Active Pickups? How do EMG do it?

The EMG 81 is now ranked as a "classic" humbucking pickup - for years it has been the go-to choice for Shred and Metal guitarists thanks to it's monstrous voltage output (making it a doddle to overdrive a preamp!) and searingly smooth high frequency response which allows guitarists to cut straight through all the sludge of the bass and drums, and, depending on the band - the singing too ;-)

Here's a description of the EMG 81 from the EMG Website;

"One of the most popular EMG’s the 81 is the one that started a revolution. Utilizing powerful ceramic magnets and close aperture coils, the tone was designed with detailed intensity, incredible amounts of high end cut and fluid sustain. Traditionally used in the bridge position, this pickup will make your leads slice right through even the densest mix. When used in both neck and bridge positions the sound can only be described as blistering."

And a demo of the EMG 81 in action and in comparison with some other EMG actives;

I've got to say - it's not exactly a sound I use very often (ever?!) but active humbuckers can be used in other genres apart from Shred and Metal. As long as you know how they work and how to alter their characteristics anything is possible (well, maybe not an active pickup designed for Country Twang!).

Luckily, someone's taken one of EMG's babies apart and they reckon they've got the whole thing worked out! Firstly here's some general specs on the EMG 81 construction from whoever took them apart;

"Magnet: Ceramic (cutting) 56x3x13mmWire: 0,06mm (Plain Enamel) (#42 AWG)
Core: 54x3x12,5mm (silicon steel?), solid steel.
Coil: 4,18KOhm (one coil), wax potted, aprox. 5700 turns, h=7,5mm
Bobbin: 64x13x9mm (or with "tube legs" 12,2mm)

Coils conection:

-opamp--------^^^^^^^^-----ground----^^^^^^^^^^---------opamp+

IC unknown, marked as EMG001"

And another video of an EMG Factory Tour (with some interesting info on the EMG 81 specs.);

And for the nosey ones here's some images of the gutted EMG 81 humbucker ;

Cover off, wax potted coils and ceramic magnet

The mystery EMG chip (suspected LM4250)

The coils and solid steel blade bobbins

Furthermore the guy who gutted the EMG 81 also traced out the circuitry inside - here's the schematic of the active circuit;

Now the freestompboxes.org forum has had a good look at this and the main proponent to it's development and implementation was the great Bajaman. Here's some of his key posts (edited slightly by myself for clarity);

First up is a post regarding the original schematic;

"Referring to this scheme it is easy to see that it is a classic differential amplifier circuit stage - any noise or hum appearing on both coils gets attenuated in the same ratio as the overall gain. In simple terms we end up with a very quiet and super humbucking pickup, BUT in the EMG81's case there exists a slight imbalance in the differential arms. Although both coils feed via 22n dc isolating coupling capacitors, the coil connected to the non inverting input is missing it's source resistor - in the schematic's case 30k. In practice this will not make much difference if any to the noise cancelling efficiency but it will create some imbalance of signal gain in the higher frequency region where the rising coil inductances come into play."

And some quick info on the LM4250;

"The LM4250 is a programmable current op amp with a specific pin out configuration. You set the operating current (and slew rate) with one external resistor - in this instance a 1M value sets the current draw to 80 uA for a very long battery life."

Here's some info on a pair of active humbuckers he constructed using the EMG plans;

"I wound two of the Stewart McDonald alnico humbucking pickups last week and attached two EMG style differential preamps on the bottom nickle silver base of each pickup. I soldered the through hole components on the copper track side of the PCB, then filed the leads flush with the plain side of the board. I inserted a small sheet of mylar overhead transparency film between the board and the baseplate to avoid shorting anything to ground. I soldered the earth bus at each end of the PCB to the baseplate of the pickup and ran a small bead of glue to hold it firmly in place. I laid the film caps and the electrolytic on their sides to keep as low a profile as possible and to assist soldering to the PCB.I wound around 8000 turns of 42 gauge enamelled copper wire (.050) on each bobbin and connected the start of each coil to the metal baseplate. 

The finish of each coil winding was fed to the non inverting and inverting inputs of my active preamp boards.

