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Acoustic Instruments and Amplification, Chapter 6

Piezoelectronics, other transducers, and dual systems
With piezo transducers, we are offered two basic styles in two different elements. The traditional crystal-based piezos are now being replaced by polymer based products. The main advantage of the polymers is a more consistent and less expensive product (at least to the manufacturer). My testing has demonstrated to me that one is not superior to the other in absolute tone quality; rather it becomes a personal choice as to which works better for the particular application.

The most common style for guitar applications is the saddle transducer. This has essentially six separate crystals (one for each string) or is a continuous strip if the base material is a polymer. Each of these has their own advantages. The six-crystal type will yield the highest gain before feedback ratio of any of the pickups I have ever used (this includes magnetics). This is accomplished through isolating the individual string through its associated crystal. The disadvantage of the 6-crystal type is a lack of the sound of the guitar as a whole being transmitted through the pickup. The installation of the 6-crystal style is extremely critical. It is not uncommon for one or more strings to be much weaker in output. The continuous strip saddle transducer, on the other hand, will pickup a bit more body vibration, but has a lower gain/feedback threshold as a result, along with being a bit easier install.

If your requirements are absolute gain, then the typical individual 6-crystal unit is really your only choice. This is the style of most of the ready made acoustic/electrics, since it offers the most gain, and the manufacturers are very scared of having the end user rejecting the instrument for feedback problems. For those of you needing a guitar that is more true to your acoustic ideal, the polymer style is recommended as an install in your acoustic. Just realize that rock and roll gain levels (or relatively high gain on a compact stage) will not be achievable without major equalization hassles and expense.

The "spot" transducer will always be more "acoustic" sounding as it picks up vibrations from the body of the instrument, rather than the string. Location on the guitar is very critical, and many times there must be experimentation done by the installer to find the "sweet" spot. You will also experience another drop in the gain/feedback threshold. These pickups can transmit a very good facsimile of the instrument if good care is given on the install. You will be limited to a lower stage gain level, although you can expect higher sound levels than what you would experience with a microphone.

What many of the piezo pickup manufacturers will fail to tell you up front is that for "full frequency response", you will be required to have a "buffer preamp" for these pickups. Without the preamp, if you just dump your signal into a typical PA mixer, the signal will be extremely harsh and brittle sounding, and no amount of EQ will correct the tone. However, the good side of this is that the preamps have really come down in price and gone up in features over the last couple of years. Expect to add at least $50 to the cost of the install to assure you of the full acoustic response available.

Even with the use of the preamp, you may find, particularly with the saddle ducers, that the tone isnt quite what you expect. Although you may be able to say the tone isnt bad, there is still something missing, or just out of alignmentand that is the "phase shift" that is very severe with piezo based devices (see Chapter 3).

Other Transducers

There are other transducers available, both condenser type and coil type. The condenser style is rather similar to a disc style transducer, although it is usually larger in area. Attached to the top in a similar manner, it has the advantage of a more aligned attack transient and arguably better frequency response. However, it does require the use of an outboard power supply; while it is not "active" in the same sense as most on board pre-amplified piezo systems are, a voltage must be supplied to the device, much the same as the interior condenser mic. This can usually be achieved through "phantom power" from the mixing console, as most modern mixers, even the less expensive all-in-one powered types, usually have this capability.

Another type is the moving coil or moving magnet style. Of the moving coil variety, the old DeArmond is probably the most common. This unit attaches to the soundboard via a rubber cup that has a coil attached. This in turn resides in a magnetic field, with the resulting signal being sent to the amp. A variation of this can be seen with a particular model of banjo pickup, where a steel chip is place under the middle foot of the banjo bridge. Under the head, tied to one of the support rods, a magnet and coil are placed in the proximity of the steel chip to provide signal. Both of these styles have the advantage of the mellow tone provided by the magnetics but are usually plagued by a less than flat frequency response.

I hold a patent on a coil/magnet device (Dynafield) that is actually very accurate to the guitar. A tiny (neodymium ferrite) magnet is glued to the bridgeplate of the guitar, and a low impedance coil is suspended beneath the magnet. As it uses the top of the instrument as a "mic diaphragm" and is very low impedance, you can actually get microphone tone quality from your basic PA system. There are critical issues involved with the installation, and a learning curve in getting high gain. But for those that need accurate amplification in a moderate gain environment, it works very well.

Dual Systems
As we are about to enter a new century, many pickup designers and higher end guitar makers are using "dual source" systems. This typically involves a saddle transducer in conjunction with an interior mounted condenser mic. The guitarist can then "blend" the output from the two transducers to arrive at the best compromise between "pickup" and "microphone" tone. Onstage, the practical outcome is piezo in your monitor, and a mix in the PA mains, which is balanced for enough mic to sound "woody" but enough piezo to give it some cutting power.

One of the more famous dual systems was the one Mike Hedges used in his D-28. His was a FRAP piezo bar (no longer manufactured) with their very expensive preamp, mixed outboard with a Sunrise magnetic soundhole unit. Fishman is marketing a lo-z soundhole unit in conjunction with an interior mic.

The previous three examples are an indication of the future of signal input transducer technology; each has its advantages and disadvantages. On the plus side, you can expect a more "natural" sound at lower gain levels, and if you need a higher gain/feedback ratio, you have the option of doing it on the gig, in real time, although some of that natural tone will go away.

The downside is the expense and learning curve. You are doubling your input signals, and along with that comes higher cost, more complexity, and probably the need for a sound engineer to achieve optimum results.

Just recently, progress has been made that uses both a disc style piezo and a high impedance soundhole pickup. By using the capacitive and inductive reactance available with these pickups, a passive dual system can be made to sound very natural. The biggest advantages (in addition to good feedback rejection) is the absolute simplicity of use and compatibility of the system with generic PA and studio components.

Next Chapter: Pre-amps and equalization

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

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