WHAT MAKES A PICKUP ACTIVE?
Essential pickup wisdom
Hello again! Now that we’ve established the majority of magnetic guitar pickups are inherently passive transducers, let’s talk about what it is that makes an active pickup ‘active’.
First, let’s begin with what makes a pickup a pickup. Magnetic guitar pickups are constructed primarily from ferromagnetic pole pieces and coils of wire. Regardless of whether they utilise round or rectangular magnets, hidden or exposed individual poles or ceramic bars, magnetic guitar pickups are passive devices known as ‘variable reluctance sensors’. Such sensors measure changes in magnetic reluctance, detecting changes in the presence or proximity of ferrous objects. For bass pickups, the job is to detect and measure the movement of the strings.
An active pickup is made in the same way, but wound with far fewer coils. The result is a low-output passive pickup. The passive signal output of an active pickup is lower than that of a conventional low-output passive pickup: in fact, their output is so low that they require an active circuit to boost and shape their output. The result is a low-impedance – in other words, low-voltage and high-current – device with a signal output that can be ‘hotter’ than even the ‘hottest’ passive pickup.
While a passive magnetic pickup generates electric potential without a need for external power, the extremely low output of an active pickup requires the external voltage of a battery to power the built-in transistor or op-amp that boosts the pickup output.
These circuits are the active part of an otherwise passive pickup and are essential to boost the signal output to a useable level. The electronic circuitry in an active pickup may be as simple as a single transistor, or can include one or multiple op-amps configured as filters, EQ, and other sound-shaping features designed to modify, filter, and attenuate the final output of a pickup.
What all this means is that pickups marketed as active are nothing more than low-output passive devices that incorporate a simple internal preamp. Some limitations of active pickups are that their active circuits can limit dynamic range, while their high output can affect an amplifier’s input circuit.
Active pickups can be a good option for some passive basses, but are not always the best complement to active preamp circuits, where their internal preamps can be redundant. This is why it’s more common to find an active bass with passive pickups, and passive basses with active pickups.