William Cumpiano's
String Instrument
Newsletter #9

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Acoustic Guitar Pickups I

William R. Cumpiano 1998, All Rights Reserved

Greetings, friends and fellow travelers;

I've been asked by a member of this newsletter list to comment on acoustic guitar pickups. I'll oblige by discussing my own approaches, priorities and prejudices, and invite folks to reply with theirs, if they wish. I hasten to admit that I am not an expert in electronic sound processing, so it is fully possible that I may misspeak herein on specific details of science or electronics. However, my approach is from the vantage point of an experienced installer and lay observer, so you will probably find what I have to say more useful than if you were to get a techie to talk to you.

The paramount considerations in selecting a pickup system are at least three: Which system will physically fit into the instrument; what will the user's performing environment be and how much the user wants to spend.

The first puzzle is, which unit will physically suit the instrument? I personally have to deal with more arcane requirements than most of you out there, since I get to service a wide gamut of world instruments. I deal with ukeleles, cavaquinhos, charangos, tiples -- instruments which, if you're lucky, you can get several fingers into the soundhole. Let alone an arm holding a pickup. Indeed, I once had to prepare a Saz with a pickup, a turkish instrument with no soundhole at all! And there are those outsized Mexican guitarrones with the medieval tie blocks. Like the guitarron, many of these traditional/ folkloric instruments have no saddle, so saddle transducers are out. But for the average installer, more pressing problems are: can I get my arm in there? and, if the user wants to control the sound output of the unit, do they want to cut out a large hole in their instrument to access the controls directly--or not?

The second consideration, the user's budget must be evaluated realistically in terms of what the user's expectations are. Generally, the price of a pickup system increases as a direct proportion to the system's fidelity to the actual sound of the guitar, and to the number and convenience of its controls. If the natural sound of the guitar to be picked up doesn't exactly warrant super-authentic duplication, then the user is wasting money purchasing an advanced, high-fidelity system. If the user can dispense with controls completely (that is, doesn't mind walking up to the amp to adjust the sound) a good 50% of the total system cost can also be dispensed with. Often a user with too large a budget will buy all the bells and whistles and come to find them too encumbering and impractical for their actual guitar-playing environment. One player I know dropped $400 on a Fishman Blender box when all he really needed on stage was a volume control, and besides while playing on a live stage surrounded with percussion and electric guitars, he had to turn down his internal microphone so much that it hardly was worth the trouble to have a pickup/microphone combination at all. He actually had better results shutting it off altogether. The expensive blender ended up in a heap of dust in his garage.

The third consideration, the player's environment is perhaps the most important. The playing environments of my customers fall into several distinct categories. The simplest requirements are those of the lone player: a guitarist simply wants to plug the guitar into an amp in a room, preferring to listen to the acoustic instrument through an amplifier. Playing a lone acoustic guitar amplified in a room is a perfectly valid creative endeavor: an amp offers an expanded dynamic range and the potential of a wide range of electronic colors added to the players' tone palette. More power to them.

Next, consider the same lone player in a studio recording situation: the prime requirement for this is usually the most complex: maximum information is required. The recording engineer often wants as much of the total sound that the guitar is producing as possible, so that all the nuances are available for processing. And although many recording engineers are adept into turning an aural pig's ear into an aural silk purse, most will tell you that they would prefer to begin with a good guitar attached to a good pickup system, than have to waste their time applying cosmetic fixes. Fidelity is key in this environment.

The next category is the stage environment, and here there are usually three sub-categories: the solo performer, the performer as part of a low-key ensemble and the performer who is part of a loud, heavily amplified ensemble, band or orchestra. Each has important and distinct requirements which I will cover shortly.

Let me preface the following, if you haven't already noticed, by pointing out that I'm carefully avoiding the word "transducer," ordinarily and loosely used to denote a strip pickup which is put under the saddle in the bridge. I don't like to use the word, because of what it really means: technically "transducer" is a generic term which describes any device which changes energy from one form to another. So it would be just as accurate to describe an audio speaker or a microphone as a transducer as would a guitar pickup. So I'll just avoid it in favor of "pickup."

Many of the solutions to the guitar amplification problem will become obvious if we take a step back and examine how a guitar produces sound. Three distinct sources of sound on the guitar can be described. Each is best "picked up" by a specific type of pickup. First, the strings alone are producing sound. The sound of the strings is the result of the natural "internal" resonances of the strings plus a complex overlay of "crosstalk" from adjacent strings and feedback from the moving soundbox (the strings shake the guitar; and the guitar then shakes the strings). As complex as this "information" is (because it has yet to affect the "nearfield" around the guitar--as well as the air and body/wood resonances of the guitar itself) it is the simplest and least troublesome information source on the guitar, and many stage performers who want to play loud or are surrounded by loud instruments will attempt to limit the "information input" into the amplification system by placing a magnetic pickup directly under the strings--usually in the sound hole. The decision to put a magnetic pickup (like a Shadow or a Sunrise) entails both advantages and costs. The advantages are simplicity, power, clarity and virtually no feedback problems. The cost: a "flat" sound with little warmth or dimension. The warmth and dimension, however can be added at the board with a touch of reverb, compression, and other alchemy. Some of the modern soundhole pickups are very popular because they add some proprietary circuitry that supplies some of the warmth and depth which are typically lacking in such systems. Another drawback is that these pickups work by having a metallic conductor (a steel string) cutting across the magnetic field of the pickup's magnetic poles. This generates the minute voltage which is carried to the amplifier. Nylon strings won't work, so you won't find soundhole pickups in classic guitars. Oh, yes, there is a third sound production mechanism on the guitar, best picked up by a microphone. I'll deal with it in the next installment.

