As you’re probably well aware, the bass guitar electronics landscape can be divided into two distinct continents: passive and active. Moreover, almost every electric bass utilizes magnetic pickups to convert the strings’ mechanical energy into an electrical signal for amplification. Sure, there are some alternative schemes; for example, some electrics include a piezo pickup that uses a small vibration-sensitive piezoelectric element to produce an electrical signal. There’s also a handful of exotic technologies, like the optical pickups popularized on Lightwave basses. But those fringe cases aside, the magnetic pickup is the crux of most basses’ transformation into an “electric” instrument.
It’s what happens after that conversion that provides the focus for this month’s column. The terms “passive” and “active” are borrowed from electrical engineering. Passive components and circuits are those that don’t require an external power source for their operation. They can store and regulate the flow of electricity, but they cannot amplify anything. Active components require a power source, and crucially, they allow one electrical signal to control the flow of another—this is the basis of amplification.
Most onboard passive circuits employ a handful of basic components to operate. Typically, the pickups are first connected to one or more potentiometers or “pots.” The pots in basses have three contacts. One is connected to a metal wiper that moves across a resistive strip, while the other two contacts are at opposite ends of the strip. Measuring the resistance across the strip at the two contacts reveals the pot’s maximum resistance, while a measurement from the beginning of the strip to the wiper depends on where the wiper is on the resistive strip, as that’s where the signal from the pickups is tapped off. In a volume control, the pickup signal is attached to one end of the strip; the output is the wiper, and the other end of the strip is connected to “ground,” an open pathway for electrons to flow. As you turn the knob (and wiper), the pot functions as a voltage divider, sending some percentage of the incoming voltage to ground depending on the position. We have a volume control!
Because passive basses have no external power, tone shaping is limited to cutting at certain frequencies. Most often, this comes from a simple circuit that employs a pot and a capacitor. The signal goes into one end of the pot’s resistive strip, while the capacitor is connected between the wiper contact and ground. When an audio signal passes through a capacitor, it functions as a lowpass filter, cutting treble at a frequency determined by the relationship between the pot’s resistance and the capacitor’s capacitance. Turning down the tone knob sends an increasing amount of the incoming signal through the capacitor, bleeding off treble frequencies. In this circuit, the pot is a variable resistor, with the knob determining the resistance between the signal and the capacitor.
Next time we’ll dive into active circuits. Until then, ponder the clever miracle of the controls on your passive bass.
BASS PLAYER Senior Contributing Editor Jonathan Herrera is the magazine’s former Editor-in-Chief. An accomplished player, Jonathan is now a full-time musician and producer. His latest endeavor is Bay Area recording studio Airship Laboratories. Catch up with him at jonherrera.com and at airshiplaboratories.com.