Adding a homemade microphone pickup to your acoustic banjo is fairly simple. The cost can range for twenty bucks to several hundred depending on the quality of the pickups you install. The job is so simple, there's very little reason to go to the expense of hiring a luthier or instrument repair guy. All you need is to have a few tools and to know where to place the pickups. There are three basic pickup designs to choose from electromagnetic coils, miniature microphones and piezo transducer pickups.
Remove the resonator from the banjo. The resonator is the wooden back found on some banjos. Unscrew the fasteners with your fingers. If you have an open-backed banjo, skip this step.
Select the spot for your audio phone jack. Either drill a hole in the resonator to mount the jack or buy a standard self-contained phone jack that can be attached to the body of the banjo.
Drill the phone jack hole. Sand the edges of the hole, insert the jack assembly from the back and screw down the keeper ring. If using a self-contained jack, screw, glue or Velcro the jack assembly to the place you've chosen. Pre-drill any screw holes to prevent cracking or splintering.
Solder the wire leads to the jack assembly if leads are not already attached.
Run the wires to the point where you will attach the pickup.
Mount the electromagnetic coil pickup on the coordinator rod or rods inside the banjo. The pickup is a magnet with a coil of wire wrapped around it in a case.
Place a small thin steel shim underneath the middle foot of the bridge on the head of the banjo. The pickup must be mounted on the rods directly below the shim using the mounting brackets that come with the pickup.
Solder the lead wires from the phone jack to the electrodes on the pickup.
Reattach the resonator; plug the cord into the phone jack and the other end into the amplifier.
Test the sound. The coil in the pickup senses the vibrating steel shim under the bridge. As you play, the pickup converts the vibrations to sound.
Mount the miniature microphone temporarily on the side of the pot so the microphone extends over the head or along the sides or mount it inside the pot so the mike extends under the head.
Wire the microphone to the jack and plug it into the amplifier.
Test the set-up for tone quality, feedback, brightness, warmth and depth.
Move the pickup to different locations and retest till you get the sound you want. Then screw the microphone in place.
Replace the resonator and test.
Test your piezo transducer pickup by taping the transducers directly to the banjo head on top or underneath. Transducers are small flat pickups about the size and shape of a nickel.
Wire the transducer to the standard phone jack leads, but leave them loose for now since you'll be moving the pickup around. It may take a while to find the sweet spot on your banjo. Start below and behind the bridge and move around till you find the right spot that gives you the sound you want.
Tape or glue the transducer in place and attach the leads permanently
Tape down the lead wires to prevent them from vibrating.
Reattach the resonator if you have one, plug in to your amplifier and play
Tape down the wire leads so they don't vibrate against hard surfaces when playing.
Thoroughly test the transducer's placement, as the pickup can dampen the banjo's sound if it's in the wrong place. Avoid mounting the phone jack by drilling through the pot or circular wooden ring that forms the body of the banjo. This can weaken the body of the banjo, which is under a lot of pressure from the head, tone ring and neck. People talking nearby can cause the head to vibrate, and a too-sensitive electronic pickup can send their voices through the PA system. Test for this when placing pickups and microphones on or near the banjo head.