Well, the strings drive the soundboard, resonances flood the guitar, and the music rises off the wood "like heat from a skillet" (colorful, evocative analogy thanks to Tim White). The sound rising from the moving wood surfaces on the guitar can be directly heard by so-called "contact" pickups: small, inexpensive devices--some as small as dots (they were first developed for the spying industry!), which are actually adhered to the plates and when moved by them, generate a minute voltage which is then fed to the amplifier. They are at best, very "directional" in the sense that they react to the acoustic activity present only in the immediate region where they are located. You usually must move them from site to site looking for the sweet spot, each spot eliciting either a subtle or dramatic difference in response. Unfortunately, you can never be quite sure that the response you are getting is the best one on a particular instrument, so you usually must settle. For many instruments, particularly the smaller ones, contact pickups are the only option: they are small, unobtrusive, cheap, work for both nylon and steel strings, and produce a reasonably acceptable response when fulfilling modest amplification demands. They will fall short in loud environments, however--they can turn the instrument into a microphone! But for $40 - $50 they're a salvation for moderate-requirement poverty jobs.

A better solution -- with an accompanying increment in cost -- are the piezo/ceramic strip pickups, which are placed in the bridge, under the saddle. These are by far the most popular pickups. They succeed where the "contact" and magnetic pickups fall short, in that due to their placement (precisely at the interface between the strings and the guitar) they receive a lot more information than the two others, and produce a more complex, balanced sound.

There are a number of technical problems which traditionally have dogged these pickups, however. Strip pickups respond to acceleration (shaking) in one direction, others to two; some to pressure. Thus, they are prone to string-to-string volume-balance problems usually caused by inexpert or inexact installation: they have to fit just right, the string pressure must be distributed adequately and evenlyover the pickup; the saddle must be shaped just right and fit the slot not too tightly or too loosely, and must protrude by the right amount. Clearly, they require thoughtful installation for optimum results. Regardless of the care exercised in their installations, traditional contact and strip pickups have a common drawback: impedance mismatch. This is a result of History: guitar amplifiers were originally made for electric guitars, which are invariably supplied with magnetic coil pickups which produce their voltage at a low impedance of about 500k ohms. Contact and strip piezo/ceramic pickups typically produce their voltage at impedance measured in millions of ohms. Thus when you plug a contact or strip pickup into your typical Fender amp, which is expecting a low impedance from a magnetic pickup, the result is a hot, "honky" sound (like somebody singing with their nose plugged up). The rather harsh, raspy sound is a result of this impedance mismatch, which actually does the same thing as if you turned the amp treble and midrange knobs to 10--all the time. (Actually, tone knobs are really impedance "mismatching" devices).

The solution: a more expensive upgrade. Add a little black box, called a buffer preamp (which requires a battery to power it) to the line, lowering the pickup's output impedance and bringing it closer to the input impedance that most guitar amplifiers are expecting. The honky sound is gone and in its place, a smooth, pleasing, balanced sound. But now that you have a battery in the system, you might as well take advantage of its presence to furnish a complement of bells and whistles: treble/bass boosting, equalization, and so forth. The only problem left is where to actually put the darn things. Most modern powered pickup systems have simplified installation by smoothly fusing the preamp with the output jack (Highlander) or by supplying a flexible strip that can be fed through a hole in the saddle slot so you don't have to solder anything (Baggs).

Where to put the bells and whistles is a problem. It is unspeakably horrible to cut a huge window sized hole in a guitar for a transducer effect box, and only slightly less horrible to drill holes for volume and tone knobs. It's bad enough that you must drill an oversized hole in the butt of the guitar to plug the thing in. So I usually beg the guitar owner to put everything on an outboard effects box which clips to the belt. In many cases, the interior battery can be dispensed with using an outboard box. Even though the latest strip pickup systems use infinitesimally small amounts of battery juice, and the engineers have gotten the battery-replacement requirements down to yearly or even bi-yearly, I'm bumping into more and more people who just don't want batteries installed inside their guitars, period. Batteries perversely insist on bumping loose and rattling around at the wrong time, or the cheezy wire clips tend to fail and shut their guitar off, again in the middle of a big show. Also, since it's near impossible to stash the wiring neatly when installing aftermarket through the soundhole, the wires tend to slosh around annoyingly all the time and even cause resonant "shadow" buzzes inside the guitar which drive people like me crazy. So I just love it when I can persuade people to use outboard effects/buffer preamp/power supply boxes that clip to their belt. By now, most pickup companies offer some variation on this theme.

That's enuf for now. I'll save my final comments on internal microphones, comments about tap tones, and other subscriber inquiry responses for next week.