I prefer using an ordinary mike sitting on a stand set at the level of the instrument while being played. Reason being is you have more control of the loudness in which you are playing the instrument. Want the audio softer and quieter, then just back away from the mike. Louder go closer to mike. Usually you don't need a pickup for a banjolele. Actually, it's loud enough and can easily be heard over a small band without an amp. all on its own.
I wondered this also, after some research it looks like a Schatten economy transducer piezo, which is only about $30. Ill be installing one sometime in the coming week and I’ll let you know how it sounds!
@chris m Actually really good!! And I’ve found not too susceptible to feedback which is usually a big concern. Did have to play with the placement good deal so start with scotch tape until you find the right spot, then use the glue!
