Impedance: How It Works In Your Pickup

In this article I want to talk about the physics behind pickups in order to explain why using the DC resistance as a measure of output is not the best way of viewing the output of a pickup. But first, what is DC in the first place? Why is it relevant at all when it comes to pickups? When those questions are answered we have to take a look at the origin of the signal itself: where does the signal come from? Why are magnets and coils so crucial?

Let’s begin with the term DC. DC is an abbreviation of the term ‘direct current.’ Consequentially there is also a term for its opposite: AC, or ‘alternating current.’ Unfortunately, giving the definition for the abbreviation doesn’t bring us closer to understanding anything, so let’s examine the basic properties of an electrical signal. An electrical signal is a flow of electrons: they move from one direction to another, and the way they move and how much electrons move give the electrical signal properties that are measurable with several instruments. If we compare the flow of electrons with a stream of water going through a tube, the amount of water that flows through the tube each second is comparable with the electric tension of the signal. That’s called the current. Since the water flows, there has to be a force that pushes the water. This force can be seen as a pressure. In electrical terms this is called the voltage. Everything that moves outside a vacuum encounters friction. The more friction, the harder it is to move. In electronics that is no different, but now it’s called resistance. These terms are joined together in a mathematical description called Ohm’s law, U(voltage)=I(current)*R(resistance).

When we talk about a ‘hot signal’ we usually refer to a signal that has a high voltage. The way pickups are designed makes it relatively hard to make a signal that has a high current.  Thankfully so, because while high currents can kill you, high voltages (usually) cannot.

Now let’s take a look at the terms direct versus alternating. A DC signal maintains a constant current and voltage. Over time, there is no difference. An AC signal, on the other hand, has a fluctuating current and voltage. At any given time you have the same current and voltage with a DC signal, but this is not the case with an AC signal. The AC signal flows like a wave (a sine wave, to be exact). A wave like a sine has an amplitude (volts in our case!) and a frequency (the number of times it vibrates per second.

In the USA and some other parts of the world, your electric socket will give you a 120V 60Hz AC signal (but since that signal is always 60 hertz and not sometimes 220, to name an arbitrary number, the physical laws for those kind of signals are practically the same as for DC signals). But on a guitar there are more complex things at work. We don’t play 60 hertz all the time. That wouldn’t be music, that would be plain and simple noise! We play high notes, low notes and a lot in between, and since the pickup creates a signal that corresponds exactly with the notes that are being played, there can be no mention of an DC signal.

Due to the properties of the coil, some frequencies are more easily ‘produced’ than others. That characteristic makes it even harder to maintain the DC resistance argument. Resistance in a coil depends on the frequencies of the signal that runs through it, and that interdependence is called impedance. That means that with some frequencies there is less resistance for the signal and with others more.

Then why do we talk about DC resistance? Because it’s an easy way to view a pickup. If there are many similar specs, the DC resistance shows some properties of the pickup. Also, if you use a multimeter to see if the pickup works or if the coil split has worked, you run a DC signal through the pickup that measures the resistance. So how can the DC resistance be seriously used as a means of information about a pickup in an AC environment?

In another article I’ll go deeper into the subject and talk about why the resistance is used as a measure of relative output, and show even more arguments to why the DC resistance doesn’t give you the full story of a pickup.

About Orpheo

Orpheo is a long-time member of the Seymour Duncan forum with an interest in the technical side of luthery and pickups and plays jazz, blues, rock and metal on predominantly carved top single cutaway guitars.
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  • Jon

    One typo in the article. In the USA, the electric socket gives 110V 60Hz AC signal

  • http://www.facebook.com/bigjoek Joe Kralicek

    VERY informative, THANK YOU!

  • darren

    Minor thing, but in Europe the AC is 230 v at 50 Hz. North America is usually 120 v at 60 Hz. (127 v @ 60 Hz in Mexico)

  • Dan Appleman

    The water analogy is wrong. Voltage is analogous to pressure, and current to flow rate. You have this the wrong way around in the text, but the image scraped from elsewhere is correct.

  • http://www.facebook.com/frank.aurigemma Frank Aurigemma

    Looks like you have your definitions out of phase here. The amount of water moving is analogus to Current (the number of electrons moving per unit time). While the force of the water is analogus to Voltage which is a measure of Electromotive Force. Although I know how the impedance defines a pickups transfer function I am not getting from the rest of the article a clear definition or description of “Impedance, How it works in your pickup” No offense intended.

  • http://www.facebook.com/frank.aurigemma Frank Aurigemma

    I don’t think pickups (transducers) produce frequencies I do believe they do attempt to detect one form of energy and convert it to another. This is our crude attempt for the past 80 or so years to capture or should I say assign a sonic signature to the electric guitar. Of course these are the sounds we have all come to know and love and therefore it should remain, Or should it?

  • 2Kings315

    Exactly wrong! Water flow is analogous to current (the motion of water or electrons). Water pressure is analogous to voltage (pressure). You can have pressure (voltage) with no flow (current) but not flow without pressure. The impedance/resistance analogy is correct. And if he doesn’t believe voltage can kill you he needs to grab a 220v line and see what happens.

  • surroundguy

    Yeah man, to not catch those eras was lame. Also, it’s E=IR, NOT U=IR!! Perhaps writing this article was a little over your head?