EMG 81
The EMG 81 is a popular active humbucker guitar pickup manufactured by EMG, Inc. It is usually considered a lead pickup for use in the bridge position, paired with EMG's 85 as a rhythm pickup in neck position (Zakk Wylde is famous for this configuration). It's not uncommon, however, to see a guitar with two EMG 81s in both bridge and neck positions (for instance, Kirk Hammett's EMG-KH21 pickup sets, which include two EMG 81 pickups for both neck and bridge positions). The EMG 81 pickup is preferred as a lead pickup because of  its high output and smooth control from having a rail magnet. Rail magnets tend to sound smoother through string bends because they have a constant "rail" that runs through the pickup, while typical guitar pickups have polepieces under each string that lose signal strength as the string bends away from the polepiece. The EMG 81 can be recognized by its dark grey humbucker form-factor and a silver embossed EMG logo. With its high output, focused mids, consistent tone, tight attack and distinct clarity even under heavy distortion, the EMG 81 is a favorite among heavy metal guitar players. History  and design. The EMG-81 was developed in 1979 and released to the market in 1981 (hence the model number). Construction is similar to traditional U-shaped pickups, but there are no separate pole pieces; steel bars (rails) are used instead. The Alnico V magnets of earlier discontinued models have been replaced with ceramic ones. While nominal output is the same as current EMG 85, lower noise ratio gives more gain opportunity. As most other modern EMG pickups, today's EMG-81 has a "Quik-connect" output, which is a five-pin header on the pickup which comes with a compatible wiring  harness. This allows for a less complicated pickup swap in the future, only requiring the removal the pickup guard and disconnecting the pickup, as opposed to melting the solder and installing the new pickup. A few variants of the EMG 81 have also been released, such as the TW and X. The 81TW version features two separate pickups and preamps in a single pickup housing, allowing for single-coil and humbucking tones. The 81-X provides increased headroom giving the voicing an organic and open tone while still maintaining clarity and response. 18V Mod. The EMG 81 power source can  be modified from 9V to 18V by adding a second 9V battery wired in series. This increases the headroom of the pickup and decreases distortion, particularly with regard to transients. Although the majority of EMG's pickups are rated for 27V operation, they recommend a maximum of 18V, citing the negligible performance increase. There are two main ways to perform this modification. One method involves using separate battery harnesses for each battery. There are several different ways to achieve this, and wiring diagrams can be found all over the internet. The other involves using a separate 9V snap leading to  the control cavity to wire two batteries in series outside of the battery compartment.