What I'm about to say could be wrong. I was the default sound guy for my later bands because I owned the PA. Knew next to nothing about the controls. Over the years people gave me advice and I read some how-tos. Some of that helped. Still knew far less than I should have.

Look at your monitors first. Be sure the SM57 is pointed away from them as much as possible. The mic should be angled up towards the banjo. Not parallel to the floor or pointed down. When I played through an SM57 (where the house was providing PA) I got best sound with the mic pointed at the lower right quadrant of the head: 3 to 6 o'clock.

Next, learn to "ring out" the room before your show. This is the process of intentionally inducing feedback in order to find and cut the offending frequencies. I can't explain it, but you can look it up. This is why having separate EQ for the monitor mix can help. Not too polite to do this if there are already customers there, so you may have to skip that part.

Best you can do instead is to try different EQ sliders when feedback happens, so you're going after individual frequency ranges and not just lowering the volume of the banjo or the band, which doesn't solve the problem. Feedback is rarely caused by volume. It's almost always a specific frequency. Remember what I said about my possibly being wrong.

Also learn (if you don't know already) how to set individual channel input gain (also called "trim") and output volume/level/fader before the show using "pre fader level" and the meters with speakers totally off. Just learning how to set channel gain (near the end of my years of running sound) improved my mixes immeasurably. Also: once gain is set, leave it alone. Point here is you can use the meters alone to set the input and output levels for all channels with speakers off  then bring up the master volume with speakers on.

If feedback seems resistant to control, you may have to consider a pickup and DI. To help with that decision, check out the ToneDexter "wave map" shootout episode of the Picky Fingers Banjo Podcast. You'll hear three different pickups with no EQ or effects added and how the ToneDexter learns and models the sound of those pickups and 5 different microphones to make the banjo played through any one of those picks sound close to the sound of the banjo through any one of those microphones. Of course ToneDexter works with any pickup and mic, but there's no way for Keith to test and record every possible combination.

Good luck.

I’ll echo Ken. Mic and monitor angle make a huge difference. One nugget of wisdom I got years ago, was imagine the sound coming out of the monitor, as a stream of ping pong balls. If they can bounce off your banjo, into your mic, you WILL get feedback. Years ago I played in a loud bar band. I always had to stand off to the side of the monitor, with the mic angled up and away from the sound coming out of the monitor. I found an old photo of this. I’m outboard of the monitor, SM57 angled up and away from it, and the monitor is pointed inward towards the bass player.

I've had the same sound problems trying to use a mic at a small VFW. I never could figure out a reliable solution. I was either not being heard or causing feedback if I turned up and it seemed to get worse the more crowded it got. I like the "ring out" idea, haven't tried that.

I ended up going with a pickup and ToneDexter setup. That solved the problem but at a small cost in tone and it took a while to get it dialed in right. It has quite a bit of EQ and feedback controls to mess with.

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Originally posted by Dean T

I’ll echo Ken. Mic and monitor angle make a huge difference. One nugget of wisdom I got years ago, was imagine the sound coming out of the monitor, as a stream of ping pong balls. If they can bounce off your banjo, into your mic, you WILL get feedback. Years ago I played in a loud bar band. I always had to stand off to the side of the monitor, with the mic angled up and away from the sound coming out of the monitor. I found an old photo of this. I’m outboard of the monitor, SM57 angled up and away from it, and the monitor is pointed inward towards the bass player.

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Yes, very important, Dean. 

Ping pong balls, great example! I used the analogy of light and mirror to clarify the same point.

I made these sketches for my  'Sound In A Nut Shell For Musicians' workshops. 

Just occurred to me: In-ear monitors.

Never used them. I haven't run PA since 2018. But I get occasional ads for these or get fed stories in social media about people using them. They seem to be especially popular in worship bands, no doubt since the absence of monitor speakers reduces the volume imposed on congregants.

If your main speakers are behind the mikes, that will add to the feedback.

Hi all,
l've signed up tonight after browsing the hangout for years just so l could reply to your post klcarte.
I agree with all previous replies but just wanted to add my solution to the same problem.
I play at a monthly dance in an old country hall. We have a caller and usually 100-120 dancers turn up. The band is me on banjo, our band leader on piano accordion, then tin whistle player, wooden flute player, keyboard player and drummer.
Stage volume is loud.
I mic up with a 57 like you and my secret to no feedback happiness is a small personal monitor(10"and horn) mounted on a stand the same height as me, so its only a few feet from my ear.
I don't need as much volume to hear because its so close and its way above the mic so no sound bouncing off the head into mic either. I'm also on stage left so its visually unobtrusive. Like a side fill. It's also helped me that the drummer has moved to stage right!
Try it, l hope it helps.
I will try and upload a pic.
Paul.