One of the features of the powerful Avid Pro Tools Carbon audio interface is the variable impedance option on four of the microphone inputs and on two of the instrument inputs. The MBOX Studio has the same Mic preamps and Hi Z inputs with the same impedance options.

It is suggested that varying the impedance on a microphone signal, particularly ribbon and dynamic microphones, can alter the sound.

There’s an excellent article on the AEA website ‘HOW IMPEDANCE CAN CHANGE THE SOUND OF YOUR RIBBON MICROPHONE’, it’s well worth a read, here’s an extract from it;

Passive ribbon microphones must be properly mated to the input impedance of the preamp to operate at their full potential. If a preamp’s impedance is too low, the microphone will have to work harder to generate a proper signal. When this happens, the low-end of the mic’s sound will be limited and the transient response will sound flat degrading the overall performance of the microphone.

The Impedance Myth

A common misconception is that the impedance of the preamp input should match the same impedance as the microphone. This idea of “matching” was born out of early telecom technology when matching the mic’s impedance to the preamp impedance was necessary to get the highest power transfer. This is no longer true.

The rule today is in order to deliver its full frequency response, a passive ribbon mic needs to see an input impedance of at least five times its output impedance. That means if the mic has an output impedance of 300 ohms, the preamp should have at least an input impedance of 1200 to 1500 ohms. We recommend a preamp with an input impedance of at least ten times the output impedance of the mic to ensure optimum performance.

We decided to make some test recordings with both a ribbon microphone and also a dynamic. The two microphones used are the SE Electronics VR1 and the Shure SM7B.

The test is as simple one, record an acoustic guitar with three passes of each microphone varying the impedence on each pass. In the case of the Carbon the impedances available are 50K, 5K, and 1K.

We conducted some tests beforehand and tried a popular booster on the signal. However, we felt using a booster may affect the results, so went back to recording them both as passive signals and setting the gain on the pre-amp to the maximum 60dB.

Recordings were noticeably different in volume depending on the impedance setting, although this is to be expected. However, our primary interest was in the sound of the microphone and if changing the impedance made a significant difference. For the purpose of the listening test we gain matched all audio to -3db before exporting them as 24bit 44.1kHz WAV files.

The recordings are untreated and do not have any EQ, compression or any other effect on them.

The audio examples are given below.

If you prefer, you can download the WAV files to audition.

Let us know what you think in the comments.