15. What's the best way to amplify my acoustic guitar? ****************************************************** [Section 15 from the FAQ (Frequently Asked Questions) document for the USENET internet newsgroup, rec.music.makers.guitar.acoustic.] NOTE! This document is in the early stages of preparation. Many sections are little more than outlines or lists. Suggestions and comments are welcome; see the final entry for contacting the author. 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 15.7 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. Finally, we mention some of the options available for those who want to buy a guitar designed by the builder to be "plugged-in." Contents ======== o 15.1 Transducers o 15.1.1 Microphones o 15.1.2 Soundhole (Electromagnetic) Pickups o 15.1.3 Top Pickups o 15.1.4 Combining Transducers o 15.2 Preamps, Buffers, and Direct Boxes o 15.3 Power Amps o 15.4 Effects o 15.5 Accessories o 15.6 Acoustic-Electric Guitars o 15.7 More Information 15.1 Tranducers =============== There are three different classes of transducers commonly used to produce an electrical signal from an acoustic guitar. Microphones convert the actual sound produced by the instrument into an electrical signal. Soundhole (Electromagnetic) Pickups convert the motion of the strings above the soundhole into an electrical signal. Contact Pickups convert the vibration of some part of the guitar's top into an electrical signal. We treat each class separately here; there are various types within each class. Each class has its strengths and weaknesses, and most professionals find it necessary to use two classes of pickups to produce a satisfactory sound. Some even use all three classes. 15.1.1 Microphones ++++++++++++++++++ Microphones are potentially the most accurate transducers for amplifying any acoustic instrument. They directly convert the sound produced by the instrument into an electrical signal. Most studio recordings of acoustic guitars rely heavily, if not entirely, on the use of one or more carefully selected and placed microphones. Nevertheless, microphones have some serious drawbacks for live amplification. Of the three classes of transducers, they are by far the most prone to feedback. To reduce feedback, directional microphones must be placed close to the guitar. However, this can result in inaccurate amplification since the microphone will "hear" the sound from a small region of the instrument. Also, to maintain consistent volume and tone, the performer's motions must be constrained. To eliminate this problem, a small microphone can be mounted inside the guitar, but the resulting tone is often not typical of that heard by the ear outside of the guitar. Finally, high quality microphones are considerably more expensive than any other class of transducer. 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 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 more 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 is 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 may be provided by a battery built into the mic that could fail during performance, or by an external power supply ("phantom power") that is sometimes (but not always!) built into mixers. 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 smaller size of condenser microphones makes them the exclusive choice for internal miking of guitars; in this application, small diaphragmed mics are mounted in small 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. 15.1.2 Soundhole (Electromagnetic) Pickups ++++++++++++++++++++++++++++++++++++++++++ These are essentially electric guitar pickups. They detect the motion of the string directly over the pickup. Since they ignore the motion of the top and the air, they are very immune to feedback. However, neither the guitar's top nor one's ear detects the string motion above the soundhole, so these pickups have the potential of reproducing an unrepresentative sound. This was true of many early soundhole pickups; but more recently some remarkably good sounding soundhole pickups have become available. They still tend to have a somewhat fatter, slightly more electric sound than alternative transducers; for some tastes, this is a benefit. Since they do not detect air or top vibrations, they 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 the easiest to install (and remove!). The most popular (and expensive!) model by far among professionals is the Sunrise ($170), which has been used by a rather impressive list of players (Leo Kottke, Phil Keaggy, Richard Thompson, Brooks Williams). In a review of 19 more affordable soundhole pickups, Guitar Player magazine found that they divided into two categories: those that sounded good and those that didn't! The five pickups in the "sounds good" category are the Bartolini 3A ($), the Seymour Duncan SA-1 Acoustic Tube ($99, used by Preston Reed), [see refs for others]. 15.1.3 Top Pickups ++++++++++++++++++ These convert the motion of some part of the top of the guitar into an electrical signal. Virtually all of them rely on piezoelectric technology, the sole exception being the Acoustech Dynamic Field Pickup which attaches a metal disc to the bridge plate and detects its motion electromagnetically (David Wilcox uses the Acoustech). Piezoelectric pickups ("piezos" for short) come in two varieties: contact pickups and under-saddle pickups. Contact pickups consist of a small disc of piezo material attached to the inside or outside of the top (usually the edge of the bridge on the top, or the end of the bridge plate inside). A few contact 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 contact pickups include the FRAP (Flat Response Audio Pickup, used by Michael Hedges) and those manufactured by Seymour Duncan (used by Preson Reed). Under-saddle pickups consist of a rigid bar or flexible "wire" of piezo material that fits under the saddle, directly detecting the motion of the string driving the top. 