Isn’t the acoustic/electric guitar an oxymoron?
“I have just recently purchased a pickup that installs under the bridge and uses a battery for power. My problem: I’m plugging the acoustic guitar into an amplifier but when I get to a certain level (not really all that loud) my strings begin to resonate. How does one control the feedback and still good gain at a live performance?”
I‘ve always questioned the wisdom of pointing speakers at microphones as is done in live stage monitoring setups. This situation is not too different from that. The problem stems from the fact that the bodies of most acoustic guitars are designed to resonate so they can acoustically amplify the strings like a speaker does. If you’ve ever used a set of headphones for a microphone in a pinch (come on, you know you have) you are aware that speakers in reverse make microphones. So the body of your guitar and its strings are picking up the energy your PA puts out and transmitting it right back to the PA like a microphone. Low and behold you have feedback.
If you can mute the strings and body you will eliminate the feedback, but of course that isn’t a very helpful solution. Basically all of the same rules for any type of feedback situation apply. First, get yourself oriented as far away from the sound of the speakers as you can. Distance is the best cure, but sometimes changing the direction you face or the direction one of the speakers is pointed can really help. Second, use a high quality equalizer to dip out frequencies that are most prone to feedback. You’ve probably noticed that some notes and chords are worse than others. Those are the frequencies that need to be cut in the EQ. Once you’ve notched out the five or six worst offenders you’ve probably done all you can do here.”
On the more esoteric side you can try things like reversing the polarity of the guitar relative to the speakers. While this is sure to have some effect it may or may not help much. The easiest way to do this is with a polarity reverse switch as found on many professional mixers. If you don’t have one it is sometimes easiest to just reverse the polarity of your speaker cables. Even though I joked about muting the strings above it really can help to add some material to the inside of the guitar so it doesn’t resonate as much. Some combination of all of these things should get you much better performance out of the system, but it is always going to be a compromise between volume, proximity (to the speakers), and tone.