Acoustic Guitar Preamps: What Each Control Does

What a preamp does

An acoustic-electric guitar starts with a pickup. Piezo systems sit under the saddle, soundhole pickups mount at the rosette, and some preamps add a small microphone inside the body. The signal these pickups produce is tiny, and it has a strong character of its own. The onboard preamp boosts that signal and applies tonal shaping before it reaches your amp, mixer or recorder. It is the difference between a thin direct sound and a usable stage tone. To understand pickup options, see acoustic guitar pickup types and the piezo pickup guide.

A preamp can be active or passive. Active systems, which need a battery, add gain and often include a full EQ and other features. Passive systems have no battery and are simpler but quieter. When a preamp is built into the guitar, as on most acoustic-electrics, the controls are the interface you will use every time you plug in. Larger-bodied guitars like dreadnoughts move more air, which changes how much signal the pickup produces and how you will want to shape it. Body size influences the tone you start with, so it helps to know what your guitar is doing before you touch the controls. The best acoustic-electric guitars guide is a good starting point for comparing preamp layouts.

Volume control

The volume control on a preamp sets the overall output level of the pickup. Martin's E1 system describes it that way: the volume knob controls overall output. The Fishman Sonitone preamp is described with similar guidance: set the volume as high as possible without causing your amp or mixer to distort. That is the practical approach. You want a strong, clean signal going into the board or amp, and the final level for the house mix should be adjusted at the mixing desk, not by cranking the guitar.

On stage, use the guitar volume as a constant reference. Do the big level changes on the mixer. If you rely on the guitar volume to cut the mix during a song, you may find yourself chasing tone. If the preamp has a battery check feature, check it before you play. Some systems, like the LR Baggs Anthem, include a battery check button. When the battery weakens, the signal can become dull or distorted, so learning how to replace the battery is worth the few minutes it takes.

Tone and EQ controls

Tone controls shape the frequency balance of the pickup signal. A single tone knob works in a few common ways. On the Fishman Sonitone, the tone control reduces treble. On the Martin E1, the tone control is described as controlling bass and treble response, a tilt-style EQ.

Larger preamp systems add separate bands. The Fishman MX is described as having a tone control that sweeps from a flat condition with no EQ to a scooped sound with emphasized bass and treble and reduced midrange. A scooped tone can make a solo guitar shimmer, but it can also fight a full band mix. The useful habit is to start flat, then make small cuts to fix problems: cut a little bass if the low end is muddy, cut a little midrange if the guitar sounds honky. Boosting aggressively is usually less natural than cutting.

Notch and feedback filters

A notch filter is a narrow dip in the frequency response of the preamp. It targets one specific frequency band and cuts it deeply, leaving everything around it alone. On an acoustic guitar it is the tool for feedback. When the sound from the speakers excites the guitar body at its resonant frequency, the preamp can ring and howl. A notch filter lets you find that frequency and cut just it. Not all preamps include a notch filter; when they do, the frequency is usually adjustable.

If feedback continues with the notch engaged, the filter is addressing the wrong frequency, or the stage sound is simply too loud for the pickup. In that case you usually need to adjust the stage setup: position yourself away from the monitors, reduce monitor volume, or change how the guitar is angled to the speakers. The acoustic-electric feedback guide covers these steps in more detail.

Phase control

The phase switch reverses the polarity of the signal by 180 degrees. Martin's E1 guide says phase inversion can decrease feedback and increase bass response. The Fishman MX-T phase switch is described as improving bass response at low volume and suppressing feedback at high volume, two useful outcomes. In practice, flipping the phase switch is a fast test. If a guitar sounds thin or hollow when both pickup and microphone are active, the phase switch often corrects the cancellation.

Phase is especially important on systems with an internal microphone, where the distance between the mic and the pickup creates timing differences. Flipping phase is free, instant, and can fix a lot of tone problems before you touch EQ.

Blend and mix controls

Some preamps combine two sources, typically an undersaddle piezo and an internal microphone. The blend control sets how much of each source reaches the output. The Fishman Aura VT Blend is described as transitioning from the undersaddle pickup through to a studio-microphone modeled sound. The LR Baggs Anthem has a Mix control for the mic and pickup balance, plus a separate Mic Level control to set the internal microphone level relative to the room.

The idea is to start with the pickup for a solid, reliable tone and bring in the mic slowly until the sound opens up. A microphone adds air and realism, but it also adds feedback risk. In a loud room, pull the mic back. On a recording, you might run more mic than pickup.

Built-in tuners

Many onboard preamps include a chromatic tuner. On the Martin E1 and Fishman MX-T, engaging the tuner mutes the output, so tuning is silent. The E1 tuner works without an instrument cable. This is a stage-friendly feature: you can tune between songs without any noise reaching the house mix. If you prefer a separate clip-on tuner, the built-in tuner vs clip-on tuner comparison explains the trade-offs.

A note on hearing

When you use a preamp to push your guitar through a PA at stage volume, the risk to your hearing goes up. The National Institute on Deafness and Other Communication Disorders notes that sounds at or below 70 dBA are unlikely to cause hearing loss, while long or repeated exposure to sounds at or above 85 dBA can cause hearing loss. Protect your ears with proper stage monitoring and hearing protection, and keep stage volume at a safe level.

What to pick for your use

Questions

What is the purpose of the volume control on an acoustic guitar preamp?

The volume control sets the overall output level of the pickup. Set it as high as possible without causing distortion at your amp or mixer, then adjust final level at the desk.

How does the phase switch help?

It reverses polarity by 180 degrees, which can reduce feedback and improve bass response, especially when a microphone is blended with a pickup.

What does a notch filter do?

A notch filter cuts a narrow frequency band to target feedback. If feedback persists, the filter may be set to the wrong frequency or the stage level is too high.

Why does my preamp have a blend control?

It balances the output between the pickup and an internal microphone. Start with more pickup for reliability, then add microphone to open up the tone.

How do I set the tone controls?

Start flat, then make small cuts to fix problems like muddy bass or honky mids. Avoid large boosts unless the guitar sounds overly dull.