Previously, we discussed impedance in the quest to gain a basic working understanding of the physical properties of pickups, all to debunk the ‘DC Resistance – output congruency’ myth. To recap, the DC Resistance is a resistance measurement of the coils, in what I called a Direct Current environment, but pickups produce an alternating current (AC), and an AC environment works differently than a DC environment. This is not the only thing that goes on in pickups. The physical properties of the coils themselves (and generally speaking, the pickups) have such a specific set of parameters that makes using DC Resistance as a measure of output somewhat incorrect. Let’s investigate further!
Simply put, a pickup is a large strand of copper wire, wrapped thousands of times around a holder (called the bobbin) forming a coil, and somewhere stuck to the coil you have a magnet. In some very simple designs the magnet itself can be the bobbin! The wires of a pickups are extremely thin – something like the thickness of hairs, maybe even thinner. But there is no such thing as a ‘one size fits all,’ standard wire. Generally speaking there are three wire thicknesses (gauges) being used. These wires have an industry standard gauge, called AWG: American Wire Gauge. The lower the number, the thicker the gauge. For example, 1 AWG is 7.35 mm thick, 10 AWG is 2.58 mm thick, etc etc.
The Custom Shop has over a dozen grades of magnets.
For guitar pickups, the most common gauges are 42 AWG, 43 AWG and 44 AWG. There are other gauges too, but the aren’t as frequently used. The effect of having a thinner wire is dramatic. Because it’s thinner, you can wrap more wire around the bobbin, in effect lengthening the strand of copper wire. And the longer the wire, the stronger the signal will be that the coil will induce (I’ll explain what that means later).
This is all nice and lovely, but how does that translate in a practical application? In other words, what do we, as guitar players and pickup pickers, experience of all of this? The differences in thickness are so small, they can hardly make a real difference. Can it? Yes, it can! Let’s take a standard humbucker bobbin and wrap it full with wire. Let’s start with 42 AWG. This is the wire that was used by Gibson to make their P-90s and Patent Applied For humbuckers in the 50s. If we wrap it around a bobbin and try to squeeze as much as possible on the bobbin, we can go as far as 4.7 kilo ohms and if we squeeze very hard, maybe even 5 kilo ohms, for each bobbin (thus, each coil). For the ease of visualization I am referring to the resistance of the coil as an indirect measure of length. It is a long and rather complicated calculation to see how many turns 4.7 kilo ohms of 42 AWG actually is. You have to take into consideration that winding the bobbin is a three-dimensional job. In order to get any perspective, I’ll compare the gauges by taking several resistances with several wire gauges. So, how long is a strand of copper wire of 42 AWG, with a resistance of 4.7 kilo ohms? (Warning here comes so math, skip below if you’d rather continue along).
Physics teaches us a relation between wire cross-sectional area (measured in square meters), resistance (measured in ohms, Ω) and the length of the wire (measured in meters), with a basic parameter called the resistivity, ρ, which is not the same as resistance. The resistivity of copper is about 1.7 x 10-8 Ωm .
The whole formula is as follows:
R = ρ x (l/A),
where R is the resistance, ρ is the resistivity of the material, l is the length of the wire and A is the cross-sectional area of the wire, (Pi x (0.5 x diameter) x ( 0.5 x diameter).
Rearranging the formula to solve for l gives us: l= R x A/ρ . All we have to do is just fill in the numbers! The result? 885 meters.
Recap. By knowing the DC resistance of the copper wire and knowing the thickness of the wire, we know how LONG the wire is. For a 4.7kΩ wound with 42 AWG, we get a length of 885 meters. By doing the same thing, only for 43 AWG (same resistance!) we get 685 meters and for doing the same for 44 AWG we get 566 meters. Thinner wire means being able to wrap more wire on a bobbin. This doesn’t mean that the pickup maker has to fill up the bobbin with wire. Sometimes it’s a conscious choice not to completely fill it up. But by having less wire on the bobbin, the pickup maker can get a sound they had in mind. The rule of thumb is that having less wire gives a clearer sound with more highs and softer mids, and using more wire, you bump up the mids but the high end has to suffer a bit. By choosing a specific winding pattern, wire gauge and magnet, the pickup maker can make a pickup that has the exact characteristics that are needed. This is because the coil acts like a capacitor and if the coil is big, the high frequencies are being ‘bled out,’ like a capacitor (compare it to a capacitor in series with your pickup’s signal to bleed out some of the highs). Again, the DC resistance doesn’t tell the whole story…
Using Lenz Law, which gives a relation between the inducted voltage, magnetic field strength and the coil itself, we can stipulate that having a bigger coil means more output of the coil (if the magnetic field remains the same).
But wait: how does that work?! Now we’re talking about coils and output again?!
That’s, right. The coil can be seen as being ‘suspended’ in a magnetic field, created by the magnet. But when the magnetic field changes (whenever you hit a string, the magnetic field changes, correspondingly with the change of the string, because the string is made of a metal that can influence the magnetic field, to put it simply), it induces an AC signal in the coil! This reaction is called inductance, and is measured in units called Henries. The higher the inductance, the more henries you have, and the more output you get, and the inductance itself gets bigger if you have a bigger coil, and a bigger coil gets back to being simply more winds. Let’s take a look at how inductance works and what it ‘looks’ like in a real day scenario:
So… pickup strength can not be accurately measured by the DC resistance alone, because we simply don’t know for sure what wire gauge is being used! A pickup that says 10k on the package can be a supertightly wound 42 AWG pickup, but also a 43 AWG pickup with less turns on the coil. And of course, the DC Resistance doesn’t take the type of magnet into account. Pickups with the exact same coil might have a huge drop of output with different magnets. Just look at Seymour Duncan’s custom- line. They all share the same coils, same winds, same wires, but the Custom Custom has noticeably less output than the Custom 5, which in turn has less output than the Custom itself.
Some pickups use two completely different coils, such as the 59/Custom Hybrid. Each coil is wound with a different gauge wire giving you different resistances and different outputs. The DC resistance of the 59/Custom Hybrid is around 10k ohms, just as the Screamin’ Demon, yet, the 59/Custom Hybrid has much more output than the Screamin’ Demon. It isn’t the magnet that’s different, they’re both alnico 5. It has to be the coils that are different, and they are.
In short. The term DC resistance as a means of viewing outputs of pickups is incomplete as their are other factors that need to be considered. Not only that, it ignores the fact that the resistance is a result of several parameters, such as wire gauge and subsequently, the length of the wire. It also ignores the tendency of the human hearing to hear midrange more easily than high frequencies: a pickup might have lots of output in the lower and higher frequencies, but might be perceived is a weaker, softer pickup than a pickup that has, objectively, less output overall, but a higher bump in the mid range. The resistance simply doesn’t tell the whole story and makes it in my book a wrong indicator of output or tonal characteristics.