How to reduce feedback

Feedback is the squealing tone you hear in a P.A. system when the volume is turned up too high. It happens when the loudspeaker’s sound at the mic is louder than the instrument’s sound at the mic.

So to prevent feedback, you want the mic to pick up a loud sound from your instrument. And you want the speaker to produce a quiet sound at the mic, but a loud sound toward the audience. Here’s how to do that.

When playing through an amp, place the amp slightly behind you and to the left so your body blocks the feedback. Or place the amp in front of you, close to the audience, so they hear a louder sound.
Feedback is worse in rooms than it is on stage. That's because the room walls reflect sound into the mics, causing feedback. You might use a pickup in rooms and use a mic on stage.
Try to play a little louder and turn down your amp to compensate.
If the feedback is a low tone, turn down the bass (low-frequency EQ) on your amp a little at a time until feedback stops.
If the feedback is a high squeal, turn down the treble (high-frequency EQ) on your amp a little at a time until feedback stops
An amp placed on the floor or near a wall can sound extra bassy and cause feedback. Put the amp on a chair and place it far from walls.
If you have a pickup, send its signal only to the monitor speakers, and send the mic signal only to the house speakers. Here's how: In your mixer's pickup channel, turn up the monitor send and turn down the fader. In your mixer's mic channel, turn down the monitor send and turn up the fader. The audience will hear the natural acoustic sound of your mic, and you will hear the pickup in your floor monitors.
Use in-ear monitors instead of floor monitor speakers, just with your own mic. This headphone amp can be used to monitor your own instrument: Behringer Powerplay P1 or Rolls PM50se.
Using a preamp with a sweepable notch filter, notch out frequencies that feed back. The Headway EDB-2 and Grace Design Felix preamps have a feedback notch filter. It works best if you don't move around much.
When using a Bose L1 type speaker, put it far from the stage and close to the audience. You might place the speaker at a front corner of the audience, aiming diagonally across. Use in-ear monitors.
Use a highpass filter (low-cut filter) on your mixer. That removes low frequencies without changing the tone of your instrument. Set the filter to 100 Hz for guitar, banjo and cello. Use 200 Hz for fiddle and mandolin.

Fiddle only: Do not place the mic directly over an f-hole because it resonates and tends to cause feedback.
Fiddle only: Try another mic position, such as under the foam block, under the tailpiece. It might not sound as natural but it will be a little louder.
Cello only: Use our Cello Mic-LB.
Banjo only: Try mounting the mic inside on the rod, and turn down the bass to compensate.