So How Do I Choose a PreAmp? – Joe Bigham
|Since you read last month’s article about pick-ups, you may have rushed out to try a few and may have even bought one. Now, you may be saying to yourself, “What am I missing?”. Or maybe you’ve used your new system in live applications, but are not completely satisfied with the tone. A preamp may be the solution to all of your quandaries.
What does a preamp do?
First, it boosts the signal level significantly. Many acoustic amplifiers and PA systems “look” for a signal that has some juice behind it.
|When the soundman boosts the signal of a non-preamped signal two things happen that are far from pleasant: the gain added by the soundman also raises the level of noise, whether it be hiss or hum caused by RF interference, and the signal will sound thin (lacking in bass and mids). The thin sound is caused by an impedance mismatch (impedance refers to resistance values measured in ohms). Piezo pickups and other styles of acoustic pickups typically have an extremely high resistance (thousands of ohms), while PA systems are always looking for low impedance sources (hundreds of ohms or less). Preamps boost the signal level of the source, and alter the impedance to match PA systems or acoustic amplifiers.
Secondly, all outboard preamps and many onboard (installed in the guitar) preamps offer varying levels of tone control. These range from simple treble and bass boosts, to finely tuned parametric equalizers. If you feel that your pickup tone (regardless of amplification) is well balanced, but lacks just a little bit of lower-mid character, you can find the frequency that needs the boost. These tone controls can also be used to eliminate feedback. Most acoustic guitars tend to feedback in the lower range at the same frequency as a G or A note on the low end of the guitar. By cutting back bass, or finding the thin band of offensive frequencies, you can “tune” out the feedback problem.
Third, many preamps also act as DI boxes. A DI box allows you to send you signal long distances by a microphone cable as opposed to a regular instrument cable. Microphone cables are less prone to RF interference and are favored by soundmen because they easily plug into any on-stage snakes (large cabling boxes that carry almost all signal to the soundboard). In fact, any instrument that is not mic’ed into the PA has to run into a DI box at some point.
Lastly, preamps give you the opportunity to mix multiple signal sources, and then apply effects to all of these signals, or individual signals via effects loops. If you have both a piezo pickup and internal microphone in your instrument, you would blend both signals at the preamp, and then add your favorite chorus or reverb to both channels. This way, you are controlling the signal that ultimately goes to the soundman or amplifier.