I wanted to retain the sound of a high output side by side humbucker, so I used a resistor in series with the non inverting input, unlike the EMG 81 which has no resistor fitted and consequently has a more single coil resonance. I used larger biasing resistors (330k each) unlike the 170k resistors fitted to the EMG 81, because i did not want to damp the resonance and shift it's frequency too far down - i suppose I could have used 1M resistors for an even brighter and higher resonance, but in practice the pickups sound very nice. I tried them out with my band mates at practice in the weekend and they were most impressed with the lack of noise and the smooth sound I was getting without any pedals.

It should be possible to fit the active preamp boards to most un-encapsulated humbucking pickups. All that is required is to separate the wire connecting the two coils in series and bring each coil winding out to the non inverting and inverting terminals of the differential preamp board - the existing output is simply grounded by connecting it to the baseplate."

One of Bajaman's active humbuckers

PCB Layout for active humbuckers

Here's a post regarding how each component can be altered to affect tone;

"The component values can be altered to tone model your pickup to your taste.I would encourage reducing the 33k in series with the non inverting input - possibly with an external trim pot to dial in the brightness of the pickup - reducing this to zero should give a more single coil like sound perhaps while still retaining the noise cancelling properties of the differential gain stage.

ALSO - the 22n capacitors can be decreased for less bottom end response OR the two 330k resistors can be reduced or increased in value - this will alter the pickups resonance - lower values should lower it's effect and possibly shift the resonst peak down in frequency - higher values (say 2.2M) should give a much more pronounced resonance at a higher frequency for a brighter sounding pickup.
The 1M resistor can be increased for still lower current drain but reduced top end response, or increased for a brighter top end but lower battery life."

And a post detailing some changes he made to his circuitry and the affect these had on the pickup's tone;

"I noticed that the high frequency end of the pickup's response was a bit restricted (perfect for jazz guitar players) but not enough "bite" for my liking. As mentioned in my last post i wound two pickups (4 coils), each coil wound with 8000 turns of .050 wire on Stewart McDonald alnico humbucking pickup kits. Most Gibson pickups used in Les Pauls and 335s etc. use 500k volume pots which load down the coil and tame it's high frequency resonance. If you have ever fitted a humbucker in a Strat style guitar with 250k pots you will know what I am talking about - the resonance and "bite" of the pickup is nowhere near as bright sounding with this lower resistance in parallel damping the coil's response.

Well - i took another look at my differential preamp design - I was using two 330k resistors to bias the non inverting op amp input to 4.5v dc. These resistors are effectively in parallel if one ignores the low impedance of the battery supply - or 165k, which is a very low resistance in parallel across the pickup coil, and here is the problem!!

For the differential amplifier to work correctly the feedback resistor from the op amp's output to the inverting input also needs to be this value ( 150k + 15k = 165k).
So how do we load the coils with 500k - simple, we use two 1M resistors as the bias resistors and two 1M resistors in parallel with each other for the feedback resistor. We do get a little more gain from the higher value resistors which is not necessarily a bad thing as it turns out because the sound is now a lot better with an almost identical "bite" to a genuine Gibson high output (not an SG) Les Paul pickup when the volume control is set to 7. The extra output gives a really nice fat response that easily overdrives most clean channels on tube amps etc.

I have A B tested my guitar fitted with this pickup and preamp configuration and another guitar i have fitted with two EMG81 pickups and it makes the 81s sound very thin and lifeless in comparison.
The humbucker "squark" is there in spades with no discernable noise whatsoever - even in front of a computer monitor.

When I turn the guitar volume from 0 to 10, I cannot hear any increase in noise whatsoever, which is eerie to say the least."

For those of you who can't find the LM4250 here's another active pickup circuit designed by Ed Tavares of Handmades from Brazil that uses the TL061;

So, you've got the schematics and the plans - go build yourself some active pickups and rock out! As a final note - people usually ask how to wire these things into their guitars so here's a diagram showing the most basic wiring diagram of an EMG 81 type active humbucker;

Here's the freestompboxes.org forum topic on the EMG 81 for reference; http://freestompboxes.org/viewtopic.php?f=3&t=880

Humbuckers - The Original Gibson P.A.F, Theory and Design Considerations

Humbuckers Dissected - image from StewMac.com  [1]

So, we've all played guitars with humbucking pickups right? The warm, chunky yet still cutting tone humbuckers can produce has lead to some of the greatest guitar tones in Rock 'n Roll. Just imagine; where would the world be with out Angus Young's signature humbucker equipped Gibson SG along with his sweet crunch tones and killer riffing on "Back in Black"? What would the Seventies have sounded like without Jimmy Page and his legendary Les Paul? Further more just imagine the grunge movement of the late Eighties and early Nineties without the searing, "wall of noise" created by Kurt Cobain's humbucker driven Fender Mustangs, Jaguars and Jagstangs?

