If the concept of impedance just doesn’t seem to get through to you, you already have some inkling of the subject, because not getting through is what impedance is all about. Simply put, impedance measures the amount of opposition to signal or current flow in an alternating-current circuit (such as an audio circuit). The amount of opposition affecting a signal passing through a circuit is the sum of resistance (which is essentially a constant value across all frequencies) and reactance (which varies with frequency).
So what does all this electrical-engineering mumbo-jumbo have to do with musicians? The key point to remember is that the reactance component of impedance tends to oppose signal flow at higher frequencies more significantly than at lower frequencies, resulting in the high-frequency attenuation and overall signal loss we guitarists affectionately refer to as “tone sucking.”
To understand how your tone can be compromised by impedance, you need to be aware of the two types of impedance that exist in audio circuits. Output impedance is the signal source, such as the output of your guitar. Input impedance is the amplifier or effect input, and this input is defined as the load. Although confusingly counterintuitive, the higher the input-impedance value relative to the output impedance value, the more signal gets through, resulting in a clearer and more robust sound. In order to optimize signal flow for maximum tone, the output impedance of the source signal (the guitar) must be lower than the input impedance of the load (amp or effect). If not, the signal will be “loaded down,” and its frequency response level, and signal-to-noise ratio will be adversely affected.
Fortunately, equipment manufacturers typically make it easy to get one piece of gear to work with another, so it isn’t necessary to research, say, your guitar’s output impedance and your amplifier’s input impedance. But there are a number of ways in which impedance issues can mess with your sound. Here are a few examples, and some tips on how to deal with them.
Stompboxes. Some stompboxes suck tone more than others, but they all have some effect, and the more of them you add to your signal chain, the more noticeable the sucking may become. The situation can be improved by using pedals with true-bypass switching (which bypasses the internal circuitry when the pedal is switched off), converting non-true-bypass units with a device such as the Switchbox True Bypass switching pedal (pedalboard.com), or giving pedals some extra kick by using a line driver/buffer such as the Axess Electronics BS2 Guitar Audio Buffer/Splitter (axess-electronics.com).
Cables. This is a huge topic, but you can never go wrong using the shortest cables possible.
Amps/Speaker Cabinets. Matching amplifier outputs to speaker-cabinet inputs is critical. If you connect an amp with an output impedance of 8ž to an 8ž cab, everything will be groovy. If the same amp is connected to a 16ž cab, the amp will not be damaged, but volume will decrease significantly. If the amp is connected to a 4ž cab, it will force the amp to produce twice its optimal current flow, resulting in distortion and possible damage to the amp.
Active pickups. Active electronics produce low-output impedances, which is why they generally have a wider frequency response than passive electronics, and it’s also why they are better able to drive longer cable runs and multiple-stompbox setups.