PreAmps
PreAmp Fast Facts
Internally installed preamp units for guitars and basses have become very popular, with many different types making their way into the marketplace. Since the installation of preamps is fast becoming a common guitar modification, players frequently ask me to explain what they are, how they function, how to interpret their specifications, and how to choose the right preamp for their playing needs. Understanding how a particular preamp operates will give you a good idea of its compatibility with your instrument and how the two can be integrated.
A Brief History of the PreAmp
Preamps are now available with a variety of functions, ranging from the single-stage buffer preamp to more involved units offering active tone controls or equalization. All of these units use transistors, FETs (field effect transistors), or IC (integrated circuit) op-amps (operational amplifiers) and depend on a battery or batteries for their operation.
The most common type of preamp used in guitars and basses is the single stage buffer or driver, which serves a variety of functions. It is usually placed directly at the output of the instrument, after the controls, converting the output of the guitar to low impedance. The high-frequency response of the signal is thereby increased (providing the pickup can produce the higher frequency range) because the pickup becomes isolated from the guitar cable and its associated filtering effect (which normally causes high-frequency loss when output is at high impedance; this effect occurs on most conventional guitar pickups, which are “passive” – i.e., use no external power source). This isolation also minimizes the line loss normally occurring when longer cords are used between the guitar or bass and the amplifier. Another advantage of a low-impedance output is that it permits direct input to a mixing board without going through a “direct box.”
Most single-stage preamps offer variable gain, thus allowing you to boost the output of the instrument with a simple adjustment. However, any noise produced by the pickup (such as hum or buzz) or by the controls on your instrument will also be amplified by the preamp proportionately with the increase in signal. Therefore, if you have problems with this type of voice you should consider this when choosing a preamp. Single-stage units can usually operate on a single 9-volt battery and ordinarily do not require a large amount of space for a clean installation.
Active tone control or EQ unit preamps are preamps offering active tone control or equalization that has been specifically designed for use with guitar or bass, the units, tone circuits being optimized for the different frequency ranges of these two basic types of instruments. Some EQ units also contain a buffered input stage identical to the single-stage preamp described earlier. However, instead of sending the signal directly out of the instrument after the buffer amp, it is sent through additional stages of circuitry required for altering tone. The variety of tone options varies from unit to unit, with some offering +-20dB boost or cut, variable frequency controls, falloff contours, and/or EQ controls. Active EQ units usually have from one to six stages of preamp circuitry. Some will require only a single 9-volt batter, while others may require two, either in series or in parallel. This is an important consideration, not only in regard to the space required for mounting two batteries inside the instrument, but also because the supply-voltage requirements for other types of active gear you may want to use may be incompatible with the requirements for the EQ unit. Also, preamps with active tone controls usually require more space in the guitar than buffer preamps, and thus may require some body modifications.
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