Impedance is a complex and nuanced topic, but I promise that when it comes to audio equipment, you only really need to get your head around a few key concepts to understand how it applies to your recording process and even use it to your creative advantage!
Sound = Pressure = Voltage
Firstly, we need a quick reminder that analogue audio signals are just electricity. Electrical current is created when we have a source of electrical “pressure” known as Voltage (V). In our world, the voltage is generated by a source like a guitar pickup or a microphone. The amount of electrical pressure goes up and down with the level of the audio you are generating; the louder your kick drum, the more voltage the microphone generates.
In audio electronics, we would say that the voltage of this electrical signal varies in sympathy with the amplitude of the audio, and that the voltage is an “analogue” of the original audio signal.
Because the voltage is constantly changing, it creates what we refer to as alternating current. The rate at which the voltage changes (i.e. the rate at which the current alternates) is called the frequency — you might have seen this expressed as Hz (i.e. 100Hz, 10kHz, etc). This idea of the frequency of the alternating current becomes important later.
Resistance vs Impedance
You might remember the concept of resistance (measured in Ohms with the symbol Ω) from your high school science class. Resistance is literally a part of the circuit “resisting” the current that is passing through it. The higher the resistance, the less of that current will make it through to the end of the circuit. Think of a pipe with water flowing through it; resistance is like a kink or a narrow section of the pipe that reduces the current moving through the system.
Impedance (represented with the symbol “Z”) is like a cousin of resistance — they both oppose the current of the electrical signal, but with a key difference: resistance works against direct current (DC), whereas impedance works against alternating current (AC). DC doesn’t have a frequency like AC does, it’s a direct and constant flow of electricity. This is great for things that just need an efficient and consistent source of power (like motors or power supplies), but we are working with audio and we need those frequencies!
So now we know that resistance and impedance both work against current flow (in engineering-speak we would say that resistance and impedance are opposition to current), there is another wrinkle: the key difference between resistance and impedance is that with impedance, the opposition to current varies with frequency.
So, while the impedance of a given circuit might push back against the current to a certain extent (i.e. at a given value) at 100Hz, it will be very different at 10kHz. This is why we rarely hear the word resistance in the audio electronics sphere; it doesn’t adequately capture the behaviour and nuances of what is actually occurring in an audio circuit.
You can think of impedance as a special, fancy kind of resistance that changes depending on what frequency is passing through it.
Input vs Output Impedance
When it comes to recording audio, the key thing we want to focus on is the impedance directly at the inputs and outputs of the equipment we are interconnecting. A quick glance at the technical specifications of a piece of audio equipment will tell you its input and output impedance, which (like resistance) is measured in Ohms (Ω).
But this can actually be a bit deceiving because the impedance value on the label is usually specified at the frequency 1kHz. The impedance at higher and lower frequencies than 1kHz will actually be different — using 1kHz is just a convenient standard.
The main thing to glean from these specs is this: is the impedance of the output of one piece of gear higher or lower than the impedance of the input of the piece of gear that it’s being plugged into?
When transferring power (i.e. audio signal) from one device to another, great results are obtained when the output impedance of the transmitting device is the same as the input impedance of the receiving device. This is referred to as impedance matching.
In the early days of commercially available pro audio equipment, the agreed standard was 600Ω. The output impedance of an original LA-2A for example is 600Ω and so is the input impedance of an original Urei 1176. Chaining these devices together results in a full and clear transfer of power including all of the frequencies we’re concerned with in the audible spectrum. Impedances were matched, signal was clear — great, right?
However, matching impedance this way is not without its drawbacks. If the cables used to connect the devices were long, high frequencies would begin to roll off (audio-speak: be attenuated) as the inherent conductive qualities of the cable itself cause it to become a capacitor. Or, if one was to split the signal from the sending device to two receiving devices, the whole signal would drop by 6dB (due to the devices being in parallel).
Because of these challenges, it wasn’t long before a more flexible standard was adopted, where output impedances were uniformly low and input impedances were higher by comparison. This is the convention most often adhered to today and is referred to as impedance bridging.
Impedance Bridging
The new impedance bridging standard brought with it many advantages. Cable runs of many meters could be made without significant loss of high frequencies, and one output could happily be shared by many inputs opening up the world of “multing” at the patchbay to create parallel chains or any number of other creative signal flow options. In my own studio I actually split one single audio interface output to 3 headphone amps and enjoy the full frequency response this allows for. It’s very convenient.
So this must be the end of it and all impedance issues have been solved forever, right? Nope.
Where It Gets Complicated
While low impedance outputs talking to high impedance inputs results in theoretically great results, the subjective concept of actually “great sound” is a little more nuanced. Some devices are specifically voiced so that a slight (or major) impedance mismatch is actually desirable!
A great example of this is the electric guitar. Passive pickups by their very nature have a pretty high output impedance. The classic guitar amps were designed to have as high an input impedance as was practical with the technology at the time, then their internal circuitry was designed to sculpt and shape the incoming sound to a pleasing sound that wasn’t too bright, nor too dull.
A Telecaster plugged into a Fender Twin sounds great, and that is in no small part due to the impedance relationship between the guitar itself and the input of the amplifier. The brightness of the amplifier itself compensates for the dulling effect the guitar cable and input impedance of the amp put on the guitar pickups.
If you take the very low impedance output of a modern audio interface and connect it directly to the Fender Twin however you might find the sound is very bright and harsh and you have to unduly compensate with the tone controls or with mic positioning.
Similarly, the classic Fuzzface pedal — with its notoriously low input impedance voiced to sound excellent with the guitar connected directly — often sounds truly awful when loaded with a low impedance buffered guitar pedal.
So the oft used adage that output impedance should always be low and input impedance should always be high falls down regularly when the devices in question have already been set up to expect a certain impedance relationship.
Variable Impedance
Some manufacturers of audio equipment, realising the varied tastes and equipment collections of their customers, have actually implemented adjustable input impedance on their devices. Most notably, the Neve 1073 mic preamp (and the subsequent variants made by a whole host of companies) included two distinct input impedances to accommodate the differing output impedance of mics ranging from vintage ribbons to modern condensers.
In our own Franklin RA-10 Studio Re-Amplifier, we wanted to open up the benefits of variable impedance to any signal chain, not just at the input of a preamp. We’ve done this with our signature “Z” control, which gives users the ability to tweak that relationship between output and input impedance for any re-amping setup. This is to acknowledge and celebrate the very wide range of devices that might be placed after the RA-10 in the signal chain.
The “Z” control ranges from a very low impedance of 600Ω (if the full frequency spectrum should be your desire) to a whopping 50kΩ if you’d like that vintage fuzz face to see the signal that it’s expecting to see directly from a guitar.
My personal hope is that you grasp the basics of impedance matching and bridging and then throw it all out the window. Experimentation is key, so don’t get caught up in technical notions of what’s “correct”; find where the wrong thing is just right for your music.