This is really a re-visited project. Some time ago I built a little pre-amp to boost the signal from my passive mando pickup (it's in the archives somewhere - check if you wish). Misdirected ambition caused me to create a compact "belt-mounted" device. It was nothing but trouble. Never again will I take on the East in the race towards miniaturisation. I will stick to what the Brits do best: Big fuckoff industrial-looking boxes with hardly anything inside but good in the long run.
Above you can see the business end of the unit. The socket marked "In" goes in and the socket marked "Out" goes out, sort of. This means you can plug things in at any level and they come out at line-level.
I'm really pleased to have discovered the secret of switching the power via the input socket so you don't need a separate power switch. Install a stereo (three-pole) jack socket for the input and connect the neg of the battery to the ring (in the tip/ring/sleeve architecture). When you plug a mono jack into the stereo socket, the sleeve and ring are shorted, the neg ends up where it's supposed to be, and, hey presto, you've got power to the circuit (this is a trick used in most effects pedals). Don't forget to unplug your cable after use else you'll drain your battery.
At the other end of the unit we have the "Smugness" knob. Although one minor side-effect is to make things louder, the primary purpose of this control is to enable geeks of electronic persuasion to treat anyone within earshot to a two-hour explanation of the difference between "gain" and "volume".
For me, the benefit is that I don't have to carry my heavy back line amp to simple sessions. I can plug my mando or bass into anything. Need more signal? You've got it...