Acoustic Guitar Feedback


Acoustic Guitar Feedback



I have a major problem with feedback on my dreadnought guitar which I had fitted with a piezo pick up.  Does the Red-Eye address this problem?




Guitar feedback…

As you may know, instruments with pickups make great microphones and are most sensitive to low frequencies.  They pick up sound from monitors and the house mains and when they pick up enough, you get low-frequency guitar body resonance and feedback.  The bigger the guitar, the more sound it picks up.   So with a dreadnought, you're a bit on the back foot already.


The type of pickup and the impedance match of the pickup to a preamp affect the frequency response of the whole system, and thus, affect low-frequency feedback.  Many preamps do not properly match the impedance of piezoelectric pickups and if the preamp's input impedance is too high, the low frequencies are boosted and the system is more prone to feedback.


We studied the physics of the piezo material used in most pickups and we terminate the pickup in a slightly different manner than other manufacturers do.  Many of our customers report that instruments with piezo pickups are less prone to feedback when used with a Red-Eye preamp.


Another factor is the type and location of the pickup.  As far as piezo pickups go, there are pickups like the K&K series that are mounted under the bridge plate inside the guitar body and pickups that fit in the groove under the saddle of the bridge.  While the internally mounted pickups, like the K&K series, are great for picking up the woody, acoustic, character of the instrument, they are more sensitive to feedback since they are "listening" to the guitar body as well as the strings.


The under-the-saddle pickups are closer to the strings and "hear" them more than the woody tones of the instrument’s body, so they are less sensitive to feedback.


Generally, if you have to amplify a lot, try a smaller-bodied guitar, an under the saddle pickup and a Red-Eye preamp.


Active pickups...

An “active pickup” system is a pickup plus a preamplifier built into the guitar.  We’ve measured several such devices and have yet to find one where the built-in preamp matches the pickup impedance correctly.  And you have to maintain a battery in your instrument.  Who hasn’t been to a performance when the instrument’s battery dies?  Nasty!  I just don’t think a fine instrument is the place to put a battery.  Please stick with a good passive pickup if you can!


Phase-reversal switches…


Actually, phase reversal is only useful if you are having trouble with very low-frequency guitar body resonance feedback.  At, for example, 100 Hz, where the wavelength is about 10 feet, the zones where the sound waves are either positive or negative phase are about a quarter wavelength or 2-1/2 feet in size and reverse every 5 feet of distance from a monitor.  So as long as you stay in the same 2 to 3 foot "box" a phase switch can have an effect.  The most practical way to help the low-frequency body resonance feedback is to cut the low bass to the monitors a bit with the sound system controls.

If feedback is in the mid-range and up, say 1,000 Hz, the wavelength is much shorter (about a foot) and the phase reverses every 6 inches distance from the monitor.  You'd have to stay within a 1/4-wavelength box (or about 3 inches) to not reverse the phase simply by your position.  


So a phase switch may be an impractical solution except for some low-frequency feedback situations.  Most of the time you can solve the problem by simply turning down the bass control.

Ground-Loop Hum

Ground-Loop Hum


What causes 60-cycle hum sometimes when I try to hook up an on-stage guitar amp?  Why doesn’t the Red-Eye normally need a Ground Lift Switch?  How do you solve the problem?




You only run into problems with AC ground loops and 60-cycle hum when AC grounds from two different AC power circuits are connected together.  Specifically, it the ground of AC power running the stage is connected to the ground of AC power running a venue’s sound system are connected together through any DI.


The Red-Eye is normally used to connect an isolated acoustic instrument to a venue’s mix board so there is no connection to any on-stage AC ground.  In the normal situation, stage ground is not connected to the Red-Eye, so there is no problem.


With the Red-Eye connected to the venue’s sound system ground via an XLR cable, connecting a pedal board to the Red-Eye's Effects Loop is not normally a problem because pedal (wall-wart) power is isolated and there is no connection to stage AC power ground.


An example that could cause 60 Hz hum would be connecting a Red-Eye to a venue's mix board ground via the XLR connection and also to an on-stage guitar amp ground via a ¼-inch cable.  The AC ground is different for each AC circuit and tying them together through a DI like the Red-Eye would induce 60 Hz hum in the grounded device connected with a single-ended ¼-inch cable, i.e. the guitar amp.


If you need to connect to an on-stage amp or some other AC-grounded device via the Red-Eye's Effects Out 1/4-inch connector and at the same time connect to a venue's remote mix board you will likely encounter 60-cycle hum.  Some guitar amps have a ground-lift switch which solves the problem.  If there is no ground lift on the guitar amp, you can use a ground-lift adapter to connect between the Red-Eye's XLR output and the cable to the mix board.


Here's a link to a popular XLR ground-lift adapter...

Others, as inexpensive as $10, are available through Amazon.


 If you do use an XLR ground lift adapter, it will interrupt Phantom Power from the mix board and the Red-Eye will need to run on its internal 9-volt battery.