Amplification

Amplification
Despite many modern pickup technology amplification of an acoustic guitar remains very difficult. The best solution for a good amplified acoustic sound is to put one (or more) quality microphone(s) in front of the guitar. No other pickup can match up to this. Such a setup however requires a big investment and it will take a lot of time to setup for a concert. Alternatives can be found in mini microphones attached in the body or around the sound hole like piezo pickups and magnetic pickups.
These are the options for amplifying an acoustic guitar.

• External microphones in front of the guitar
• Mini microphones (internal / body mounted)
• Piezo pickup
• Piezo pickup in combination with mini microphone
• Sound hole Electro Magnetic pickups

The complexity of the sound of an acoustic guitar

The sound of a typical acoustic instrument is a complex and ever-changing blend of many components. To use the most common example, the natural sound of an acoustic guitar incorporates the effect of the soundboard vibrating as a whole, which includes many different sections of the soundboard vibrating quasi-independently, with varying phase, amplitude and frequency relationships. In addition, an essential component of guitar sound is the air resonance coming from the sound-hole. Added to this are the quiet but finely delineated sounds coming directly from the strings themselves. A well placed air microphone will pick up all of this. But a contact pickup will transcribe only the movement of a single point on the sound board, a rather one-dimensional representation of what is in fact a many-dimensional sound event. In spite of this, you can still manage to get a very good sound from a contact pickup.

Microphone
Although microphones are the best options for amplifying an acoustic guitar they have also some drawbacks. They are 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. Despite these drawbacks, microphones are very commonly used, very often in conjunction with a piezo 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 percussive 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.

Piezo

In general there are two kind of piezo pickups.

• contact pickup
• under saddle pickup

Contact pickups are in direct contact with some part of the guitar, and convert the motion at that location into an electrical signal. Virtually all such pickups rely on piezoelectric technology. Piezoelectric pickups (“piezos” for short) use one or more piezoelectric crystals to detect motion. These crystals produce a voltage upon being stressed.

Contact

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. They offer good reproduction of the strummed or picked guitar string sound. Disadvantages of contact pickups: They are less subject to feedback than air mics, but they aren’t feedback-proof, especially when attached to soundboards. They may produce an exaggerated and disconcerting response to any unintentional knocking or scraping on the body of the instrument. Most important disadvantage: contact pickups often produce an unattractive and unnatural sound, which requires a good deal of tweaking and signal processing to make it more acceptable. However, recent years have seen improvements, and it is possible nowadays to get a very good sound from them.

Under saddle

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. They are almost as immune to feedback as sound hole 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 noticeably 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.

Big Tone

In the Selmer/Maccaferri style guitars the so called “Big Tone” piezo pickup (photo) is very populair. It’s a passive piezo pickup (no battery required). It is from origin an under saddle pickup. Wasso Grunholz, an uncle of the Rosenberg family was the first to use the pickup differently. He build it in the excisting solid bridge not using a saddle. Advantages are the easy of use (plug and play) and reliability. The Big Tone piezo has a distinctive sound which makes this pickup popular amongst gypsy jazz players.
Many companies manufacture under-saddle transducers, but three brands are dominant among professional performers: L. R. Baggs, Fishman, and Highlander.

Combining a piezo pickup and a microphone

Various types of transducers have different, complementary strengths and weaknesses. Therefore, many if not most professionals combine the signals of two or more transducers to create their amplified sound. 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, the Fishman Acoustic Blender and Pocket Blender the L. R. Baggs Micro Duet. A similar, and similarly popular, combination is to combine the signal from a sound hole 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. Stochelo Rosenberg has used this approach.

Preamp: Needed or Not?

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. Piezo-type contact pickups produce a frequency-dependent output impedance that doesn’t travel well over long cable distances. With increasing cable lengths, there tend to be losses in both signal strength and fidelity. In addition, amplifier inputs generally are not optimized for such extra-high-impedance signals. For this reason, the use of a pre-amplifier, located as near as possible to the pickup itself, is often recommended. The pre-amp increases the signal strength and also converts the signal to low impedance, making it less subject to loss over long distances and more suitable for typical amplifier inputs. The pre-amp is not essential.

No Pre-amp
This is the way to go if you want an inexpensive, hassle-free, easy-to-install system. You’ll do fine without the pre-amp if you keep the cable lengths (the distance from the pick-up to your amplifier input) to a minimum – 2 to 3 meters should be OK. It also helps if you’ve got a strong signal to begin with: different instruments and different mounting configurations produce different signal strengths. In many cases, you’ll need to to make sure you’re going into a “mic-level” input on your amplifier – if you go into a “line-level” input, the signal may be too weak to provide much volume. While most amplifier inputs are not optimized for the ultra-high impedance of a piezo pickup, typical instrument inputs will still produce acceptable results. (These inputs are usually made to take a quarter-inch plug and may be labeled something like “instrument in.”) Some of the new breed of acoustic guitar amplifiers (as distinct from traditional electric guitar amplifiers) include one input which is optimized for piezo pickups.

Yes Pre-amp:
This is the way to go if you are very concerned about avoiding any loss of sound quality and signal strength, particularly when you need to use longer cable lengths. It costs more (you have to buy the pre-amp). The installation may be very simple, or may be quite a bit more involved, depending on the configuration you use (more on this in the discussion of pre-amps below). But tiny pre-amps are available, and these can make for a very elegant set-up with the pre-amp either on the instrument itself, or clipped to the player’s belt, or in some other convenient arrangement. The pre-amp will require a power source. Typically they use a 9-volt battery, and as power consumption is minimal, battery life is generally quite long. Some have the capability to operate off of phantom power, which is a system by which a tiny amount of power can be taken from the main power amplifier through the instrument cable. Here at Experimental Musical Instruments we carry two excellent, very small pre-amps, the K&K Pure Preamp and the Fishman Powerjack.

Conditioning Microphone Signals

Can usually be handled by PA mic input.

Acoustic guitar power amplifiers

An electric guitar amp is not suitable for amplifying an acoustic guitar. These amps are essentially “lo-fi” amps. Many use circuitry that is 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. Also, guitar speaker cones are constructed differently from hi fidelity speaker cones; the cone material and speaker surrounds (the material connecting the cone to the frame) are designed to encourage “break-up modes” in the motion of the cone that create a characteristic speaker distortion that is pleasing to electric guitarists. 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. 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.

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