What happens inside an active pickup?

Active pickups seem to be very mysterious, hiding all their secrets under their plain black covers. All the innards are sealed in an opaque black epoxy making them just about impenetrable. But in this article we'll be having a look inside to see just what interesting contraption hides inside. Watch our video below alongside reading this article for a full teardown of an active pickup.

Firstly, the core of the pickup works exactly alike to any standard passive humbucker - two coils of wire each with a steel core and a magnet underneath charging the strings. They don't have much in similar in construction to a standard humbucker though, more so a mini humbucker. Thin, tall coils lend the pickup a brighter, more defined tone than a standard humbucker's squat coils. The steel rails add additional inductance (output) to the passive design, and prevent note drop out when strings are bent. The coils are generally wound about 10% more than a vintage PAF - much more than most people would think (most believing active pickups are very low output pickups with the preamp doing the bulk of the work). These pickups in passive mode are actually reasonably powerful. 

Then this passive pickup is fed into the preamp to mould the tone into what we know and love (or not, maybe). The preamp is a relatively simple design in terms of how many components there are, but every component has been very specifically tested and selected for exactly the tone requirements of the design. The first part of where an active pickup differs is that the preamp is actually fed by the coils individually. Our standard humbucker is our two coils connected in series, essentially making one big pickup, with the two coils interacting together electronically. Our active pickup is essentially two single coil pickups combined  in our preamp.

Our first component that our inputs see is a coupling capacitor - this capacitor performs two functions; it blocks the DC voltage from our pickup coils, and it trims off some bass end, brightening the tone. This capacitor is chosen to be a small value to have a significant effect on the tone. Second in our signal path is the series resistor. This resistor interacts with the inherent capacitance of the coil (from the winds laying on top of each other in close proximity), to darken the tone. The resistor shifts the resonant frequency of each of the coils depending on the resistance of it - and for this reason, they're commonly chosen to be of different values. The more asymmetrical they are, the more open and dynamic the pickup sounds and feels. Higher asymmetry also adds a little bit of hum into the design, but they can be pretty asymmetrical before it becomes a problem.

After the resistors is where the two inputs diverge in design. The inverting input also has a negative feedback resistor. This controls how much of the signal is fed back to reduce the amplification of the amplifier. The non-inverting input has bias resistors to both the battery voltage, and ground. These resistors are a parallel resistance acting on the input. All of these resistors act to load their respective coil. For tone purposes, the lower the value of these resistors, the lower the output, the darker the tone, and the more compressed the pickup is. These resistors are very low values, and are primarily what is responsible for the characteristic compressed output of active pickups. The negative feedback resistor interacts with the series resistor of the inverting input to set the amplification factor of the amplifier. The voltage is distributed between the two bias resistors - for maximum headroom these resistors are matched to give half the maximum voltage at the input.

At the end of the amplifier there's just three simple components. There's a coupling capacitor, an impedence setting resistor, and a loading resistor. The coupling capacitor is of a high enough value that it's transparent to the tone of the pickup - it's only function is to block the DC voltage from the output of the amplifier from getting to the guitar. The impedence setting resistor is a low resistance. This resistor influences how the pickup will be loaded by external loads. A low output impedence means that external loads that could leach off a lot of the output of the pickup will affect it significantly less than it would a typical passive pickup (which has a high output impedence). This is most notably seen with long cable runs. An active pickup will see a negligible loss of treble, whereas a passive pickup will have a noticeable loss of treble with the same cable. The last resistor loads the output of the pickup, trimming off a little bit of the output and interacting with external loads, just as the impedence resistor did.