In this three part series, I will explain the significance and interpretation of pickup-design variables and their effect on resistance, inductance, and resonance, in the context of the string centric model of pickup function. I’ll try to keep it as straightforward as possible, but some of these concepts can get a little deep so there’s only so much you can simplify it. It’ll also jump around a little bit as these parameters all interact to form the complete system that is a guitar pickup.
Before we get into the simplest parameter to understand, resistance, let’s look at what makes up a pickup and how it really works.
The simplest pickup that we typically encounter is the Stratocaster-style single-coil.
The Strat pickup consists of a coil wound around six cylindrical pole pieces. In the classic Strat-style pickup, the pole pieces are also magnets and they serve to magnetize the string in the region directly above the pole piece. This apparently simple system is actually deceivingly complex. The coil/pole piece system has associated with it: an electrical Resistance (R), an Inductance (L) and a Capacitance (C) (I’m generally not splitting capacitance out as a separate parameter, but I will talk a little bit about it here in the context of the coil, and a lot about it in the third part where we cover resonance). This makes the pickup an RLC circuit all by itself, and as such the pickup will have a resonant frequency. Due to the values of these three parameters the resonance typically falls in the audio frequency range (20Hz-20kHz), and it’s the position and shape of this resonance, along with the absolute value of the output, that mostly define the “tone” or “timbre” of an electric guitar pickup.
Even more, the pickup is connected to a system of variable resistors (tone and volume potentiometers, or “pots”) and typically at least one capacitor. I’m not really going to talk about the implications of adjusting the pots, that’s a whole different discussion, so for the purposes of this series we’re only going to consider what happens when the pots are on “10”, that’s complicated enough.
Other pickup designs get more complicated than the basic Stratocaster pickup. Some pickups, like P90s or so-called “ceramic” import Strat pickups, utilize ferro-magnetic metal pole pieces, usually steel, with external magnets to charge them. Humbuckers utilize two opposing coils (to cancel hum) each with their own metal pole pieces and charged to opposite magnetic polarities with an external (to the coils) magnet or magnets, as illustrated in Figure 1. Covers, baseplates, shields or other metal parts may be added for various design and cosmetic reasons.
But in terms of thinking about all of these complex design variables, a very simple physical picture of pickup function can help inform our understanding. It seems that people can get hung up on the idea that “a pickup doesn’t need a magnet”. While basically true, this isn’t the point. The point is to think of a pickup as a receiver of flux, not as a generator of a magnetic field. The magnetic field of a pickup is simply there to magnetize the string. Once the string is magnetized, and especially in terms of how signal gets generated, we can simply accept that the string is magnetized, and think about the pickup as a receiver of the flux from this vibrating magnet.
Theoretically, yes, it’s true that a pickup does not need a magnet to make a signal. Practically though, the string needs to be magnetized, and the most efficient way to do this is to incorporate a magnet into the pickup. From a design perspective, making the pole pieces magnets themselves in order to magnetize the string directly is a great idea, so that’s why we do it.
The strength of the field that the string is in, and hence how strongly it becomes magnetized, will also be a function of its position relative to the magnets that are charging it. So obviously and from a practical perspective, the magnetic function of the pickup is important and necessary. But again, the important part in how a pickup develops a signal, and in how pickups sound, is to think of the pickup as a receiver. Even here, magnets are important, not because of their properties as magnets, but because of how they interact with the magnetic flux emanating from the string.
In fact, the only absolutely necessary part to make a pickup is a coil. If we could magically magnetize a string above an empty pickup coil with no other parts, just a coil: the pickup would produce a signal when the string was vibrating. And even further, the only information that is emanating from the vibrating, magnetized string that gets turned into signal is the flux that passes through the center of the coil. Nothing outside the boundary encompassed by the coil gets “picked up”. So from a signal generation perspective, all of the other parts of the pickup are only important in so much as how they affect the magnetic flux that goes through the center of the coil.
None of this should be a revelation, really. It’s simply the most straightforward interpretation of pickup function, informed by the basic physics that were performed by the guys that discovered this stuff. Faraday’s Law of Induction states: “The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.”
That’s all a pickup is, an inductive sensor. An electrical signal is induced in the coil by the time varying magnetic flux that passes through the coil (provided by the vibrating string). At the most basic level this is all that happens; a variable magnetic flux passes through the volume encompassed by a coil and a signal is generated. Everything else is just modifying that basic physics.
For example, we can think of pole pieces and other metallic parts of the pickup as a funnel that gathers up this flux and a system of magnetic pipes that help direct it through the center of the coil and back around to the string (remember that all magnetic flux must form a complete loop back to the source).
