Red-Eye Preamp - Acoustic Guitar Feedback

Question…

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?

Response…

Guitar feedback…

As you 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.  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…

Feedback that occurs at mid-range frequencies or higher typically happens to instruments playing with a microphone.  It’s a result of the microphone directly picking up the sound from monitor speakers and the mains.  The wavelength at those frequencies is very short and if you moved just a few inches you would totally counter the effect of a phase-reversal switch.  For example, at 1,000 Hz the wavelength of sound is about a foot.  If you move 6 inches on stage, the acoustic phase will be completely reversed and feedback will be promoted, not cancelled.  A phase-reversal switch is useless for controlling feedback at these frequencies.

Low frequency guitar-body feedback is typically around 100 Hz.  At that frequency, the wavelength of sound is about 10 feet.  Again, if you move ½ wavelength, or 5 feet, you have reversed the phase and feedback is promoted.  If you move ¼ wavelength, or 2-1/2 feet, you will be at a point where a phase-reversal switch will make no difference.  So even at low frequencies, a phase-reversal switch has limited utility.