I had a Harman/Kardon HK395 2.1 computer speaker system sitting on my garage workbench for years. One day I looked at it and thought, "Man, I need more room on my workbench. What am I gonna do with that bulky, old speaker system?" It occurred to me that I could probably turn it into a nice, Bluetooth enabled speaker that the family could use again.
The first order of business was to disassemble the speakers and take a look around. The speakers seemed to be of decent quality: a couple of 2" full range drivers and a 5 1/4" subwoofer. The amp and power supply construction looked pretty straightforward as well. The only issue that presented a challenge was the multi-pin umbilical cord that went to the speaker on the left in the photo below. The wiring for power on/off, volume control, left and right speaker connection, line in, and power LED were all contained in that cord. Since I wanted to eliminate as many unnecessary connections as possible, I needed to figure out how to get rid of the umbilical.
I set about drawing a wiring diagram of the umbilical and the components it served. The original volume potentiometer was mounted in such a way as to be unusable in my new design. So I took some resistance measurements of the pot and purchased a new one.
I had some scrap 1/2" bamboo plywood lurking around my workbench for several years and I thought that the look of bamboo would compliment the new enclosure nicely. Also, bamboo ply machines cleanly to fairly tight tolerances so the material was a good fit for the project. However, I did not have enough material to build an enclosure identical in volume to the original Harman design. Knowing that a difference in enclosure volume would affect the low frequency response, I went over to The12Volt.com to calculate a new enclosure port length.
+5V power and signal in are the only connections needed to make the module work. Since the module draws very little power, I just tapped the +12V, filtered DC from the Harman power supply, added a 7805 5V voltage regulator with some 100uf decoupling caps and presto ... we have 5V to power the module.
The finished project turned out well; it sounds pretty good and the Bluetooth connection is reliable. Its low-end response is a bit weak but I believe that's an artifact of the smaller enclosure size. The case is finished in tung oil and wax and the enclosure fits nicely on a bookshelf. I love the fact that I can stream Pandora to this system and still carry my phone in my pocket! Oh, and the cool, chrome volume control knob can be found at PartsExpress too.