Microphone Impedance Explained

 In TSM Studio Blog

In this article, we’ll briefly discuss key terms associated with the impedance specs of microphones. The three terms we’ll be seeking to provide clarity on are impedance, output impedance and load impedance.

 

What is Impedance?

Impedance, as represented by the symbol Z, is simply defined as the total opposition to alternating current (AC) in an electrical circuit. To take it one step further, impedance can be described as a form of AC resistance that depends on frequency and controls the flow of the audio signal.

 

Output Impedance

To understand the differences between output impedance and load impedance, we’ll first need to understand that microphone signals are AC voltages. Most low impedance microphones measure between 50 and 600 ohms, however more common output impedances tend to fall short of 200 ohms. The benefit of such low impedances allow for cable runs to extend further before signal degradation leads to a drop in quality. Typically, as cable runs extend further, the high frequencies are the first to experience attenuation or loss. Conversely, instruments that carry high output impedances (electric guitars) can experience a significant loss in sound quality after just 20-30 feet.

 

Load Impedance

In a signal chain, a load impedance can be defined as the input impedance necessary for the device following the microphone to operate at its highest standard. Ideally, the load impedance should be at least five times higher than the microphone’s output impedance in order to function optimally (ie: a microphone with an output impedance of 100 ohms should connect to a preamplifier with an input impedance of at least 500 ohms). The reason for this is because an output circuit needs to heavily drive a low impedance input. To avoid any sort of saturation or distortion, it’s in the best interest of the preamplifier to drive a light load.

 

In Closing

Ideally, you’ll want to make sure that your preamplifier has a higher input impedance than your microphone. If your preamplifier has a lower input impedance than your microphone, it’s likely that signal degradation will occur, resulting in a more limited frequency response. It’s not uncommon to lose high frequencies, low frequencies, or both in the event that the load impedance is less than optimal.

Fortunately, most of this is just tech jargon and it’s more than likely that just about any microphone you’ll use in a studio will have an output impedance that’s low enough to supply a proper load to the preamplifier. In the event that you find yourself using a preamplifier with adjustable input impedance, just crank it up all the way. You can do more harm than good stressing about the small details!