Whether it's classic crunch or full on tonal meltdown you're looking for the Humbucker can get you there - with the added bonus of no "added" 60Hz hum that is associated with P90 and Single Coil pickups :-D

Jimmy Page of Led Zeppelin with his Humbucker equipped Les Paul

The Original - the Gibson P.A.F Humbucker

So what is a "humbucker" and where did the design original from?

By the early 1950's Gibson was being directly challenged by Fender as the major producer of quality solid body electric guitars, to counter the Fender charge on the market Gibson decided to develop a high quality, low noise pickup with which to equip their newly introduced "Les Paul" model guitar. Seth Lover was the engineer assigned with the task of developing this new product. In order to solve the inherent noise problems of the P90 and single coil pickups Lover decided upon connecting two side by side single coil pickups in series but with opposite magnetic polarities and opposite current travel in either coil. The product he developed changed the sound of electric guitar forever - the Humbucker was born!

Original "Zebra" P.A.F Humbuckers [2]

Gibson filed their patent for the pickup design on June 22, 1955. And by 1957 the humbucker was standard issue on most of Gibson's products. However - the patent wasn't fully granted until July 28, 1959. This led to Gibson sticking on the little black "Patent Applied For" decals on the back of those early humbuckers (from late '57 till mid/late '62) hence the "P.A.F" abbreviation. [2]

Original Gibson P.A.F humbuckers with black decal [2]

Here's another interesting piece of info regarding those early Gibson pickups:

"Seth Lover received his pickup patent #2,896,491 on July 28, 1959. By mid to late 1962, Gibson changed the pickup decal to read, "PATENT NO 2,737,842". Interestingly the patent number listed on the decal was not for Seth's pickup design but was for Les Paul's trapeze tailpiece! Perhaps this was a research roadblock for the competition, or maybe just a mistake?" [2]
And an extract from the actual humbucker patent granted to Seth Lover:

An extract of the original Humbucker Patent filed by Seth Lover

How do Humbuckers buck the hum?

Now we know where the original P.A.F came from I'll offer a quick explanation of how the technology works to give us a general idea of what is going on between those coils!

The image below shows how the humbucker pickup is laid out:

(fig.1) Basic Drawing of a Humbucker Pickup [3]

As you can see the two coils have opposite magnetic polarities; the top coil is "North" and the lower coil is "South" (in a regular humbucker the South pole coil is the one with adjustable screws, the North has non-adjustable metal slugs). Further more note the coil windings; they are both wound in a clockwise direction from their starts to finish but, and here's the key, look at the direction the current travels in each coil; in the North coil it travels in a clockwise direction from red to black yet in the South coil it travels in an anti-clockwise direction from the "series connection" (the point at which the two coils are electrically joined by the black and white wires) and the white wire through to the green wire. In a standard humbucker the red wire would be your "hot" or signal connection and the green would be the grounding wire (note that different humbucker manufacturers use different coloured wires so your connection wires won't always be red and green).

Single coils Vs Humbuckers

So, how does this configuration allow the humbucker to perform it's duties and "buck the hum" associated with it's single coil brothers? Well I turn to this handy explanation:

(with reference to fig.1) "Coil 1 and coil 2 are electrically out of phase and since electrical interference only travels through the guitar pickup's windings, when the hum travels from one coil to the other, it gets canceled due to the opposite coil windings. (The magnets play no part in hum cancelation.)

Unlike hum, an electric guitar's sound (the signal) is generated from the pickup's wiring and magnets. So, when the guitar signal travels through the coils, no cancelation occurs because the coils are out of phase electrically and magnetically.

