By Tom Mulhern
Until several years ago, the concept of an amp dedicated to acoustic guitars was considered pretty twisted. After all, acoustic purists practically regarded amps as instruments of the devil, and electric players often dubbed the acoustic their "practice instrument." More than a decade ago, some bold pioneers, including Peavey and Shubb, made the first bold steps into acoustic amplification, only to be greeted by a void instead of a marketplace.
Today, the story is very different, with acoustic guitars selling at high speeds, many of them with built-in pickups begging to be amplified. Of course, anyone who’s plugged an acoustic into their electric guitar’s rig knows that there’s more than just inserting a plug to get a tone worth cranking up. The fact is, electric and acoustic amps–when properly designed–are as different as trumpets and tubas. They may look somewhat alike and function similarly, but there’s a lot more going on.
Take the preamp section of an acoustic amp, for example: An acoustic guitar’s pickup has an extremely high output impedance, which means that the amp’s input has to be designed to work with it. Electric guitar amps are designed for a lower-impedance signal from a typical single-coil or humbucking pickup. The result of plugging an acoustic into most electric guitar amps is a weird, thin, often brittle tone. An acoustic’s signal doesn’t usually fare much better once inside an electric guitar amp. The preamp in your usual electric guitar amp is engineered to give you lots of gain, since the idea is to get crunch and overdrive happening. It also runs contrary to what you want to do to the average acoustic signal. Combine the extreme gain with the warming effect of the preamp’s coupling to the power amp section, and you’ve got one gritty mess of an acoustic sound. The final tweak to an acoustic signal pumped through an electric amp is the speaker. Speakers are usually selected by amp designers for a combination of reasons, including the ability to color the distortion produced by the amp, and to break up on their own when driven to extremes. We’re not talking high fidelity here; we’re in the realm of amp voodoo designed to make Strats, Les Pauls, and V’s sound like they have an attitude.
So much for why an electric guitar amp is usually unsuitable for acoustic guitar–unless you use an outboard EQ, settle for low gain, and don’t mind changing the speakers to full-range, high-fidelity models.
If you plan to plug and play your acoustic before an audience, a real, live acoustic guitar amp is going to make you sound your best. The two routes you can travel are (1) a full integrated preamp/amp/speaker combo or (2) a preamp for plugging into a power amp and speakers, or into a P.A. Regardless of which way you decide to go, always try out the gear with your own guitar. Assuming that there are only 50 different acoustic guitars and 50 different pickups (bridge, stick-on, soundhole magnetic), then you should realize that there are more than 2,000 possible pickup/instrument combinations, and you can bet that the amp makers had to choose which combinations they felt sounded best to them. Unfortunately, their choices may not sound great to you, or you might have a guitar and/or pickup that didn’t fit their scheme.
Here are features that you need in an acoustic amp (other features are nice, but these are essential):
Lots of power. You don’t necessarily want to blast out the audience, but you do want to have what’s known as headroom–volume to spare. That way, if you’re tooling along, playing your favorite fingerstyle passages at a good volume level, and you decide to whack the strings, your amp and speaker won’t distort. (If you’re planning to buy a preamp instead of an integrated combo, make sure that it has a wide dynamic range.)
Feedback control. One of the main bugs in the acoustic phenomenon is the constant battle with feedback. Acoustic guitars are designed to vibrate, and when a pickup is attached, the vibrations go through the amp and often are recycled through the guitar and pickup again, resulting in woofs and squeals. Make sure that the amp has a notch filter–a circuit that literally cuts a notch out in the feedback zone–and/or a phase switch. A phase switch, as its name implies, inverts the signal’s phase, so that if a sound reaches the great recycling guitar, it will be inverted (when a signal and its inverted copy merge, they cancel). A combination of a notch filter and a phase switch can kill practically any feedback.
Good EQ controls. Try the EQ, or tone, controls and listen to determine that they do for your guitar what you want them to do. If you want the thump taken out of the bottom end, can they do it? If you want to sweeten the pick noise, are they up to the task? A good complement of EQ controls can also help you to keep feedback under control and give your guitar cutting power–very important if you’re onstage competing with drums, bass, keyboards, and a singer.
Beyond those basics, there are other "perks" that you should consider, including portability, the company’s reputation, etc. Some acoustic amps have two channels–useful if your guitar has a pickup and a mic, or if you want to put vocals through it along with your guitar. An effects loop, reverb, line outputs, and other additional features usually contribute to a higher price, but they may be indispensable for interfacing the amp with recording or P.A. gear. No matter what kind of acoustic amp you try, though, if possible audition it and at least one other acoustic amp. You may have your heart set on one for reasons of price, word of mouth endorsement, etc., but it’s not inconceivable that something more suitable to your guitar and style is waiting for you to discover it.