Accurate live amplification of the acoustic guitar may well be the audio equivalent of the search for the Holy Grail. Few if any serious guitarists seem completely satisfied with any of the available solutions. This being said, there are many very effective ways of producing a convincingly realistic amplified acoustic guitar sound. They vary greatly both in terms of the equipment required and in terms of expense. The best we can do here is provide a brief introduction to this complicated problem. Item 15.8 lists sources where you can find more complete information.
It's useful to distinguish three parts of the amplification problem: converting the guitar's sound into an electrical signal (Transducers); conditioning this signal so that it is appropriate for amplifiers (Preamps); and finally, amplifying the signal (Power Amps). We treat these aspects in separate articles below, and in addition we discuss Effects that can be used to alter or enhance the amplified sound. Some commercially available devices perform several of these functions at once, but it remains useful to distinguish them in a general discussion. We also discuss some accessories that vary from being necessary (cables!) to merely useful (tuners) for the "plugged-in" acoustic guitarist. Then we mention some of the options available for those who want to buy a guitar designed by the builder to be "plugged-in." Finally, we describe the choices of various well-known performing professionals.
All prices listed in this summary are approximate "street" prices based on mail order catalogs, or list prices from manufacturer's literature circa 1994. You may be able to get some of this equipment at substantially lower prices from retail and phone order dealers.
Despite these drawbacks, microphones are very commonly used, very often in conjunction with a contact pickup or soundhole pickup. They pick up certain characteristic sounds of the guitar better than any other transducer, particular the highest frequency sounds associated with high harmonics and finger and pick noise (which are important parts of accurate acoustic guitar sound reproduction), and percusive sounds produced by tapping or hitting the guitar body. In multiple transducer setups, they are often used at low levels to pick up these aspects of the guitar's sound, with the main part of the tone coming from another transducer. This greatly reduces feedback problems.
Two types of microphone are commonly used. Dynamic microphones consist of a coil of wire attached to a diaphragm. Sound hitting the diaphragm causes the coil to move in the field of a magnet, producing an electrical signal much in the manner of an electric generator. Condenser microphones consist of two conductive plates separated by an insulator (possibly air), forming a capacitor (or "condenser"). When a voltage is applied across the capacitor, any change in the distance between the plates, such as that resulting from sound waves, produces a varying current. Circuitry built into the microphone converts this current signal into a voltage signal.
The sensing element of a condenser mic can have a very small mass, enabling it to respond quickly and accurately to high frequency sounds. Thus condenser mics can have more accurate transient and high frequency response than dynamic mics, whose moving coil must by its very nature be more massive than the sensing element of a condenser mic. For this reason, condenser mics (or somewhat similar ribbon mics) are most frequently used in the studio. However, they have some drawbacks that lead many to prefer dynamic mics in live situations. They require a constant DC voltage to charge the sensing element; this voltage must be provided by a battery built into the mic or by an external power supply ("phantom power") that is sometimes (but not always!) built into mixers. It can be a nuisance to have to change batteries or worry about a battery failing or the availability of phantom power at the next gig. Also, the small and delicate sensing element in a condenser mic is more susceptible to harm from an accidental drop or from intense sound than is the durable diaphragm and coil of a dynamic mic. Dynamic mics thus dominate live vocal and external instrument miking, although many performers use condenser mics in live performance. However, the potentially smaller size of condenser microphones makes them the almost exclusive choice for internal miking of guitars; for this application, small diaphragmed mics are mounted in miniature cases, and are called mini-microphones, lapel microphones, or lavalier microphones.
Common microphones used for external live amplification of acoustic guitars are the Shure SM57 dynamic instrument mic (an industry standard instrument mic, $99), the Shure SM81 phantom powered condenser mic ($299), and the AKG 460 phantom powered modular condenser mic ($519).
Popular choices for internal guitar mics (all condensers) are the Shure SM 98 lapel mic, the Crown GLM200 mini-mic ($120; used with the Fishman Acoustic Blender), the AKG C 406 instrument mic ($180; used with Pendulum preamps), the Audio-Technica AT831b guitar mic ($120 with belt clip preamp and soundhole mounting clip), and the Countryman Isomax.
Since they do not detect air or top vibrations, soundhole pickups are favored by players who use techniques like right-hand tapping and harmonic slapping that can generate undesirable tapping and squeaking noises that are sometimes excessively amplified by other transducers. They are also easier to install (and remove!) than most other guitar-mounted transducers.
The most popular (and expensive!) model by far among professionals is the Sunrise ($170), which has been used by a rather impressive list of players (Leo Kottke, Phil Keaggy, Richard Thompson, Shawn Colvin, David Lindley, Michael Hedges, Alex de Grassi, Brooks Williams). In addition to the rather expensive pickup, the ``Sunrise sound'' requires that one use a high quality, high impedance preamp; depending on the model, this can add from $100 to $500 or more to the price.
