Geek Speak: All About Onboard Preamps

January 13, 2015

THERE ARE PERHAPS HUNDREDS OF QUALITIES THAT DISTINGUISH ONE BASS FROM ANOTHER. Yet there’s one characteristic that divides instruments into two distinct camps: whether or not they have passive or active electronics. The ubiquity of active electronics over the years demands that we all develop a keen sense of what they do, and why they do it. Moreover, understanding active electronics could even give you reasonable cause to avoid them, if their purpose isn’t aligned with your sonic vision.

To understand what makes a bass “active,” it’s critical to understand why we call some basses “passive.” In essence, the nomenclature refers to whether or not a bass’s onboard electronics include a source of power or not. The easiest way to ID an active bass is to look for a battery. By including a power source, an active bass has the ability to add gain (and more) to the output of your pickups. Without this external power source, the pickups are working passively. There is no additional power available in the circuit, so any adjustment to the sound is only ever subtractive. Most active basses provide a means of additive tone sculpting.

An onboard preamp offers a degree of flexibility that a passive system can’t possibly match, but don’t let that fool you into believing it’s necessarily better to use one. It’s just a different means of achieving sound. The first important contribution of a preamp is related to the purpose revealed in its name. As a preamp, the circuit onboard an active bass conditions the signal to make it more appropriate for the much greater amplification residing in your amp. It can boost and regulate the overall gain of the bass, and also buffer the signal, an important factor. Most preamps contain a buffer, and its purpose is twofold: First, it lowers the output impedance of the bass. Second, a well-designed buffer makes the signal less vulnerable to interference from other components in the system. Without going too far down Geek Lane, think of impedance specs like a measure of the potential efficiency of a given circuit. The ideal impedance relationship is one wherein a low-impedance output feeds a high-impedance input. For our purposes, a buffered bass signal is less vulnerable to impedance mismatching (which can result in attenuated high-frequency response) and the degradation associated with long cables.

The other advantage of an active bass is the one we most commonly associate with onboard preamps: equalization. EQ is just frequency-specific boosting and cutting. Through clever design and with the help of integrated circuits designed for this purpose, an onboard EQ allows you to add gain only to certain ranges of the frequency spectrum. Typically, the onboard EQ on a bass contains two or three bands, which in total cover the breadth of the bass’s frequency response. Whereas a passive bass can attenuate high frequencies with its tone control, an active bass can, for example, boost the bass, cut the mids, and boost the treble. This results in a tone color that’s essentially unavailable through onboard manipulation on a passive bass.

Some preamps, like models from Audere and Glockenklang, add an additional feature: active blending. On a multi-pickup bass, the output of each pickup is typically blended by means of one blend potentiometer or two separate volume controls, as on a J-style bass. This approach works well, but it does result in interactions between the pickups as they’re blended that alter their individual tones. An active-blended preamp avoids this, resulting in a range of tones that more authentically reveal the sound of each pickup’s true output, regardless of the position of the blend control.

By this point, perhaps you’re wondering: Why aren’t all basses active? The simple answer is that they sound different from passive basses. We bass players are interested in sound, and perhaps less concerned overall with the technology behind that sound. If you know that passive basses have a different flavor compared to active basses, then it’s easy to imagine that they merely represent additional sounds—in the case of passive basses, this typically means one that’s perhaps a little darker, to summarize as simply as possible.

Whatever you think about the contrast, be encouraged that the aftermarket is rife with options for those seeking to try out an active system. Companies like Aguilar, Bartolini, Nordstrand, Audere, John East, Demeter, Delano, EMG, Seymour Duncan, and Michael Pope Designs all offer excellent onboard preamps for you to give a whirl. And, finally, if modding your bass ain’t your bag, understand that there’s an equally large selection of outboard preamps (usually in form) for you to enjoy.

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