In effect, having opposite electrical and magnetic polarities is similar to putting two phase switches on two pickups. When both switches are set to the out of phase positions, no cancelations occur because the 2 "out of phase" conditions cancel each other out." [3 - http://www.1728.org/guitar1b.htm]

Wikipedia (although I do dislike referencing wikipedia!) also has some useful info (al be it slightly incorrect - my edits in brackets []): 

"In any magnetic pickup, a vibrating guitar string, magnetized by a fixed magnet within the pickup, induces an alternating voltage across its coil(s). However, wire coils also make excellent antennae and are therefore sensitive to electromagnetic interference caused by alternating magnetic fields from mains wiring (mains hum [60 Hz hum]) and electrical appliances like transformers, motors, and computer screens. Guitar pickups reproduce this noise, which can be quite audible, sounding like a constant hum or buzz.

The direction of a voltage induced across a coil by the moving string depends on both the coil winding direction and the direction of the fixed magnets. A humbucker has two coils wound in the [same direction but with opposite directions of current travel]. The magnets in the two coils are arranged in opposite directions so that the string motion induces voltages across both coils in the same direction. Electromagnetic interference, on the other hand, induces voltages in opposing directions across the coils because it is only sensitive to the winding direction. When the signals from both coils are summed together, usually by connecting the coils in series, the two noise voltages cancel, while the signal voltages add thus dramatically improving the signal-to-noise ratio."  [4 - http://en.wikipedia.org/wiki/Humbucker#How_humbuckers_work]

Still not sure what differentiates a single coil pickup from a humbucker? Check this video out:

Back to the P.A.F - Design considerations

Now, many guitarists go crazy for the original P.A.F tone, the original pickups sell for hundreds and examples of late 50s and early 60s Gibson guitars with P.A.F humbuckers sell for tens of thousands. Many of todays boutique pickup manufacturers offer a straight clone of the P.A.F and charge a healthy price for it. Below is a video of the (very nice!) Bare Knuckle Pickups "The Mule" which is a faithful reproduction of the original P.A.F:

So how were the originals made and what makes them sound the way they do? Firstly lets refresh ourselves with the humbucker and it's components, fig 2 shows you all the internal parts of the humbucker:

fig 2. Internal humbucker structure

As you can see from fig 2 there aren't a huge amount of parts involved in the construction of a humbucker. However the parts do need to be put together concisely, taking into account a number of key tonal factors to ensure a decent sounding pickup is produced. These tonal factors are wide ranging and cover all aspects of the pickup's design and construction and, indeed, many guitarists will insist that very strange factors (with no scientific or technical evidence to support their claims) will affect a pickup's tone (e.g - it was made on a Tuesday by "Ron" - it's all in the "mojo"). However, we're not interested in the far out theories on tonal excellence regarding the P.A.F humbuckers, we're interested in the *real* reasons the P.A.F humbuckers sound the way they do and these, with reference to various sources, are [5][6][7]:

• Magnet type,
• Wire quality, gauge and insulation,
• Number of coil windings,
• Baseplate and covering,
• Overall coil shape,
• Coil winding pattern, and
• Potting.

Now, lets investigate each of these factor in more depth. Firstly;

Magnet Type

Humbucker magnets - from left; ceramic, alnico 5, 3 & 2. [8]

The type of magnet that a humbucker uses is one of the most important factors in deciding it's overall tone. There are essentially two types of magnet that are widely used in humbucking pickups; ceramic and alnico (aluminium, nickel and cobalt alloy). The key factor in determining the tone that a magnet will impart upon a pick is it's overall magnetic strength. The strength of a magnet is noted by the number after it's material; the higher the number the higher the magnet strength. Ceramic magnets are more powerful than alnico and due to the increased cost of higher strength alnico magnets they are usually selected when you need more power than an alnico 5 magnet (although you will see Alnico 6 and 8 pickups about). So what difference does this increase in magnet power make to the tone of the humbucker? Generally the higher the strength of the magnet the more emphasis on the highs and upper mids, this usually sees higher strength magnets being described as "bright" and "brittle".

Humbuckers that utilise coils wound with a higher DC resistance and therefore a hotter output often use stronger magnets, and their increased high end response, to compensate for the inherent high end losses caused by the hotter coil windings.