Some pickups, like Stratocaster pickups with their AlNiCo pole pieces, act more like an empty coil, where the magnetic flux from the vibrating string is barely affected by the presence of the pickup materials.
But even in this case, there are more subtle effects, and these effects are important in how pickups sound and what we like about that sound.
So that’s the gist of it. I’d like you to keep that picture in mind as we discuss pickup design, specifications, and function. Before we talk about resistance specifically, let’s look at one example of some of these effects in action to hopefully bring it home a bit.
Figure 2 (below) illustrates three cases that cover a lot of ground, including most of the ground covered by the most popular pickup designs.
In the case of no pole piece, the flux field radiating from the string is symmetric about the string with nice, even field lines and field contours (a contour is a line of constant flux density) that form concentric circles around the magnetized portion of the string (in this 2D simulation, anyway – note that in this visualization we are looking straight down the string, such that the string appears as a circle at the center of the flux field).
Basically in this case, the coil is sitting in a field, but not influencing the field in any way. If we put something like an AlNiCo 5 pole piece inside the coil (like a Strat-style pickup), the field starts to get pulled into the pole piece a little bit, increasing the flux density in the volume encompassed by the coil. If we put a piece of steel inside the coil (like one coil of a humbucker), the field gets pulled into the core of the coil even more, and we can even see an effect of blocks of AlNiCo 5 at the base of the pole piece.
And remember, in this model we’re only looking at the AlNiCo as a block of metal, not a magnet at all. The only source of magnetic flux in Figure 2 is from the string. So you can see pretty clearly how construction differences in pickups can influence the flux that goes through the interior of the core, and it is these differences in the magnitude of the flux as well as how this flux gets, in effect, “filtered” by these metal parts, that define what pickups sound like.
Let’s get into resistance, then. Resistance is a spec that is tied pretty tightly to the coil itself, since it is a measure of the difficulty in passing electrical current through the coil. Resistance is measured in Ohms, and the definition of an Ohm is the resistance of a circuit where one volt of potential gives rise to a current of 1 Ampere. You can think about the coil wire as a pipe that the signal has to get through. The narrower the pipe, the harder it is to push the signal through and the higher the resistance will be.
Figure 3 (below)shows the relative cross sectional area of the typical wire gauges used in guitar pickups.
The fattest wire, 42 gauge, has roughly 50-percent more area to transmit signal than the 44 gauge. Consequently, the 44 gauge wire exhibits about 50-percent higher resistance per unit length compared to the 42 gauge wire. Pickup coils use a lot of wire too, with roughly half a mile of it wound on a typical Strat pickup (7500-9000 turns of wire in a typical strat pickup at a few inches per turn), so even though the resistance per foot of wire is pretty low, it adds up. Guitar pickups have a resistance of thousands of Ohms, even tens of thousands of Ohms in some cases.
So what does resistance tell us about the coil?
If we know what kind of wire was used, then the resistance of the coil is a pretty good indicator of how much wire is in the coil. Since the length of wire in the coil is directly related to the number of turns in the coil, under certain limiting conditions resistance can be a very good indicator of pickup output and tone. As we’ve said, the only thing that is absolutely necessary to have in a pickup is a coil (assuming a magnetized string). So obviously the characteristics of the coil: including the shape, which determines the area of magnetic flux encompassed by the coil; and the amount of wire, which determines number of turns around the encompassed flux area with each turn generating another “packet of signal”, will determine how the coil responds to a large extent.
But, as we saw in Figure 2, magnetic metal in the encompassed area of the coil can have a dramatic effect on how much flux goes through the coil. So, the limiting case for when resistance is a good metric for comparing pickup output would be when the shape and construction materials of the pickup are identical, including the gauge of the wire used to make the coil.
The good news is that this condition holds for a pretty big chunk of the historically popular pickups. That’s one reason that resistance is so widely used as an indicator. Probably the most applicable case where resistance can be used as a good indicator is in Strat-style single coil pickups. These pickups have typically uniform coil dimensions between manufacturers and use predominantly 42 gauge coil wire and cylindrical AlNiCo 5 pole pieces. The resistance of most Strat-style pickups falls in the range of about 5500-7500 Ω. At the lower end of this range the response is more hollow or “mid-scooped”, like the 50’s and early 60’s vintage pickups. In the middle are the more full sounding examples like the mid 60’s, and at the high end of the range are fatter, more “modern” sounding pickups.
