I went looking for an affordable tiny full range driver with decent bass and found the Dayton Audio ND-65 at Parts Express. I love this driver, with it's aluminum cone it can better withstand getting stuffed into a pack. I already had a TA2024 2x15w amplifier but had to design a charging circuit to round out the required electronics. I went through 2 iterations on the box design, the first one was to test how low the speakers would get in a small enclosure which seems to drop off right around 105hz. But I found I could really shrink the enclosure and tune it to about 75Hz without any major impact on the 105Hz I was getting before. The final enclosure ended up being 12.25"L x 4.25"W x 3.25"H (outside dimensions) of 1/2" birch plywood. The length and width were decided by the size of the solar panel I wanted to use, the height was dictated by the height of the driver.
The exterior was painted with a rubberized undercoating for cars to help withstand some abuse and the elements.
Power is supplied by 8 AA NiMh 2200mAh batteries which are in turn charged by a 12v 1.5A solar panel. The batteries are fully charged at 1.45V per cell or 11.6V total. Open voltage on the solar panel indicated 22V in full sunlight, so I needed to regulate it down to my maximum voltage of 11.6V. This was accomplished by using an LM317 voltage regulator with two resistors and included a diode. The value of the resistors will vary somewhat as you'll need to account for a 0.7V drop across the diode. I believe my actual resistors wound up being 240 and 2100.
The USB charging circuit is another simple circuit using only an LM7805 to output 5V to a pair of UBS ports. This works really well but tends to get warm if you have a really power hungry device, since it's only really good for about 1 amp. I'll be installing a switch on the LM7805 since it's always on and draining the batteries with a tiny current draw.
The audio input is via an 1/8" audio jack from the headphone jack on the phone. This may eventually get replaced with a bluetooth input device since audio fidelity isn't of paramount importance. But as it is I can have my phone plugged in and charging and listen to some tunes at the same time.
My overall listening impressions are that it is way better than one of those small bluetooth portable speakers. It still doesn't get as deep as I'd like but isn't bad for a 2-1/2" driver. Mids are really good and the highs are bit bright but it's smoothing out with use. It does get rather loud, volume is currently completely controlled by your device, though I may add a volume knob because the lowest volume on my phone is still too loud for my cube neighbors at work.
With a full charge I had this playing at near max volume for about 8 hours, the phone was not being charged, and the solar panel wasn't connected.