Here's a concise description of the differences between alnico and ceramic from the GuitarTechCraig website:

"Alnico was used in all popular vintage pickups and generally has a warm and smooth response. Ceramic magnets tend to boost treble response and have become popular for pickups with extra coil winding to compensate for the treble loss. Alnico magnets also lend to a smoother distortion tone with a prominent midrange while ceramic magnets can improve the clarity and high end grind of a distorted tone. There are many different formulas of Alnico and Ceramic magnets. In general, the stronger the magnet can be magnetized, The brighter it will sound in a pickup." [9]
Furthermore, Tim, from Bare Knuckle Pickups, offers this more in depth break down and description of the various magnet types:

"Magnets do add to the character of a pickup although it must be understood that a magnet doesn't have a sound on it's own, it contributes by the way it accentuates certain frequencies as current is produced in the coil windings. 

Alnico II is the softest and generally has a smooth bass and treble although this is more pronounced the hotter the windings get.

Alnico III is very transparent, low output and clean,sounds great for rounded fat jazz applications-typical of '50s tone.

Alnico IV is probably the best vintage tone IMHO(for humbuckers) and along with II and III was used in the earliest PAFs-this is a fact and not myth as we've had them analysed and a collegue of mine has also seen original Gibson purchase orders that clearly state AIV bar stock being purchased.The tone of AIV is balanced and extremely organic, it produces the most authentic vintage tone and sits better in slightly hotter vintage winds than AII which tends to get very soft in the bass and highs if used incorrectly. 

Finally Alnico V is the hottest producing more highs and lows, great for rock applications or where power and cut are important.

Different companies use different grades for personal reasons, we use all of the applicable Alnico grades to suit the correct design, both to be historically correct but more importantly to have the best sound.

Changing magnets in a humbucker can give dramatic results, you soon find the ones that really don't sit right and others that are head and shoulders better........

[regarding ceramic magnets]

We use ceramic 8 as do most pickup makers-it is more powerful and essentially more efficient so the resulting tone usually has a very fast tracking bass response with a distinct cut in the highs.Some players find them cold/hard when run clean and they can cause alot of compression from their relatively hot output-again, depending on how you use them, they are capable of good clean tones too but the general consensus is that Alnicos are sweeter run clean." [10]

This Bare Knuckle Pickup's video provides and insight into the differences between ceramic and alnico magnets and gives you and idea of how magnet strength affects the overall tone of the humbucker:

So, there are the differences in magnet types. Getting back to the P.A.F; what did the original use? Well, inconsistency was king at Gibson in the late 50s and 60s and a range of magnets were used; Alnico 2, 3, 4 & 5. It is suspected that the majority of P.A.F era humbuckers utilized alnico 4 (IV) magnets as backed up by Tim from Bare Knuckle Pickups;

"Ask Gibson historian Walter Carter to look up the purchase orders for the mid to late '50s for magnet bar stock or ask Tom Holmes(ex Gibson) who's also seen the records which show Alnico IV bar stock being purchased during this period.The X ray spectrograph analysis showed a 1959 PAF humbucker magnet as being Alnico IV."

Tim also uses the Alnico 4 magnets in his "The Mule" P.A.F replica. This inconsistency in magnet type continued at Gibson right through the late 50s and early 60s, however by 1965 Gibson was using Alnico 5 magnets in all their humbuckers. [2]

You should also take into account the size of the magnet; "The original PAF magnet length was 2.5" long, which was decreased by 1/8" to 1/4" to around 2.25" in July 1961." [2]

And further more;

"Dimensions of PAF magnets follow (measured using a micrometer, and obviously this will vary a bit from magnet to magnet): 2.509" long ("long magnet" version), .506" wide, .131" thick. The "short magnet" PAF length was the a bit different: 2.371" long, .491" wide, and .121" thick." [2]

Wire Quality, Gauge and Insulation

Different wire types - note the plain enamel insulated and it's purple colour wire to the right.

Take this description from skguitar.com, which covers everything in general about humbucker wire selection:

"The finer the wire the fewer highs pass through. Finer wire also has a higher resistance for a given length. For this reason a pickup wound to 6k with 43awg [thinner] wire will have less output than a pickup wound to 6k using 42awg [thicker] wire. Regardless of what has been said I do no believe there is any difference based upon type of insulation alone. However, wire with a heavier insulation (i.e. "double build") will result in a wider coil at the same resistance/output [altering the coil shape alters the tone too].