In a review of 19 other, more affordable, soundhole pickups, Guitar Player magazine found that they fell into two categories: those that sounded good and those that didn't! The five pickups that fell in the "sounds good" category are the Bartolini 3A or 3AV ($99/$111), the Seymour Duncan SA-1 Acoustic Tube ($99, used by Preston Reed), the Seymour Duncan Woody XL ($129), the Dean Markley Pro-Mag ($85), and the DiMarzio Quickmount ($84).
Piezoelectric pickups ("piezos" for short) use one or more piezoelectric crystals to detect motion. These crystals produce a voltage upon being stressed (also, they will move when a voltage is applied to them, and are thus used in buzzers and alarms as small speaker-like transducers). They come in two varieties: top pickups and under-saddle pickups.
Top pickups consist of a small disc of piezo material attached to the inside or outside of the top (usually on the treble end of the bridge on the outside of the top, or at or near the treble end of the bridge plate inside). A few top pickups consist of two or three such discs that can be placed at different places to get a more representative sound. Besides offering very good reproduction of the strummed or picked guitar string sound, these pickups also faithfully reproduce the sound of percussive taps on the guitar body. Popular pro-quality top pickups include the FRAP (Flat Response Audio Pickup, used by Michael Hedges), models produced by Barcus Berry, and the SA-2 manufactured by Seymour Duncan (used by Preston Reed).
Under-saddle pickups consist of a rigid bar or flexible "wire" or "ribbon" of piezo material that fits under the saddle, directly detecting the motion of the strings driving the top via the saddle. Under-saddle pickups are probably the most popular transducers among performing professionals. They are almost as immune to feedback as soundhole pickups, but tend to offer a more recognizably acoustic sound. However, the sound one hears after a string is played is not simply the vibration of the string at the saddle, but the result of the entire guitar body and the air it contains responding to and processing that vibration. It takes a little bit of time for the guitar to respond to the driving force of a vibrating string; as a result, the attack of the signal from a saddle transducer is often noticably shorter and sharper than that heard acoustically, leading to an amplified sound that may be somewhat harsher than the guitar's natural sound. Also, the various resonance and filter effects of the top and body that give a guitar its characteristic warmth and "boxiness" are not always faithfully reproduced by under-saddle pickups. Still, the best under-saddle transducers, when carefully installed and amplified, probably produce the next-best sound to that of a good external mic.
Under-saddle transducers are by far the most difficult to install. They may require modification of the bridge (drilling a hole for the signal wire, and deepening or widening the saddle slot). Their sound can depend quite sensitively on details of the installation, such as how smooth and flat the bottom of the saddle slot is. These details can also affect the balance of the signal from string to string. Only a professional repairman or luthier should install and voice these pickups. A recent article in Acoustic Guitar magazine discusses in detail some of the issues and techniques involved ("Pickup Voicing," by Rick Turner, in the Jul/Aug 1994 issue).
Many companies manufacture under-saddle transducers, but three brands are dominant among professional performers: L. R. Baggs, Fishman (who sell under their own name and also under the Martin Thinline name and through Carvin), and Highlander.
rec.music.makers.guitar.acoustic newsgroup are quite
favorable. You can email L. R. Baggs for more information about their
products (which include preamps and pickup/mic systems) at
BaggsCo@aol.com
Prominent players use all three brands (Baggs: James Taylor, Phil Keaggy, David Wilcox, Cliff Eberhardt; Fishman: Harvey Reid, Leo Kottke, Tim O'Brien, Tom Prasada-Rao; Highlander: Martin Simpson, David Crosby, Graham Nash). The Shadow company also manufactures a wide variety of piezo pickups for classical and steel-string acoustics, including some that combine the piezo signal with a microphone or soundhole pickup. They appear to be popular among amateurs, but I do not know of any professionals who use Shadow products.
Piezo pickups of either variety require more care in terms of signal handling than other transducers. A preamp or buffer designed especially for very hi-Z transducer signals is a necessity, unless the pickup includes a preamp (as do the Fishman Acoustic Matrix and the Highlander).
The most popular combination is an internal mic and an under-saddle piezo pickup. Mics are most prone to feedback at low frequencies (particulary near the body resonance, typically in the range of 100 to 200 Hz), and also tend to give an excessively boxy sound at bass and lower midrange frequencies. Under-saddle pickups, on the other hand, can give strong, warm low frequencies without feedback. Their weakness lies in the upper midrange and high frequencies, just where mics are most accurate. Thus combining their sounds can give a more faithful reproduction of the true acoustic sound than either transducer can give on its own. Several commercially available preamps are designed to work with a saddle/mic combination, giving the player control over the mix and EQ of each pickup with preamp stages optimized for each. Popular units include systems available from Pendulum Audio (used by James Taylor, David Wilcox, Brooks Williams, Jonatha Brooks and on occassion by Leo Kottke, Phil Keaggy, Michael Hedges, and Alex de Grassi), the Fishman Acoustic Blender and Pocket Blender (designed in collaboration with Harvey Reid and used by many singer-songwriters), and the L. R. Baggs Micro Duet (used on occassion by Phil Keaggy and Alex de Grassi). The newer Rane AP 13 rack mount preamp, designed to combine a microphone and pickup, is also gaining popularity (used by Tim O'Brien, Tom Prasada-Rao and Alison Krauss; the more capable and expensive Rane MAP33 is used by Leo Kottke and Tim O'Brien).