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 body and the air it contains "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, giving the signal a somewhat harsher than 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 amplified, probably produce the next-best sound to a good external mic. Under-saddle transducers are by far the most difficult to install. They may require modification of the bridge (widening the saddle slot). Their sound can depend quiet 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. Three brands of under-saddle transducers dominate the market: L. R. Baggs, Fishman (who sell under their own name and also under the Martin Thinline name and through Carvin), and Highlander. The Baggs models are pickups bonded to a saddle (and most if not all models are wider than the typical guitar saddle, so they require routing for installation). Fishman transducers are rigid, thin transducers that fit under an existing saddle (but a new saddle will be necessary due to the added height). The Highlander is a felixible coaxial piezo transducer that fits under the saddle; it comes with a small preamp built into an endpin jack, with battery clips for mounting in the guitar body. Prominent players use all three brands (Baggs: James Taylor, Phil Keaggy, David Wilcox, Cliff Eberhardt; Fishman: Harvey Reid, Leo Kottke; Highlander: Martin Simpson). 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. 15.1.4 Combining Transducers ++++++++++++++++++++++++++++ You may have noted that the various types of transducers have different, complementary strengths and weaknesses. It will come as no surprise, then that many if not most professionals combine the signals of two or more transducers in their performances. The most popular combination is an internal mic and an under-saddle piezo pickup. Mics are most prone to feedback at low frequencies, and also tend to give an excessively boxy sound at those 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 Leo Kottke, James Taylor, David Wilcox, Brooks Williams, Jonatha Brooks and on occassion by 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). Some players that use right-hand tapping and percussive techniques seem to favor a combination of a soundhole pickup and a piezo contact pickup. Michael Hedges (Sunrise soundhole pickup and FRAP contact pickup) and Preston Reed (Seymour Duncan SA-1 soundhole pickup and Seymour Duncan contact pickup) fall into this category. Finally, some players rely on three transducers. Phil Keaggy, for example, has performed with a combination of a Sunrise soundhole pickup, an L. R. Baggs under-saddle pickup, and a Fender FM-1 internal condensor mic. 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, the internal 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. 15.2 Preamps, Buffers, and Direct Boxes ======================================= These are crucial for getting a convincingly acoustic sound! They offer one or more 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 level). 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 levels 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. Active direct boxes use battery-powered circuitry to convert either hi-Z or line level signals. Those designed to convert hi-Z signals (such as the Countryman 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 transformers. They are designed to convert line level signals to balanced lo-Z, and are thus not appropriate for connection to an unbuffered pickup. 15.3 Power Amps =============== An electric guitar amp is not suitable for amplifying an acoustic guitar. These amps are essentially "lo-fi" amps. Some are designed to distort in a manner that is pleasing in an electric setting, but not so in an acoustic setting. All have large speakers that are essentially just "woofers," incapable of reproducing the high harmonics that give the acoustic guitar its characteristic sheen and brilliance. In performance, it is likely that the audience will hear the guitar through a high-fidelity PA system, 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 vocal can be combined with the guitar sound. 15.4 Effects ============ Equalization (EQ) Delay-Based Effects (Reverb, Chorus, Delay) Dynamics Processors (Compressors, Limiters, Gates) Enhancers Distortion Multi-Effects 15.5 Accessories ================ Endpin Jacks These get the signal from internal transducers out of the guitar! They usually combine the functions of a 1/4" phono jack and a strap button endpin. If you have two transducers, you will need a stereo endpin jack, which provides three connections (signal 1, signal 2, ground) rather than the two connections (signal, ground) of a standard phono jack. Stereo jacks are also called TRS jacks, for "Tip-Ring-Sleeve," indicating the part of the mating male plug that connects to the two signal connections and ground connection, respectively. Note that some condenser mics require three connections by themselves (the third is for the power that charges the condenser). If you combine such a mic with another source, you will need to use a special endpin jack (the L. R. Baggs Micro Duet, combining a mic and a Baggs saddle transducer, uses a 5-pin jack), or install a second jack (that need not offer an endpin function) somewhere on the guitar. Cords and Cables These get the signal from the guitar to the preamp. You need a stereo cord for use with a stereo (TRS) jack. Specialized jacks require specialized cords; it's a good idea to buy an extra! Some preamps (like the Pendulum and the stereo version of the Baggs micro duet) require use of special cords. A/B Boxes These let you switch the signal from the PA signal path to the tuner, so you can tune without the audience hearing. Some preamps provide a separate tuner output and a mute function, so you do not need an A/B box to perform this function. Tuners These are affordable enough nowadays that they are almost not optional; there is now no reason to not be in tune. Even the best ears sometimes just can't figure out what that sour note is; but the tuner's "ear" never changes. There are several varieties: guitar tuners (which only work for the six notes of a guitar in standard tuning) and chromatic tuners (which will tune any note), each in manual (set the note you want to tune to with a switch) or automatic (figures out what note you want to play). If you use alternate tunings, you need the chromatic variety; for on-stage tuning, the automatic variety is a must. Many will let you tune to correct relative pitch relative to some non-concert standard, which is handy if you have to play with a non-tunable accompanying instrument (like a piano!) that may have drifted away from A440 concert tuning. 15.6 Acoustic-Electric Guitars ============================== 15.7 More Information ===================== See the separate annotated reference list. This FAQ section is maintained by Tom Loredo. Comments, corrections, and additions would be greatly appreciated; send them to him by email to loredo@astrosun.tn.cornell.edu.