There is a characteristic of wire called "skin effect" which is more pronounced in smaller diameter wires due to thier higher restance. "Skin effect" is where the higher frequencies travel along the surface of the wire rather than travel thru the center of the wire. This leads to that "brittle" characteristic of high output pickups which must be wound with the finer wires." [5]

As you can see the choice of insulation has no direct impact on the tone of a humbucker, it may, however, have an indirect impact on tone due to alterations in coil shape and thickness.

So, once again - back to the original P.A.F; what type of wire was used in their construction? 

It is widely acknowledged that the original P.A.Fs were wound with 42AWG wire with a plain enamel (which gives it a purply colour); 

"The pickups were wound with #42 plain enamel wire. On original PAFs the bobbin wire appears purple, versus later PAF and patent# pickups that appear reddish. Gibson eventually switched to polyurethane coated wire around 1963." [2]

Also note you want good quality wire that has a regular gauge without large fluctuations and without any voids associated with the tooling of wire. If you can get wire that has been annealed (heat treated to remove voids and defects associated with tooling) you're onto a winner!

Number of Coil Windings / DC Resistance

Winding a pickup bobbin

The more windings on a coil the higher it's DC resistance, inductance and output. A higher number of windings gives a more girthy, mid range dominated pickup while more highs are rolled off due to an increased coil inductance and capacitance. Take this explanation from the MusicalIlluminism blog;

"Fewer winds will have an audible effect, because the pickup will have less inductance, which affects the frequency response – making the pickup brighter. The pickup inductance interacts with the guitar volume/tone controls, guitar cable capacitance, and amplifier input load to create an EQ network. More inductance causes more highs to be lost in this EQ circuit."

So what are the specs on the vintage P.A.F?

"Due to the human factor and the wide tolerance of the manually-run pickup winding machines used by Gibson from 1956-1961, PAF pickups usually measure between 7.5 and 9.0 thousand ohms (K ohms). By 1962 (the end of the PAF era), Gibson was making pickups very consistently with 7.5k ohms of wire (give or take .25k ohms).

The separate bobbins of a PAF can measure very differently due to Gibson's manufacturing techniques. For example one bobbin could measure 3.5k, and the other 4.5k ohms (for a total of 8k ohms). This mis-matched ohms is actually a good thing, as certain frequencies will stand out if both bobbins have different resistance. This contributes to why two PAF pickups can sound quite different." [2]

As you can see there is a mention of mis-matched coils, usually humbuckers sound best if the the bridge humbucker has it's slug coil wound with a higher resistance than the screw side, this gives it slightly more mid grunt and avoids it becoming to treble heavy. For a neck pickup the opposite is true, with the screw side overwound the emphasis is more on the open strings of the neck and gives a more open and natural tone (the emphasis is on the part of the string that you want to hear!). [13] This is personal preference though and if you play about with which coil you offset you'll find your favourite. I'd try winding with offsets from a few hundred turns right up to a thousand turns (I, personally, usually go for about 250-300 turns offset). For further insight take this explanation into coil offsets from Tim, of Bare Knuckle Pickups;

"Coil offsets do effect mid range but not quite to the extremes you may think. What's actually happening is the moment there is asymmetry between the coils phase cancellation isn't 100% any more. Aurally this means more highs come through and the 'perception' of midrange alters.If I wound 2 coils with 5000 turns each they would be perfect humbuckers and cancel out 50/60 cycle hum efficiently. If I wound 5150 and 4850 which would produce almost the same DC reading, you'd hear high end which balances against the mid range giving the impression of it dropping back. As you're no doubt aware when you use a graphic EQ, altering the phase of one cycle tends to knock the phase of the others-how much is perceived and how much actual I wouldn't like to hazard a guess as it'd take some serious analysis to verify.

Bottom line, anything to do with altering coil shape, size, no. of winds, offsets between coils etc will effect tone.

Before anyone panics about offset coil pickups not being 100% true humbuckers, panic not..........in my experience you can offset coils by as much as 1000 turns before noise becomes audible.This is dependant on wire gauge and overall no. of turns.................generally the offsets we use are nothing as extreme and no noise is audible at all." [14]

And here's another explaination of the offset effect from MDV over at the Bareknucklepickups Forum;

"The hum cancelling effect of humbuckers is a 1:1 turn per turn effect. Turns wound in the opposite direction and powered by magnetic fields going in the opposite direction (RPRW; reverse wound, reverse polarity) cancel the natural noise that each turn makes.

