A bit of history
- Number of coils
- Type of poles (posts, bars, vertical bars, etc).
- Type of magnets (alnico II, alnico III, alnico IV, alnico V, alnico VIII)
- Output (low, medium, high)
- Vibe (vintage, hot, high gain)
Single Coil Pickups
We already mentioned their drawbacks and, that's the reason why there are some tries to minimize the noise (noiseless single coil pickup) coming from practically every well know pickups Maker.
- Some are using a dummy coil below the real one just to cancel the noise.
- Some are using two half coils inside, one to cover one half of pickup and other to cover the other half.
- Some are using some special magnets (as the Samarian-Cobalt in Noiseless).
- Some are using some special trick, as Shure.
Since single coils were so noisy (and specially if they were unpotted), Les Paul models started to work with another approach. Seth Lover designed a way to cancel the hum inside a pickup and, since it bucked the hum, it was called humbucker.
Each coil has an opposite magnetized field and, coils are connect with inverse electrical polarity. What at the end happens is that everything that the magnetic field catches is being cancelled while everything that the pole pieces get is being accepted.
When humbuckers have twin coils if when they have the major hum rejection ratio. Even that the original design was like this, nowadays, a lot of humbucker models have differently voiced coils (specially if they are used for splitting tasks) and, therefore, some floor noise can pop up.
First humbucker designs, made by Seth Lover, had an sticker or writing on the back plate that read P.A.F. (Patent Applied For) and, you will hear this term very often to people that is trying to get that vintage vibe that the early humbuckers (PAF) had.
Original PAFs had their coils in series and that made their signal powerfuler than a single pickup, what added more sustain and a warmer and thicker sound. While the first PAFs have an output level very close to first single coils, nowadays they are humbuckers with an output up to three times the vintage one.
In pickups' world, everything matters. The height of the coils (taller = treblier), capacity of bobbins, turns of wire, wire gauge, wire's isolant type, wounding method (auto, scattered), magnet's size, magnet's material, magnet's molecular alignment (rough, casted, aligned, ...), etc. This results in very different voiced pickups and, the slightest change can make a big difference.
Traditionally, the pair of bobbins inside a humbucker were connected in series (imagine a train with two wagons) but, lately, they started to release humbuckers with 4 conductors, each pair corresponding to the start and finish of each of the two coils. In that way, with a tricky wiring, we can select any of the two coils or to arrange them in a different way (parallel-series, in phase-out of phase).
The ground wire is always soldered to the base plate.
Nowadays, you can see humbuckers with and without cover. The cover was part of the trick to make the humbucker to reject the noise. As we know, any electronic component enclosed in a metallic cage that is connected to ground (Faraday's cage) will reject most of noise. Humbuckers without cover aren't inside a Faraday's cage and therefore, are more prone to catch noise and interferences, while covered humbuckers are quieter.
Even splitting a humbucker (that is, selecting just one coil), you will never get the same sound as with a traditionally single coil. The coils of a humbucker lie over a magnetic bar and, therefore, the magnetic field is in principle horizontal. The trick of using pole pieces and screws inside the bobbins helps to project the magnetic field over the pickup, in the area of vibration of strings. But, since the pure single coils have magnetic pole pieces, in vertical position, the magnetic field that they create is vertical and react in a different way as a coil inside a humbucker.
The opposite is also true, if you put together two pure single coils, to create a humbucker, you will never get same sound that you achieve with a pure humbucker and, reason is exactly the same: different shape-orientation of the magnetic field, results in different induced electrical signals.
EMG is a very well known Maker who is mainly delivering Active Pickups. For a long time, it covered practically the whole market with their models, specially EMG 81 and EMG 85.
But, while those kind of pickups, with a very similar look to a passive pickup (except for the absence of visible pole pieces or screws) is what we usually know as an Active Pickup, any pickup that needs active electronic components is an active pickup.
As we said, this kind of pickups need a pre-amp circuit able to raise their low signal level up to the level that traditional passive pickups deliver. So, they way as those pickups are wired inside a guitar is different and, it includes the use of one or more batteries and one or more PCB (Printed Card Boards) with one or more circuits (pre-amp, middles booster, etc.).
While with passive pickups we use switches to select wires and get the results we are looking for, in active pickups some changes can be made just if we add some more PCB circuit or any other active device.
We already discussed pros and cons of using such a kind of pickups.
Piezo criytals are able to generate electrical signals when suffering pressure differences and, that's the principle behind Piezo pickups. Those pickups need to be in touch with a vibrating area that creates slight pressure differences that generate very low electrical signals. Due to the low signal level, those pickups need active electronics to pre amplify the signal up to the typical levels of passive pickups.
Even that those pickups were mainly used in acoustic guitars, it's not so strange to see them in an electrical guitar, specially in semi-acoustic guitars.
The most usual is to see those pickups delivered under the saddles of a guitar's bridge so, they can directly catch the vibration from the strings (Ghost piezo saddles, by example) but, they can also consist in some kind of bar that sets horizontally under the bridge or under the fretboard (Epiphone Ultra II).
The signal of each saddle is the mixed together and sent to a pre-amplification stage, before it can exit the guitar.
You cannot directly mix a piezo or active pickup signal with a passive pickup signal, you need first to pre amplify any active signal before you can mix both types. The preamplifier includes also the proper filters to get exactly the kind of targeted sound.
Piezo pickups deliver an acoustic-like tone, very different from the typical electrical-like tone that is being achieved with typical passive and active pickups. Guitars with piezo pickups usually have a blend control that allows to mix the electrical and acoustic sound to get a wide range of nuances in the sound. Also, it is very common to have or different outputs (electric and acoustic) or an stereo output (electrical and acoustic in separate stereo channels) to allow to run every signal in a different amp, by example.
There are variate designs currently available, as the 4-coils Motherbucker, and some hybrids that try to get the sound of a P90 when split and the sound of a humbucker when full (P-Rails), or a strato-like single coil when split and a single-sized humbucker when full, etc.
But, the thing that will determine which possibilities of wiring a pickup has is if it is Active or Passive, because rest of components in the circuit should be used accordingly to this.
There is always a way to mix both worlds, after the active signal has been preamplified.
Some pickups are humbuckers (two coils) that will be a direct replacement for a strato single-coil pickup, those are being called single-sized humbuckers. Each coil can cover half of the pickup or, they can be stacked (vertically).