Okay, i was too ambitious. Turns out i wanted to squeeze too much electronics in a too small package, which broke stuff.
What it comes down to is that the pcb is a breadboard. The solder kind. and it was connected to the lcd with some wires and a header. This was aparently too fragile, because the lcd broke.
I am not going to fix it because the speaker still works and the volume control as well. I will just us it as-is from now on.
So now i know for next time. calculate more space for pcb's and heatsinks and try to use actual pcb's to save space.
Also another thing that i noticed. People on ebay oversell their batteries. Who would have thought. Basically i bought a battery pack described as 4 Ah 12v. My guess however (i didn't measure) is that it is 1 Ah max.
The heart of the whole machine is of course the amplifier board. It uses the tda7677 amplifier. This is on itself a complete power amplifier, which makes matters easier. It has either four chanels, or two bridged ones. I used the two bridged ones. one for my subwoofer, and one for the other speakers.
Easy however, is not my middle name (that would require some very cruel parents). So i put some a preamp before the power amplifier. This has two purposes: to jack up the volume a bit, and to separate the volume-control from the filter stage.
The filter stage, i am a bit proud of, because it is the first filter i designed myself. It is simple however, it is just two linear rc-filters. one high-pass, and one low-pass. Both their cutoff point are at about 80Hz. I'm not sure if this is good, but it results in good sound so i'm fine with it.
Before the preamp, there is the volume control. It uses microchips mcp4231 digital potentiometer. They work like a charm, and also allow me to detach the terminals digitally. I used to have three of them, but then i fried one by putting 12 volts on it, so now i have two channels. The digital potentiometers have two channels, which go to the both channels of the preamp, which gives me control over the bass and trebble of both channels.
The micro-controller is a pic32 series, which is huge overkill for this project. That is mainly because i originally designed this board for another (abandoned) project, which would have had network and stuff. It drives the two digital pots and is controlled by two buttons and a rotary encoder.
The buttons are those chrome buttons of ebay, with a led in it. I just didn't like the chrome (chrome and wood is just wrong). So i sanded them down, and it turns out they are brass underneath, which is way more awesome. After some polishing they looked really shiny
The lcd was a blue one with white letters. This also didn't really fit in. So i made the easiest modification ever. I put a peace of red paper in between, which makes the letters red, and the background really dark blue. (It look a lot better than on this picture.)
The rotary encoder i'm still not really happy with. For now it is a five eurocent coin embedded in wood. It is easy to replace if i find (out) what i want for it.
The speakers are salvaged as well. The subwoofer comes, not surprisingly, from an old subwoofer. The other three (two midrange and a tweeter) come from a surroundsound system speaker. They have double the impedance, but i didn't have space to put two sets in.
The casing used to be a small ikea desktop organiser thingy which i didn't need anymore. It is quite cheap wood, so if i ever do a project like this, i will not be cheap and invest in nice wood (if i have proper tools by then). It is basically a multiplex faceplate, with holes for the speakers, buttons and screen, which i painted with a paint-mask make out of scotch tape.
After painting i glued the faceplate into the sides, into the already existing grooves. (Thanks ikea.)
After that it was just a matter of lots of linseed oil, which, on a sidenode, i found in a glass bottle with a cork in my parents place, and i think it is over fourty years old.