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Acoustic Instruments and Amplification, Chapter 2

If you ever read stories about Les Paul, the great guitarist, inventor, and innovator, you are reading a capsule history of the amplification of our favorite instrument. His earliest tries at amplifying his guitar were to take a phonograph needle and tape it to the top of his guitars, in an effort to make it louder. In those days, the early to mid thirties, the pickup systems were almost identical(at least in the principle of how they work) to what is in use today, that is, a crystal(piezo) or magnetic(string pickup or dynamic microphone). Of course, our pickups today are much more sophisticated in manufacture, if not design.

The original phono cartridges(if you could call them that) were just a stylus (needle) attached via a fulcrum point, to a ceramic disc. As the disc was stressed it produced a voltage strong enough for a tube amplification stage to hear. This signal correlated roughly to the vibration at the stylus and we had the first pickups. The problems with these early piezo designs were twofold: The location on the instrument is an indication of the frequency response on just that area of the guitar, and the crystals themselves rarely had a predictable frequency response due to inaccuracies in the available manufacturing methods.

The magnetic transducer was taking a couple of different coursesthe microphone, where a diaphragm with a coil of wire attached to it vibrates within a magnetic field, and the string pickup, which uses the ferrous(steel) strings vibrating within a magnetic field to excite the flow of electrons in a coil of copper wire. Due to the already established wire manufacturing methods, frequency response could be much more predictable, at least in the microphone. However, magnet technology had not advanced to the point where the capsule assembly could be of use in anything more than a mounted microphone.

About 1930, George Beauchamp developed the first successful string pickup for guitar. The first guitars to use this design were the Rickenbacker "frying pan" lap steels. These used a pair of large horseshoe magnets surrounding the strings. While the guitars did make a useable signal, good tone was not necessarily part of the sound. Gibsons first electric was also a lap steel and was sold in conjunction with their first tube amps. Contrary to popular belief, the first electric guitars were actually solidbody instruments!

During the mid 30s, Gibson led the way with amplifying the "Spanish" guitar, which was at that time a medium sized, arch top, f-hole guitar. The pickup and coil were very large and most of it was hidden inside of the instrument. Charlie Christian made this instrument famous on several recordings, in addition to being a featured soloist with groups like the Benny Goodman Sextet. This was really the first widely accepted use of the amplified guitar as we know it today. The pickups and amps of the time didnt have the wide frequency response that we take for granted today. If youve played a good archtop acoustic, you can tell that the tone and cutting power of the archtop has little to do with the sound of modern recorded jazz guitar. The dark, mellow tone from these early Gibsons and in particular Christian, set the standard that many jazz players follow more than sixty years later. Meanwhile, in Nashville and the southeast, two brothers from Kentucky were setting the stage for the signature "acoustic" sound of their day.

Bill and Charlie Monroe established the quintessential form of American country music through the popularity of their acoustic string band, the Bluegrass Boys. Their standard, carried through to today, is a totally acoustic sound, due to the makeup of the instruments that play it. The group generally assembled around one microphone for large performances, with the soloist stepping forward to emphasize his solos. Because the "flattop" doesnt have the cutting power of the other instruments, the guitar was relegated to a supporting rhythmic role. With the folk music boom of the early sixties, this was beginning to change. Many groups began to use the acoustic guitar as a lead instrument in itself, and this required better forms of amplification.

While the deArmond company was building magnetic pickups that fit on the guitar, the natural tone that we look for was still missing. During the middle sixties, a large US aerospace defense contractor was owned by a man who was also a guitarist(Charles Kaman). The Ovation guitar was born. While many of us prefer an all wood guitar as a matter of taste, they did introduce a new type of crystal pickupthe piezoelectric SADDLE TRANSDUCER. Even though the crystal elements the transducer used had been around for years, the arrangement under the saddle was a whole new way of looking at amplifying.

For more than twenty years prior, development had been occurring in the field of microphones. By charging a diaphragm capacitor with a dc voltage, minute voltage changes could be detected by an audio amplifier when the diaphragm was stressed.the condenser mic. By the mid 60s, these microphones were becoming small and accurate enough to actually be placed inside of an acoustic instrument.

We now have all of the basic, modern elements in placeFirst and foremost, the dynamic microphone, then the magnetic pickup, the saddle transducer, the condenser mic. Each has its own foibles and virtues. Each has its own voice. And each presents its own set of problems when learning how to amplify the guitar to the preference of the user.

Before the next chapter, I would suggest boning up a bit on all of the varying distortions that we deal with when amplifying our guitars. Harmonic distortion, intermodulation distortion, clipping distortion, transient distortion, and maybe the most important, phase distortion1. The first that generally comes to mind in any group of guitar players is clipping distortion. This is the typical "fuzzed out" tone from the mid 60s that begat loud electric guitars. Nearly all of us can appreciate the tone of a distorted electric guitar. Think of Duane Allman playing slide or Jimi Hendrix taking us to Mars and back. There is something about that smooth, distorted soundits called "psychoacoustics". Since the sound of the electric guitar is harmonically simple, at least as compared to acoustics, I believe we want to hear that complex interplay of harmonics that a clipped tube amp provides.

The dry sterile, tone of a Tele just doesnt cut itunless we sweeten up that tone with a wash of artificial reverb. These are just a couple of tools that are available to the electric guitarist who wants to make his sound more palatable to the listener. We could talk distorting the time domain(reverb and delay), the harmonic domain(distortion), the transient domain(limiting/compression). I cant think of an EFX device that simulates intermodulation distortion.

What does all of this electric guitar stuff have to do with an acoustic guitar? Stay tuned..Chapter 3 will deal with where the distortions come from, in particular how they relate to your acoustic guitar sound.

To learn more about Dave and the guitars he builds, please visit http://www.electrocoustic.com

Glossary of Distortions

Clipping: occurs when an amp stage can no longer respond the higher gain levels it is being asked to achieve. The result is a "square wave"(as seen on an oscilloscope) that generates a harmonic signature of its own.

Harmonic: The ability of a transducer or amplifier to reproduce the upper harmonic partials of any given fundamental tone. Tube amps tend to emphasize "even order" harmonics while solid state amps tend to emphasize "odd order" harmonics. The Western European system of music(12 tone) is based on this harmonic series of overtones.

Intermodulation: the ability of an amplifier(or ANY acoustic device) to produce harmonically dense and complex material. This is evident in a poorly made violin as a "wolftone". Early tube amps have a poor damping factor and this gives rise to this distortion.

Phase: this is a type of time delay distortion related to the frequency that is being amplified, where a lower frequency is passed down the amp chain sooner than a higher tone or vice versa.

Transient: the ability of an amplifier to reproduce accurately the attack transient of a signal, such as the striking of a loud drum.

Dave Wendler is a luthier at Ozark Instrument, and holds a US patent in the field of acoustic instrument amplification technology.