A preamp is essentially a device that can increase the signal level, color the tone, lower the output impedance, convert the signal from unbalanced to balanced, or do any combination of the above. Each of these functions has its use, but an onboard preamp is a very different animal from a rack-mounted model. Making sense of the different preamp designs and functions will help you decide which preamp you need.
If you have an onboard preamp installed directly in your bass, your instrument is considered to be “active.” The evolution of active electronics began in the late 1960s, when Rick Turner and Ron Wickersham joined forces at Alembic. Their early work with two Bay Area rock bassists, Phil Lesh of the Grateful Dead and Jack Casady of Jefferson Airplane, paved the way for the now-ubiquitous active circuits found in today’s basses. In addition to adding gain, onboard preamps feature active EQ that’s either 2-band (bass, treble), 3-band (bass, midrange, treble), or—as in the case of a bass like the Ernie Ball Music Man Reflex— 4-band (bass, low mids, high mids, treble).
Many stompbox preamps work with the magnetic pickups found on most electric basses, but using these preamps with piezo-pickup-equipped basses can be a problem. The output impedance of a piezo pickup is often too high to match the input impedance of an amp designed for the magnetic pickups of an electric bass. This mismatch results in lessthan- optimal tone, which is why preamps such as the Fishman Platinum Bass are designed primarily to provide a “buffer” for piezo pickups—they alter your signal’s impedance level to match an electric bass amp’s input. In recent years, some amp manufacturers have adopted higherinput impedances, making buffer preamps unnecessary.
Another common type of preamp is the rack-mounted unit, which most players use to drive a power amp in a live situation (many models come equipped for the studio, too). Several companies build rackmount models that are nearly identical to the preamp section of their popular amps; units like Eden’s WP100 Navigator allow you to get a brand’s signature tone while using a separate power amp or plugging directly into a studio console. Other preamps, like the Demeter VTBP-201S, are designed to be more transparent. While not devoid of personality, devices like this strive to attain a more pristine response, letting the natural tone of the bass take center stage.
There are perhaps hundreds of qualities that distinguish one bass from another.