There are four types of acoustic guitar pickups :
"Under the saddle" pickups are thin strips of material which are seated under the saddle, in the slot of the bridge. A wire runs from the pickup element to an output jack, usually located at the strap button area at the end of the guitar. The "saddle" is a piece of bone or plastic which sits in a slot located on the wooden "bridge". And it's removable.
Some acoustic guitar pickups will include a pre-amp assembly which must be mounted inside the guitar. This is usually done proximate to the soundhole. It can either be adhered with Velcro tape, glued to the inside of the guitar, or screwed on to the neck block. Systems with pre-amps need 9 volt power and in many cases a battery clip must also be located, again, proximate to the soundhole.
Under the saddle pickups are of two types:
1. Piezo crystals - literally, crystals, encased in a sheath of varying materials, and
2. thin wrapped strips of sensing material.
Piezo crystals are the hardest material used for sound reproduction, having the potential to produce the brightest, clearest tone. An under the saddle acoustic guitar pickup have the distinct possibility of affecting the "natural" acoustic sound of your guitar.
Under the saddle pickups provide a relatively high degree of feedback resistance because the pickup is seated under the saddle and surrounded by the wood of the bridge. This resistance to feedback will vary from instrument to instrument because of a couple of factors - The overall resonance of the box The depth of the saddle slot in the bridge. The higher the setting, the more likely to feedback.
Contact acoustic guitar pickups sense the vibrations of the top of the guitar. The pickup attached to the top of the guitar with strong tape or putty. They are lightweight and generally will not interfere with the natural sound of the guitar. A "contact" pickup is adhered to the top of the guitar with a type of putty.
For this installation a hole must be drilled for the jack. Putty holds the pickup in place on the inside of the top of the guitar, usually in the area of the bridge. In the ideal installation, the final location of the pickup is determined by trying a few spots to achieve the best tone vs. feedback resistance, which varies depending on the particular guitar in question. The location of the pickup on a Dreadnaught sized guitar is going to be different than on a cedar topped classical guitar.
Contact acoustic guitar pickups provide little resistance to feedback, because they are seated directly on the soundboard and are greatly affected by any ambient sound which hits the soundboard.
Some companies manufacture pickups which are clamped or wedged into the soundhole of the guitar. An endpin jack is an option with these pickups as well. A wire can be left dangling out the soundhole if the preference is to minimize affectation of the instrument, but, frankly, if you plan to use this pickup with any frequency, an internal mounted endpin jack is preferable.
Soundhole acoustic guitar pickups, which seat across the soundhole, both tighten the top in this area and block the soundhole. Anything that is affixed to the body of the guitar adds weight, and in some areas, such as the back and to a lesser degree the sides, can affect the tone and volume of the instrument. While some parts of a guitar are more directly affecting the sound (top and back), the whole box vibrates when you pluck the strings. Therefore, pre-amps, battery clips installed to the back sides and even the neck block, soundhole pickups and contact pickups adhered to the top, can indeed have an affect on the tone and volume of the instrument.
Soundhole acoustic guitar pickups are certainly the most feedback resistant. They are electrically charged magnets, similar to those used in electric guitars. While they do pick up some vibration from the instrument, most of the sound is triggered by the vibrating strings. This provides a great resistance to feedback caused by ambient vibrations, a problem that the open box of an acoustic guitar is quite susceptible to. The drawback of the magnetic pickup, though, is that, for many people, the sound is not "natural" enough.
Some microphones are integrated with an end pin jack. Others clip on to the bracing inside the box of the guitar with the option of running a cord out the soundhole. Some adhere to the inside of the guitar with velcro and connect to an internally mounted pre-amp which also has an end pin jack.
Microphones are usually clipped to a brace or mounted with tape or velcro inside the guitar. They are usually very lightweight, and tend to have little affect on the natural sound of the guitar.
Microphones, for obvious reasons, are the most prone to feedback. The best way to prevent feedback, regardless of the pickup, is to keep the volume at the pickup as low as possible and let the amp, boost the signal.
Combinations of these acoustic guitar pickups can also blended into one system by various guitar manufacturers.
The longer your signal chain, the more circuits you place between your strings and the amp, the further you are getting from the "natural" sound of the box. There is no doubt that a pre-amp gives you more control over the sound of your guitar, and in some onstage situations the ability to contour tone for feedback prevention is essential.
The bottom line is that Pre-amps give you more control over the tone of your amplified sound, but the least amount of circuitry between the guitar and the amp is likely to provide a more "natural" sound.
Regarding how to get the most natural sound; A good argument can be made that for a quality acoustic guitar a well placed contact pickup combined with a microphone can provide the most "natural" sound. But the sensitivity of this combination to feedback can make it unacceptable for high volume settings.
Ask these questions as you make your choice:
- Why are you amplifying your guitar and what situations will you be playing in?
- How much modification will you accept?
- Have you thought out the entire signal chain that you will play through?