One of the most mystifying topic of the studio is impedance. Impedance is basically what it sounds like it is anything that impedes the flow electrical current. Impedance manifests itself in three ways: Resistance, Capacitive Reactance, and Inductive Reactance, the most commonly referred to of these is Resistance. The letter assigned to impedance is “Z” which is why you’ve probably heard the terms Hi-Z and Lo-Z fly around from time to time. Hopefully the following will clear out some of the fog.

For the time being lets call the microphone the source, and the mic pre-amp the load. A typical dynamic microphone like a Shure SM58 (R), will have a source impedance of approximately 200 Ohms. As a general rule you would like to have a load impedance at least five times the value of the source impedance, otherwise signal strength is lost. The mic pre of a Digidesign(R) Mbox 2(R) is 3.5k Ohms which is well beyond the desired 1k Ohms. Similarly if you would like to connect a synthesizer such as a Yamaha Motif to a line input on the Mbox 2 (R) is good to check the impedances. The Mbox 2 (R) line impedance is 10k Ohms and the Motif’s output impedance is XXXXXX. Some equipment (especially some vintage and specialty synths) need to be passed through a DI (Direct Injection) box to achieve an appropriate mic level.

This brings us the the issue of DI boxes and guitars. The nature of guitar pickups makes it difficult to adhere to the load five times greater than the source rule. Guitar pickups are inherently much higher impedance (10k to 50k Ohms). This means that even though the typical instrument cable you connect to your guitar will also fit in the line input, you will likely get little or no signal. This is because you likely have a higher source impedance the load impedance, thus grossly breaking our general rule (load impedance > 5 X source impedance). The DI box was designed as a solution to this problem.

The DI box is an impedance converter allowing you to pass the same signal through, but creating a new source impedance. The Art Zdirect passive DI box offers an input impedance of 50k Ohms and an output impedance of 600 Ohms. Due to the fact that the signal is typically hotter coming from a guitar (higher in voltage, especially from humbuckers) the 50k Ohms input is acceptable as the mic pre will not be expecting the hotter signal off of the guitar pickups. Therefore it is possible to record a guitar straight into the Mbox 2 (R) through the DI box (600 X 5 = 3k Ohms < 3.5k Ohms, OK!)

Some audio interfaces offer instrument inputs as well. These inputs are designed to accept a guitar or other hi-Z source and often have a load impedance in the ball park of 2 Mega-ohms or greater.

Having a good grip of how impedance works in the studio is the first step to attaining a good signal to noise ratio.