Matching impedance as far as microphones and preamplifiers goes, is a widely misunderstood term. Most microphones can be used very satisfactorily with most preamplifiers and as we’ll see later, true “matching” is actually undesirable. However if you are seeking a particular quality, tone, coloration, character, then picking the right combination of equipment can be very helpful.
What is Impedance?
Every microphone inevitably has output impedance, and every microphone preamplifier has input impedance. These characteristics simply describe the “resistance” to signal current flow out of the microphone circuitry and into the preamplifier. The symbol for impedance is “Z,” hence the term “Hi-Z” as applied to inputs and outputs. (Guitarists should be familiar with Hi-Z.)
In practice the input impedance of a microphone pre-amplifier can have a great effect on the sound of the recorded signal. It is mostly the interaction between the output impedance of the mic and the input impedance of the mic preamp that causes audible differences, such as different sounding “EQ” as well as different attack characteristics (among other things). Also, the (often quite complex) impedance of a mic-pre will interact in a unique way with the output impedance of an individual design of microphone. (This is why certain engineers will have a mic and preamp of choice for different usage, such as the Neumann U87 with a Focusrite Red 7.)
It helps to think of impedance in terms of having a nozzle at the end of a garden hose. The garden hose (mic) is a low impedance source (there is little resistance to the flow of water) and the nozzle (preamp) is the higher impedance of the input being fed by the hose. When the nozzle valve is closed (open circuit): Input impedance is very high, pressure (voltage) is at maximum, and flow (current) is zero. Now open the nozzle just a little: Input impedance reduces but remains high, pressure reduces but remains high, flow is small; you can hear lots of hiss from the spray (high frequencies). As you continue to open up the nozzle: input impedance reduces further, pressure reduces, flow increases; hiss from spray becomes less noticeable. With the nozzle open all the way: input impedance is very low, pressure falls dramatically, flow is greatest; hiss from spray all but disappears. As you can see from this example, as the preamp’s (nozzle) impedance is lowered, the high frequency content is lowered as well.
Matching microphone and pre-amp impedances to the same value (power-matching) is not desirable since it reduces both the level and the signal to noise ratio by 6dB. For dynamic and condenser microphones, the preferred preamp input impedance is generally about ten times that of the microphone output.
A feature on some newer pre-amps (e.g. Focusrite ISA 428, Summit 2BA-221) is variable input impedance, which allows the level of transformer and microphone interaction to be adjusted. It is therefore possible to create a variety of mic/mic-pre ‘colors’ due to the aforementioned EQ-like effect. The advantage of this is the ability to shape frequency content while recording without adding an EQ device in the signal path, and the inherent noise and signal degradation that follows with more components in the signal path. Keep in mind that if you have a preamp with variable impedance, beyond the rule of thumb, 10:1 ratio of preamp input impedance to mic output impedance: settings become a matter of taste. You won’t destroy your mic or preamp through experimentation, but you may very well find exactly the sound you’re looking for.
