Impedance describes an electric phenomenon that’s relevant to the bass in two primary ways. This issue, we’ll dive into the impedance relationship between a bass and the input of an amplifier or recording device. In the next column, we’ll explore the impedance relationship between an amplifier and any connected speaker cabinets.
To understand impedance, you must first understand resistance. Electrical resistance (represented by the letter R) is measured in units called ohms (symbol: Ω ). Imagine a simple circuit consisting of a battery with wires connected to both terminals and then connected to opposite ends of a resistor. As the battery’s voltage pushes current around the circuit, the resistor resists the current, resulting in a voltage difference between the resistor’s two leads, referred to as a “voltage drop across the resistor.” In this example, we can consider the battery to be the circuit’s “source” and the resistor to be its “load.” Due to the mathematical interrelationship among voltage, current, and resistance, if we know the battery’s voltage and the resistor’s resistance, we can easily calculate the current going through the circuit and the voltage drop across the resistor.
The situation becomes substantially more complicated when dealing with alternating current (AC) signals. Batteries are direct-current (DC) power sources—there is no periodic fluctuation in the voltage over time. But a bass is an AC source, with a periodic change in amplitude that also switches direction at a particular rate (i.e., the signal’s frequency). It’s this alternation in direction that’s later amplified to push a speaker in-and-out to create sound waves. A DC source would merely push the speaker in one direction and keep it there. With AC signals, other aspects of the circuit become relevant. In addition to resistance, the properties of inductance (interaction between currents, such as between the two paths in an instrument cable) and capacitance (temporary storage of charge) influence the circuit’s performance. Since all audio devices include components that exhibit resistance, inductance, and capacitance, the term impendance emerged to measure the collective impact of all of these variables on an AC signal, which is (a bit confusingly, I admit) also measured in ohms. Impedance includes resistance as described above, but also factors in reactance, which (for our purposes) can be thought of as resistance that varies with frequency.
Practically, here’s why it matters: Passive bass guitars have a high output impedance (basically a measure of the impedance contained within the components of an instrument’s electronics). This is the result of the relatively high resistance of the pickups and pots and the induction necessary to create a signal in magnetic pickups. The moment a bass (our source) is plugged into anything (our load), the source and the load are now part of the same circuit. Magnetic pickups operate through the transference of the mechanical energy of a vibrating string into analogous electrical energy in the pickup (measured in volts). For the voltage in a circuit to be faithfully preserved, the load impedance should be at least ten times higher than the source impedance. Since the average passive bass can have an output impedance of 7k Ω –10k Ω, the optimal input impedance of our load should be ten times that, or more. That’s why you see passive inputs on amps, which have impedances of 1M Ω or higher. Such a high-impedance input best preserves the voltage of the bass’ signal. Active basses have much lower output impedances than passive basses, due to the “buffering” action of the onboard preamp. As a result, they’re not as sensitive to the input impedance of a load (i.e., an amp’s input jack). Keep this in mind when considering your live or studio signal chain.
BASS PLAYER Senior Contributing Editor Jonathan Herrera is the magazine’s former Editor-in-Chief. An accomplished player, Jonathan is now a full-time musician and producer. His latest endeavor is Bay Area recording studio Airship Laboratories. Catch up with him at jonherrera.com and at airshiplaboratories.com.