LR Baggs Acoustic Reference Amplifier - Manual (Page 16)

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9) poWeR amplifieR
As with all power amplifiers, the ambient temperature may have an effect on the performance of the power amplifier in the Acoustic
Reference Amplifier. It is always best to have the amplifier sitting away from other heat sources and, in an outdoor venue, out of the direct
sun. The rear aluminum panel of the amplifier is designed to act like a heat sink and it may feel warm. This draws heat away from the
power amp. In most playing conditions, the ambient temperature will not be high enough to interfere with the power amplifier’s normal
operation.
10) acoustic GuitaR feeDBacK
Feedback is caused by a particular resonance on a guitar being so strong that it is excited by the output from the speaker, which then
causes that frequency to be amplified louder by the pickup and thus exciting the guitar more. This is a loop that causes the volume to
increase at that frequency until it hits an equilibrium that is often very loud.
our first tool in feedback prevention is the phase button (sometimes called “invert”). An acoustic guitar will usually feed back more often
in one phase versus the other phase at any given location. If you move around, the phase that causes feedback may switch because
moving about 3-6 feet towards or away from the speaker will cause the phase to flip in the primary feedback range for an acoustic guitar.
so, the first thing to do when fighting feedback is to try both phases by flipping the phase button on that channel in and out or simply
moving towards or away from the amp. The best phase is usually obvious unless you are positioned between the phase extremes.
our second tool in fighting feedback is the notch filter. The frequencies that are most likely to feed back on an acoustic guitar are
determined by the size and the construction of the guitar. A larger bodied guitar will usually have a lower top resonance because of its
size and will usually feed back lower than smaller guitars that often have slightly higher-pitched fundamental resonance. The typical top
feedback frequency range is from 180 Hz–260 Hz.
The notch is designed to decrease the gain of the amplifier only at that specific frequency that is feeding back. This way, you can get more
volume without having that resonance feedback. The best way we have found to set a notch filter is to get the guitar to start feeding
back or “ringing” just slightly. It is never good to allow a prolonged or loud burst of feedback through a speaker as that can damage it.
However, just a light beginning of feedback for a short time is okay. Then, sweep the notch control from the counterclockwise position
slowly towards the clockwise position. As the very narrow notch passes the frequency that is feeding back, the notch will cause the
feedback to drop out. now you can get more volume-before-feedback out of the amplifier. This specially designed garrett null notch that
we use on our products is almost transparent to the tone of the guitar (but of course improves it by getting rid of the feedback). The full
counterclockwise position will “click” to engage a hard-wire bypass to disable the notch.
If you are using these two controls and still need more volume before feedback then you can begin by reducing the low frequencies,
which will usually help reduce some of the feedback. However, this will also affect your tone. The key to fighting feedback is to remember
that you are trying to prevent feedback at lower levels but, as volume increases, you can always get to a situation where feedback can
occur. At high volumes, standing farther away from the amp can reduce the sound pressure level on the guitar top.
11) micRopHone feeDBacK
The Phase and notch controls are designed only for suppressing the primary feedback frequencies encountered in typical acoustic
guitars. They are not in any way intended to modify, suppress or eliminate feedback of a vocal microphone. Dealing with this sort of
microphone feedback is well understood in the craft and is beyond the scope of this manual. However the mic pre has substantial gain
so be very careful when using a vocal microphone.
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