C-bow (cymbal electronic bow) is a feedback device that works exactly like a guitar e-bow but it is capable of exciting heavier resonant surfaces like metal percussion (cymbals, gongs, etc). It is made of a contact mic used as pick up, a transducer that re-projects the sound onto the same surface where the contact mic takes it, and a simple analog amplifier with a lowpass filter. The sound comes entirely from the object c-bow makes resonate.
As a contact mic I used a normal 35mm piezo. The transducer is a Dayton Audio DAEX19CT-4 (4 ohm / 5W). The first transducers I tried weren't even remotely powerful enough and Matthew Goodheart's platform reembodiedsound.org spared me a lot of trial and error. The circuit has a single op-amp (TL071) used as a unity gain buffer, a lowpass filter, and the LM386 used to drive the transducer. A tricky part was the isolation of the case: simply attaching a transducer and a contact mic to the case causes the case to feedback onto itself, and the resonances of a little plastic box are far less pleasant than the delicate complexities of metal percussion. To isolate the contact mic, I tried wood, softer plastic, silicone layers, but the first easy solution was supermarket sponge. I cut a layer as high as the height of the transducer so that c-bow stays planted on the surface with two legs (see pic of the bottom part).
Features of c-bow: