to get in the way of your playing. Best to place it somewhere imbetween the bamboo and the near end of the wood.
You can now use an ordinary guitar lead to plug the dan bau into an amplifier (preferably a high-impedance input). Play around with it! You'll need to get used to finding the string's harmonics - I made a lot of pencil-marks on the wood to guide me.
N.B. I'm pretty sure that my way of playing it (plucking) isn't the traditional way. I'm told that proper dan dau players use a short wooden thing - perhaps as a plectrum or a mallet, I'm not sure - but I haven't been able to get good results out that way.
As of the time of writing, I can't play it very well. I'll get better though. If you're desperate to get a rough idea of what the thing might sound like then here's a quick improvisation of beatbox and dan bau (MP3, 935kb).
Here's a very nice dan-bau site with instructions for playing, as well as dan bau (and other instruments) for sale.