“The bass player in my band insists on using his old upright bass on some of the songs. We have a Barcus Berry pickup on it and have a graphic EQ inserted in that channel of the mixer, but we still have lots of problems with low end feedback. We can usually pull enough out of the EQ to make it work, but the bass sounds terrible at that point. Do you have any suggestions?”
This is difficult. Some players use a towel down near the bridge to dampen the strings. This greatly reduces feedback problems. A graphic EQ, no matter how good it is, is not the ideal tool for dialing into specific problem frequencies like this. Invariably by the time you make the cuts you need to reduce the feedback you have also removed a significant portion of the musical signal that you need to carry the low end for the band. Try a high quality parametric EQ and experiment with very steep notches at problem frequencies. You’ll be able to take more out and leave the tone of the bass relatively in tact. If you don’t think you can handle the parametric try using a feedback eliminator, as they do similar things automatically.
You can also experiment with the polarity of the signal. Flipping the polarity of that bass signal somewhere (just reverse pins 2 and 3 on an XLR cable) can at least move the feedback to points that are easier to get to with EQ. Your stage monitors can also contribute to triggering feedback, even if the bass isn’t coming through them. Polarity reversal of the bass and/or all the monitors can sometimes make a big difference.