That means that, as nolly alludes to, if wind is assymetric then the excess turns on one side behave as single coil turns because they dont have corresponding turns to cancel them, and have more top and more bottom, but noise is not cancelled. This gives more clarity and depth to the pickup (but mainly clarity/top end).

Its also true that the noise isnt noticable and bk offsets are well below whatever the offset threshold for noise is." [15]

With a little searching I found a few specs for Gibson "Burstbucker" Humbuckers (The Gibson replica of the original P.A.F) and their individual coil windings;

Pickup---------Screw Coil---------Slug Coil
BB-1 ---------- 3.90k -------------- 3.99k
BB-2 ---------- 4.16k -------------- 4.22k
BB-3 ---------- 4.41k -------------- 4.54k
BB-3 ---------- 4.30k -------------- 4.80k

As you can see from these figures the offsets aren't huge; just enough to introduce a little play in the high frequencies. Remember - to much offset and you'll reintroduce 50/60Hz hum back into the pickup!

If you play with higher gain settings you may like both coils to be matched as closely as possible for a smoother more mid dominated humbucker.

Regarding the total DC resistance, you can see the original P.A.Fs vary between 7.5K and 9K. I wind my bridge humbuckers to 8.3-8.5K and my neck humbuckers to 7.3-7.5K. This gives a decent output in the bridge position that breaks up well yet still cleans up with the guitar volume control. The lower output in the neck position gives a more rounded, natural tone with the warmth associated with the neck position.

Baseplate and Covering

If the baseplate is metal and not grounded it will increase the inductance and capacitance of the humbucker, therefore rolling off more highs and adding lows and mids. The cover has the same effect. Now "early P.A.F. pickups as used on the 1956 lapsteels and 1957 Les Paul Standard had brushed stainless steel pickup covers (brushed to make them look nickel plated). This quickly changed to brass covers with a nickel plating. If the cover was gold, the brass was first nickel plated and then gold plated. Early PAFs also have four brass bobbin attachment screws, instead of steel screws." [2]

On pickups I've built I've always used Nickel Silver base plates and Nickel Silver covers and they've always sounded good. The difference between covered and uncovered humbuckers is illustrated nicely in this video:

You can hear the subtle differences - the slight increase in highs of an uncovered pickup versus the rolled off highs and increase in mids of the covered pickup. The choice is yours!

Overall Coil Shape

Short fat coil bobbins

Tall thin coil bobbins

"A coil which is taller and narrower will be clearer, more focused and slightly brighter than another coil which is shorter and wider if they have the same output. This is because the shorter wider pickup is sensing a wider section of the string, which gives it more variance in the signal it's sensing (due to variance in oscillation size/ pattern of the string along the length of string sensed)." [5]
Now, all humbucker bobbins are made a standard height, the only difference will be in the width of the coil. The use of a thicker gauge wire will increase the width of the coil, a lower or higher winding tension will also alter the shape of the coil as will the use of wire with thick insulation.

A well wound medium tension coil from 42AWG plain enamel insulated wire should produce the classic  P.A.F coil shape; a full but not overwound bobbin.

Coil Winding Pattern

A pickup wound by machine will have a very regular and even wind pattern, a pickup wound "by hand" (i.e guided onto the bobbin by hand) will have a much less regular wind pattern - this is called "scatter winding" and many guitarists believe it gives a much superior tone when compared to regular machine wound pickups. Tim, of Bare Knuckle Pickups, scatter winds all of his pickups;

"Scatter-winding can only truly be done by hand and represents a high degree of skill by the person winding the coil. Although time consuming, it has many advantages over conventional machine winding and mass-production, not least the far superior tone and dynamics produced. We deliberately scatter the wire as we build up the windings of the coils so the wire isn't as even turn on turn, layer on layer, as with the more uniform wind of an automated machine with pre-tensioning. This lowers the distributed capacitance that exists between the turns of wire. Lower capacitance means improved high-end clarity, the resonant peak increases slightly and frequency response is greatly extended. The tension of the wire is also varied as it moves through the operator's fingers reflecting the ability to control the tension within the coil by the person winding the pickup. The result is a clearer, more open tone that has the impression of being louder purely by the amount of extra detail and dynamics present." [12]
The original P.A.Fs were machine wound but the use of scatter winding is certainly one way to improve upon the tone. Lets face it - if we're winding our own pickups they are going to be "scatter wound" unless we have access to a proper pickup winding machine.

