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# Impedance
- URL: https://wishingtree.audio/impedance/
- Published: 2026-08-17T12:00:55.000Z
- Updated: 2026-08-17T12:00:54.000Z
- Author: Mark Rudolph

There's a good chance you've heard of Ohm's Law - **Voltage** equals **Current** multiplied by **Resistance**. 

$$V=I \\times R$$

We can think of a simple battery driven **Direct Current** circuit, a resistor and LED for example, where we are pushing a current across and limited by a resistor. For a given voltage, the larger the resistor, the less current will pass through.

It's also likely you've heard of **impedance**, and things like that "Guitars are high impedance, or *high-Z*" - but what does that mean? Well, impedance is also a resistance, and even shares the same Unit of $\\Omega$ (Ohms). In audio, our signal is an **Alternating Current** \- impedance is the opposition to the flow of current in a circuit to an AC signal, and determines how efficiently power moves between gear. It can also be frequency dependent! 

Impedance is usually given the symbol $Z$. There is also a component of interaction from capacitors and inductors call **reactance** ($X$), and for simplicity we might say "impedance is the resistance to an AC signal", but it is technically a complex number which is the sum of the reactance and standard resistance.

$$Z = R \\times \\imath X$$

## It's All Connected

When we say "resistance of a circuit", we mean the total circuit - so this can also be viewed as the "black box of our gear", e.g. a guitar amp is a "circuit" and so is the speaker cab plugged into it. Gear that generates a voltage has an output impedance, and gear that accepts a voltage has an input impedance.

Let's take a moment to think about a mis-matched guitar amp and cab example (a **power transfer** issue). Have you ever had something like an amp with a $4 \\Omega$ output impedance, and were confused about which cab was safe to use - a lower $2 \\Omega$ one, or a higher $8 \\Omega$ one? If we think about impedance as "resistance to our AC signal", and "more resistance means less power transfer", then we can reason that the amp will have a harder time pushing the signal through the larger one - which is what we want! It will be *too easy* to push a signal through the $2 \\Omega$ one, which would result it too much current/over heating/damaging our gear. It's safer to push a signal into a higher impedance.

Now let's consider a microphone that's sitting in front of that cab. We don't really want to transfer a lot of **power** from our mic to our preamp (i.e. *current*), but we do want a strong clean *voltage* signal. This is now a **voltage bridging** issue. We can do this by matching a low impedance mic with a much higher impedance preamp - this has lead to a typical rule of about 10 to 1 higher - so a $100 \\Omega$ mic would work well with a $10 k\\Omega$ preamp.

But what about those *high-Z* guitar pickups going into the amp? This is still a **voltage bridging** issue. A guitar amp is expecting those high impedance picks of 20-50 $k\\Omega$, but what happens when you plug into that $10 k\\Omega$ interface you're using with the mic? It's lower! Welcome to "tone suck"! This is why lot's of interfaces have a setting (or dedicated input) for high-Z gear, which has an input impedance in that 10:1 ratio - typically in the $M\\Omega$ range.

## Wrapping Up

Knowing the output and input impedance of your gear is important to get the best signal - and so is knowing if you're concerned with **power transfer** or **voltage bridging**. An end-to-end guitar rig is a good example to keep in mind for this.