All about pickups and impedance
Guitar pickups and "impedance" are the subject of much mystery. I'll try to dispell some of the misunderstandings with respect to conventional magnetic pickups, and to define impedance in a way that makes sense for the ways in which a guitarist might encounter the term, at least with respect to lower-level signals. I won't discuss impedance as it relates to amplifier-to-speaker connections.
Let's begin with a Q&A format:
Q1: Guitar pickups have an impedance that ranges from typically 6K ohms to over 17K ohms; the impedance usually seems to correlate to output. What is going on with this?
A1: This is actually a measurement of DC resistance, which is different from impedance. I'll describe the difference in greater detail after a few more questions.
Q2: The typical guitar amp has from 220K ohm to 1M ohm input resistance. Why is this so much higher than the pickup rating?
A2: Again, this is the DC resistance of the amp's input. The reason it's so high is that the amp input is part of a circuit that involves properties of the pickup (again, more on this later) other than DC resistance. The short answer is: the higher resistance makes the pickup sound better.
Q3: Are there guitars that aren't compatible with certain amps, or sound terrible because of impedance mismatches?
A3: So long as we're still talking about magnetic pickups or active pickups, no.
Q4: How does higher or lower input impedance affect overall tone?
A4: A higher input resistance (in general, resistance and impedance are not interchangeable) allows the resonant peak (typically around 2 to 3 KHz) of the pickup to be more prominent. A lower input resistance tends to flatten that peak. Sonically, the more prominent resonant peak tends to give the sound more "bite" or "sparkle", depending upon its frequency.
Q5: How does this relate to the impedances involved when connecting a rack preamp to a separate poweramp?
A5: This is a very different situation since there's no resonant peak created by the components involved.
Q6: What would be the optimal preamp output impedance of the preamp and what would be the optimal poweramp input impedance?
A6: In this case the impedance is more closely related to the DC resistance, so I'm going to use the term impedance in this discussion. The rule of thumb is that the input impedance should be about ten times the output impedance.
Q7: If mismatching is happening between a preamp and a power amp, what is the tonal result?
A7: As the input (receiver) impedance approaches the output (sender) impedance you'll notice a loss in amplitude of the transferred signal. You may also notice an increase in distortion as the output tries to drive a load for which it wasn't designed. Finally, you may notice some loss of low-frequency content if the output is capacitor-coupled.
Q8: If I was using an MFX into an FX return, could the possibility of an impedance mismatch occur? If yes, what would be this result?
A8: Its unlikely that you'd have problems with an MFX driving an FX return. You might - depending upon the design of the FX loop - have problems with the FX send driving a piece of gear with a lower input impedance. I've only seen this in all-tube FX loops. Level matching is a much bigger problem in FX loops. There's no standard for FX loop levels. They tend to expect either professional studio levels (+4 dBU) or "guitar" levels (-10 dBV). Despite the seemingly large discrepancy, these two levels have different reference points; the absolute magnitude of the voltage differs by about a factor of 3. Still, running a +4 loop through a guitar pedal can cause unwanted distortion and running -10 loop through rack gear can cause unwanted hiss to be more noticeable.
OK, that's enough of the Q&A format. It's time for you to fetch your favorite beverage while we get into specifics about guitar pickups. In particular, let's examine why the DC resistance is not the same as the pickup's impedance. I'll try to keep the technical mumbo-jumbo to a minimum.
A guitar pickup has some kind of magnetic assembly and one or two coils of wire. The coils, together with the magnetic properties of the metal parts and magnets that interact with the coils, determine both the DC resistance and AC impedance of the pickup.
The DC resistance is easier to understand. A long piece of wire tends to resist the flow of a steady stream of electrons. A longer piece of the same wire resists that flow even more. We measure that resistance in ohms.
The coil is usually wound on a bobbin. The dimensions of the bobbin limit how much wire you can wind. The more turns you can wind around the bobbin, the more output you'll get from the pickup. But to wind more turns around the same bobbin you need to use thinner wire, which has a higher resistance. This is why high-output passive pickups have a higher DC resistance.
That's the easy part.
A wire also has the property of opposing changes in current. This is because a change in current creates a changing magnetic field around the wire which in turn induces a current in the wire to oppose the current that created the magnetic field in the first place. (Don't worry: they don't completely cancel.) The amount of opposition to a changing current depends upon a property called "inductance". A coil of wire has more inductance than a straight piece of wire. Wrapping the coil around a piece of ferrous metal further increases the coil's inductance.
The more inductance, the lower the frequency at which the coil's opposition is effective. This is why hot pickups (having more turns of wire) have less "sparkle" and "sheen" in the high frequencies. They're simply not able to produce as much output in the higher frequencies because the higher inductance of the "hot" coils prevents it.
Still with me?
OK, there's another property called capacitance. Imagine two pieces of flat metal that are parallel to each other but not touching. If you apply a changing voltage to one plate, you'll get a changing electric field. The other plate picks up some of that changing field. The larger the plates, or the closer together they are, the more that field gets coupled from one plate to the other. That degree of coupling is called capacitance.
Now imagine that any conductor carrying a changing voltage also has a changing electric field, and any other nearby conductor will be influenced by that changing field. There's still capacitance, even though we're not dealing with flat plates.
If you took a coil out of a pickup and sliced it across its midsection, you'd see a bunch of wires laying next to each other. They're electrically insulated from one another because of a thin coating of plastic or enamel on the wire, but they're all next to each other. Guess what? There's capacitance between those wires.
So the coil of wire in the pickup has both inductance and capacitance. The capacitance is distributed throughout the coil, but it's there. More importantly, the capacitance acts as if it's in parallel with the coil's inductance.
This is where it gets interesting. A capacitor and an inductor in parallel form a tuned circuit. This is the principle by which a radio receiver, for example, picks one signal out of the air and ignores all the others. The tuned circuit is responsible for the pickup's resonant peak. The values of the inductance and the capacitance determine a single frequency at which the circuit has its highest impedance. Above that resonant frequency the capacitance dominates and reduce the impedance. Below the resonant frequency the inductance dominates to reduce the impedance.
That high impedance at the resonant frequency causes the pickup to emphasize the close-by frequencies. I'm going to ask you take that on faith. Otherwise I'd have to get deeper into the equivalent circuit of the pickup and start talking about generators and voltage dividers. I suspect I've already gone a bit too deep...
Finally, the input resistance of the amp takes some of the energy out of the pickup's tuned circuit. The lower the resistance of the amp's input, the more energy it takes. Since the pickups impedance is highest at the resonant frequency, the amp's input resistance affects the resonant peak the most.
This concludes our survey of the inner workings of pickups and the meaning of "impedance". I hope you've found this useful.