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(
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.