Potting

Potting humbuckers in wax

Potting involves dipping the finished pickups in wax or other similar materials (epoxy or lacquer) to solidfy the coil windings and remove unwanted microphonic feedback (those horrible squeals you sometimes get at high gain). The original Gibson P.A.F humbuckers were not potted but most modern reproductions are. I always pot the humbuckers I wind, it's a simple process (submerge the pickups for 10 mins in parafin wax at 65 degrees centigrade) and in my opinion the positives out weigh the negatives. But don't take my word for it. Once again with reference to Bare Knuckle Pickups;

"Unpotted coils have a very dynamic and touch-sensitive feel with a brighter edge which is well-suited to low gain playing, while potted coils have better feedback rejection and are much more practical for any playing style where high amounts of preamp gain and loud volume are used. We do make pickups with unpotted coils as an option, although we lacquer the magnet and use paper tape inside the cover to help reduce squealing, just as in the early humbuckers.

Historically, not all pickups were potted but with modern high gain amplifiers and performance this isn't always practical. Nearly all our finished coils are potted in a mixture of paraffin wax and bees wax to remove unwanted air trapped in the coil, solidify the windings and prevent microphonic feedback. All of our high gain models or any pickups that require metal covers are wax potted a second time to eliminate any chance of microphony." [12]
Once again, this choice is yours!

So there you have it; how humbuckers came about, how they work and how the various design considerations effect the overall tone that the humbucker will produce. Below are some specs for the various Gibson P.A.F era humbuckers.

Various Gibson P.A.F era Humbucker specs

"1956 – 1957 (“PAF”): Long (2.5”) Alnico 2, 3, 4 and 5 magnets used randomly, brushed stainless steel cover, *no* PAF sticker, automatic traverse wound with manual-stop (until bobbin was “full”), #42 plain enamel wire (purple/maroon), individual coil ohm differences, black leads on coils, ohms vary from low 7k to high 9k, black PAF-style bobbins (“square in circle” with holes). PAFs first installed on Gibson lap-steels in ‘56 and then guitars in ‘57.

1957 – 1961 (“PAF”): Long Alnico 2, 3, 4 and 5 used randomly (A2 most common), nickel cover, “Patent Applied For” sticker, automatic traverse-wound with manual-stop, #42 plain enamel wire (purple/maroon), individual coil ohm differences, black leads on both coils, ohms vary greatly – generally between 7k and 10k, black and cream (early-’59 thru mid-‘60), all bobbins black again by late ’60, PAF-style pickup bobbins.

1961 – 1962 (Late “PAF”): Smaller (2.37”) Alnico 5 magnet used for remaining production (all transitioned by July ’61), nickel cover, PAF sticker, automatic traverse-wound with manual-stop, #42 plain enamel wire (purple/maroon), black leads on both coils, individual coil ohm differences, ohms averaged 8.0k by ‘62, PAF-style bobbins." [11]

References;

[1] - http://www.stewmac.com/shopby/product/5961?tab=Pictures
[2] - http://home.provide.net/~cfh/paf.html
[3] - http://www.1728.org/guitar1b.htm
[4] - http://en.wikipedia.org/wiki/Humbucker#How_humbuckers_work
[5] - http://www.skguitar.com/SKGS/sk/pickup_factors.htm
[6] - http://www.seymourduncan.com/tales-from-custom-shop/
[7] - https://bareknucklepickups.co.uk/main/howwemakethem.php
[8] - http://www.projectguitar.com/tut/barmagswap.htm
[9] - http://www.guitartechcraig.com/techpckp/pickups.htm
[10] - https://bareknucklepickups.co.uk/forum/index.php?topic=8005.0
[11] - http://musicalilluminism.wordpress.com/2009/04/28/vintage-gibson-humbucker-specs-and-general-pickup-tech/
[12] - https://bareknucklepickups.co.uk/main/howwemakethem.php
[13] - http://www.zhangbucker.com/humbuckers.html
[14] - https://bareknucklepickups.co.uk/forum/index.php?topic=4582.0
[15] - https://bareknucklepickups.co.uk/forum/index.php?topic=22683.0