By Tom Mulhern
Back in the 1970s, I heard some really crazy stuff. No, it wasn’t coming from the mouths of psychedelic-drugged stoners or excited inventors. It came from musicians--guitarists, to be exact. They were in love with the sound that Larry Carlton and Lee Ritenour and Robben Ford were getting. Who wouldn’t? It was sustainy, but it had a smooth, silky texture that just hadn’t been heard in rock and pop before. The guitars? Well, a hint is that they didn’t call Carlton "Mr. 335" for nothing. And the craze for semi-solidbody guitars such as Gibson’s ES-335, ES-345, ES-347, and related instruments was on.
What a lot of pickers didn’t realize (or refused to believe) was that these great session aces were getting their tone in controlled environments and weren’t wrestling with the same kind of feedback problems that you get when you’re standing in front of a stack running at full overdrive. So, here’s where the craziness comes in: To fight feedback, they were stuffing all sorts of stuff in their guitars, including rags, foam rubber, and just about anything else that could be crammed through the little f-holes. Fiberglass insulation? Sure, try it! Silly Putty? Why not! Then the stuffing went to full-bodied arch-tops and roundhole acoustic guitars. Yikes!
Luckily, it eventually dawned on people that the very essence of what made these semi-solidbody and hollowbody instruments sound unique was the fact that they weren’t solidbodies. But feedback problems remained. Standing far away from the amp was one easy solution for many players. Using a graphic equalizer to de-emphasize the frequencies causing resonance in the guitar was another solution.
Today, thanks mostly to the amplified acoustic guitar craze, we have technologies that make it pretty easy to thwart feedback in even the most feedback-prone guitars. Here’s a look at them.
Start simple. For years, jazz guitarists have used a piece of felt to damp the strings at the nut. By lightly touching the strings on the playing side of the nut (as opposed to the side where the tuners are), it’s possible to keep open strings from ringing too much and causing the guitar to erupt into feedback. If you can find them, the Van Eps String Damper or the Kleen Axe string damper work well. Otherwise, just try attaching a strip of felt with a rubber band at the nut and making sure that the felt lightly comes into contact with the strings.
Some amps (and even some guitars’ onboard preamps) have a phase-reverse switch, which inverts the phase of the signal coming out of the guitar. Why do this? When a sound wave meets another just like it that it’s in phase with, they add together, making a stronger wave. If your guitar is producing an A note and the amp is producing an A note from its speaker, if they meet in phase, that A is gonna be louder. And if that combined A is picked up by the body of the guitar and fed back through the pickup and amp again, it’s going to combine with the note once more (and once more and once more), and that buildup will break out into feedback. Inverting the phase of the signal coming from the guitar causes identical waves hitting each other to cancel each other out–at least, that’s the theory. It isn’t perfect, and it can make your guitar sound different, because inverting the phase affects all frequencies, and not just a specific one.
Using a graphic equalizer can help, too, if you have one with enough frequency bands. If your graphic EQ has too few bands, then it’s hard to remove an offensive resonance without having a detrimental effect on other frequencies. But even if you have only, say, a 6- or 7-band EQ, give it a try. Set all of the sliders to the zero point (no boost or cut), and then slowly turn up the amp until it feeds back, and then back off the volume just a little. At that point, move one slider at a time to the boost position (a little bit at a time) until you find the right slider and the right amount to push the system into feedback. Now move that slider to the "cut" position, turn up the amp a bit and cut until you don’t get feedback. This takes patience, so work slowly. And remember that because of the imprecise nature of most graphic EQs, big cuts mean wide cuts, affecting a wider range of frequencies than you may want to mess with.
Probably the easiest signal processors to use are the Feedback Eliminators from Sabine. They really are "plug ’n’ play" units. Their internal computers sense aberrant peaks in the frequency spectrum and spring into action, filtering them out. This approach isn’t foolproof, but it’s very good, especially for instruments with an open microphone on or inside of them. If there’s any downside, it would be expense, but Sabine makes Feedback Eliminators in a few price ranges, so you should be able to find something to suit your budget.
Finally, there are parametric equalizers, which are sometimes tricky to use, but can do surgically precise feedback notching, which makes them worthwhile. Professional (rackmount, studio-grade) models can be expensive, but there are some at prices that guitarists can handle. One of my favorites is the Fishman Dual Parametric D.I., which (as its name implies) is two parametrics in one, plus a direct box–handy for recording or onstage use. The box is compact (about 4" x 6" x 2") really rugged. The way you use it is typical for parametrics: Set the boost/cut to zero, turn the frequency knob to its lowest setting, and set the bandwidth to somewhere close to its narrowest setting. Now turn up the amp’s volume to the brink of feedback and back off a little. Next, raise the parametric EQ’s boost setting and start turning the frequency knob until feedback begins to emerge. With a little feedback ringing, narrow the bandwidth and gently turn the frequency knob back and forth to zero in on the frequency where the feedback occurs with greatest intensity. Keep narrowing the bandwidth until it’s as narrow as you can get it. Now turn the boost/cut knob toward cut (gently!) until the feedback disappears. You may have to repeat this procedure until you get the feedback to go away, but after a few times, you’ll find it simple and effective.
I hope these tips help. Hollowbody and semi-solidbody guitars have so much tonal character; don’t mess them up by stuffing them full of junk.