A similar, and similarly popular, combination is to combine the signal from a soundhole or contact transducer with that of an external mic. In this case, the external mic signal usually goes straight to the house PA mixer, with the soundman providing the monitor and house mix, EQ, and effects for the mic signal. Leo Kottke has used this approach.
Some players that use right-hand tapping and percussive techniques seem to favor a combination of a soundhole pickup and a piezo top pickup. Michael Hedges (Sunrise soundhole pickup and FRAP contact pickup) and Preston Reed (Seymour Duncan SA-1 soundhole pickup and Seymour Duncan SA-2 contact pickup) fall into this category.
Leo Kottke equips his performing instruments with both soundhole and under-saddle transducers. Which he uses, and whether he decides to combine its signal with that of an external mic, depends on the acoustic character of the venue he is performing in.
Finally, some players rely on three transducers. Phil Keaggy, for example, has often performed with a combination of a Sunrise soundhole pickup, an L. R. Baggs under-saddle pickup, and a Fender FM-1 internal condensor mic. David Wilcox has often performed with an L. R. Baggs under-saddle pickup, an Acoustech internal dynamic pickup, and an external AKG 460 microphone.
The most versatile multi-transducer configurations allow you to keep the signals separate. One advantage of this is that you can have a different mix in the monitors from that in the house. In particular, an internal or external mic signal can be low or even absent from the monitor mix, preventing feedback, while the house mix can have more mic signal, letting the audience hear a more faithful amplified sound.
In a professional setup, the pickup signal almost always passes through a preamp or direct box before heading to the PA mixer or an amplifier. These are crucial for getting a convincingly acoustic sound.
It is a common misconception that pickups need preamps because the amplitude or level of the signal they produce is too small for most amplifiers. In fact, many pickups produce a signal with a quite large amplitude. The problem with guitar pickups is that they have a large, frequency-dependent output impedance, not that they have a low output signal.
Preamps designed for acoustic guitar pickups provide some or all of the following functions: buffering (converting impedance), gain (amplifying the signal level), equalization ("EQ", adjusting the timbre and controlling feedback), mixing (combining the sounds of two or more transducers), and an effects loop (matching levels for external effects, and controlling the effect amount). They come in various sizes and degrees of complexity and expense, from small, inexpensive onboard or belt-clip units, to expensive, feature-laden rack-mount units.
Three signal types are involved: transducer output (usually hi-Z, up to several Mohm, two conductors), line level (10-50 Kohm, two conductors), and balanced lo-Z (150-300 ohm, three conductors in 3-pin XLR configuration or TRS 1/4" phono jack). A buffer converts a signal from one impedance to another.
A direct box converts signals to balanced lo-Z, so they can be sent long distances on a balanced cable (which is relatively immune to hum and RF pickup). It may also offer ground lifting to help alleviate hum from ground loops. Most direct boxes are designed to convert line-level signals, and are thus not suitable for direct connection to a high-Z pickup output (although they are fine for connection to the output of a preamp). Active direct boxes use battery- or line-powered circuitry to convert either hi-Z or line-level signals. Those designed to convert hi-Z signals (such as the Countryman Direct Box and the Demeter tube direct box) can be used with an otherwise unbuffered pickup output, but those designed for converting line level signals are appropriate only for the output of a preamp or buffer. Passive direct boxes are simply high quality transformers. They are designed to convert line level signals to balanced lo-Z, and are thus not appropriate for connection to an unbuffered pickup.
In performance, it is likely that the audience will hear the guitar through a high-fidelity PA system provided by the venue, so the player need not be concerned about power amplification. However, one may wish to carry a power amp for monitoring, or for performing in a small venue. The cheapest option is to use a keyboard amp in conjunction with whatever preamps and effects you may be using.
A more self-contained approach is to use an amplifier designed especially for amplifying an acoustic guitar. Several of these are now available. They combine some or all of the features of a good preamp with a power amp and wide-range speakers. They often also include extra equalization for controlling feedback, and reverb or other effects. Some include an additional mic input so that vocals can be combined with the guitar sound.
loredo@astrosun.tn.cornell.edu. As you might guess from
this email address, he is an astronomer. But sometimes he wishes he
were a guitarist of the caliber of Phil Keaggy or Martin Simpson!
If you really want to know more about him, check out his home page,
Way Back Home
(the title is from a Phil Keaggy album!).
All original material in this document is copyright (C) 1995 by Thomas J. Loredo; all rights reserved. Duplication for nonprofit private use is not only permitted, but encouraged! But if you want to publish this anywhere, please contact me about it.