PAF style humbuckers are another design where resistance is a good measure. This design also uses 42 gauge coil wire wound on bobbins with uniform dimensions between manufacturers, with steel pole pieces (slugs in one coil and screws in the other) and AlNiCo magnets charging the pole pieces in order to magnetize the string. The resistance of most PAF style humbuckers is between about 7000 and 9000 Ω Ohms. At the lower end of this range is the more bright, “Tele on steroids” tone, while the higher end of the range is a bit fatter and darker. In matched neck/bridge pairs the neck pickup will tend to be towards the lower end of the range and the bridge pickup towards the higher end.
But, if you change one thing in the design equation, the ready comparison of pickup performance breaks down. For instance, changing wire gauge will alter the relationship no matter what else is going on as the resistance will change for the same length of wire and number of turns around the encompassed area. Lower gauge, larger diameter wire will have less resistance but take up more space for a given number of turns. Higher gauge, smaller diameter wire will have more resistance for the same number of turns, but will take up less space. That’s why smaller diameter wire is typically used to make higher output pickups in both the Strat and PAF style designs, the designer simply runs out of space on the bobbin at some point and higher gauge, smaller wire enables more turns in less space, at the expense of higher resistance. If you see very high resistance numbers for a given pickup, relative to the typical ranges, you can be pretty sure that it is using higher gauge wire and the resistance number is not directly comparable to the values of a 42 gauge design.
In the Strat style design, and even more in Tele style pickups, different AlNiCo alloys were used at times historically, and are often utilized in aftermarket pickups. These different alloys concentrate the magnetic flux to different degrees (although they all generally concentrate it more than AlNICo 5), as well as imparting a different magnetic strength to the string. So, in the case where the magnet alloy is changed, the ready comparison also breaks down; AlNiCo 2 or 3 pickup resistance values can’t be compared directly to an AlNiCo 5 value, or to each other for that matter. In PAF type designs, the magnet type is less important, but it still has an effect, and one that many claim they can hear. As Figure 2 suggests, there is a physical basis for why we might expect this to be true. We’ll talk more about magnet types and tonal responses in the next installment.
And, of course, design variables can make a big difference. Stacked noiseless or dual rail Strat style pickups can’t be directly compared with conventional design Strat pickups, for example.
So as a measure of pickup output and performance, resistance can be a good number to reference, but only in limited circumstances, as we’ve outlined above.
How else do we need to be concerned about resistance?
One way is in how pickups interact with the volume and tone controls. A volume or tone pot is a variable resister or “potentiometer” (“pot” is just short for potentiometer). When connected as a volume or tone pot, and when the knob is set to “10”, the full value of the potentiometer resistance is connected to ground, in parallel with the pickup. Physically, what this means is that the electrical signal has two paths it can go through. In the first path the signal can go through the pickup and out to the amplifier. This path has a resistance equal to the pickup resistance, let’s say it’s a single coil pickup at around 6500 Ω. The second path is a “short circuit” to ground where the signal can escape without going to the amplifier, but the resistance in this path is the full potentiometer resistance, which for this type of pickup would typically be 250,000 Ω. That’s almost 40 times more resistance than the pickup, so most of the signal will go through the pickup, but still some of it will get through the 250 kΩ pot (“k” is shorthand for 1,000s, so 250k = 250,000). Because of the way the physics work out, (and we’ll talk more about this in the last installment where we’ll deal with resonance) the high frequencies tend to go first, so most of what you lose through the very large pot resistance is high end.
One way to think about it is like a garden hose with a pinhole in it. The pickup is the garden hose and the water coming out of the nozzle is the signal making it to the amplifier. The pin hole is the volume pot, and the spray coming out of the pinhole is the small amount of signal bleeding off to ground through the pot resistance. The pinhole, with its small diameter, represents a much higher resistance to flow than the larger diameter of the hose, just like the pot compared to the pickup. The spray is like the little bit of mostly high end signal you lose through the volume pot.
With a higher pickup resistance, the difference in resistance between the path to the amplifier and the short circuit to ground becomes less, so unless the pot value is also increased, more signal will be lost to ground. This is why higher resistance pickups like humbuckers tend to use 500 kΩ pots.
Most people think humbuckers sound “muddy” with 250 kΩ pots, and that’s because the lower resistance to ground lets out more signal, and more high end. With a 500 kΩ pot, more of that signal is retained, and especially the high end signal. This phenomenon is what is referred to as electrical loading. A 250 kΩ pot represents a higher load on the circuit compared to a 500 kΩ pot, because it lets more of the signal bleed off to ground.
So that’s a bit more detail on what we believe is the correct physical picture in which to interpret pickup design and function, and a discussion of resistance as a measure of pickup performance and how we need to be concerned about resistance in spec’ing out a pickup control circuit.
Wrap your heads around what I’m saying about how a pickup works, because next time we will draw on this heavily in a discussion about inductance!