Microphone Impedance - How does it matter?

It's time to focus. And by that, I mean we've decided to implement Focus Months within the blog! This month's topic: PREAMPS

Now, many of you in the community have been asking recently, 'what impedance should I use with my _____?’ and 'how does impedance work?’. Impedance is both very simple, and very complicated all at once, but to understand how it affects us in the audio world, I've done my best to break it down for you in a couple of ways. 

You can both listen and see the test we've done with 2 of our own mics, and I discuss the results with my friend Luc, who was also kind enough to help me do this by playing acoustic guitar for the video:

In any case, let's break this down and dive in. 

What is Impedance?

In the simplest way to put it, impedance is another way of saying resistance. Microphones have very low output impedance, which allows the signal to travel for a long stretch of cable without signal degradation or loss of signal, which, compared to the very high output impedances you can usually find on guitar or bass signals, these struggle to maintain the same signal and fidelity across even 30 meters of cables (hence the need for a buffer pedal if you have a big pedal board!). 

So now we understand that, we can look at Load impedance, which is simply the resistance of the preamps input. We need to think about say a transformer for example: If the input impedance of the transformer is too low for the mic's output impedance, then the rest of the circuit of the preamp has to work much harder to pass that signal along to the output of the preamp. So, in general, we want the preamps Load impedance to do a lot of the heavy lifting on a lower impedance of say a microphone, right at the start of the circuit to make it easier to pass the signal through. 

How does it affect the sound of the microphone? 

Hopefully now you can see why this is simple and complicated all at the same time now! But don't worry, if you're confused, there's only really a need for us as engineers or producers to understand the impact on the sound and how that varies from lower load impedances to higher ones. In short, this should help you going forward and make it much easier to refer back to if you need to: 

  • High Load Impedances (the impedance on your preamp) will give you more overall level
  • Higher impedances also tend to make the low-end and low-midrange content of the mic much flatter, as well as have no dampening effect on the higher frequencies  
  • Low Load Impedances (again, relating to the input of the preamp) will result in a lower output signal
  • Low Load Impedances also add more emphasis on the low end and lower midrange frequencies, as well as picking up the resonances lower down the spectrum much more. 
  • You'll sometimes find that the upper frequencies are slightly more diminished and the mic will lack that upper detail. 

How do I choose the right impedance? 

First off, let's dispel a myth that has been ongoing for too long now; matching impedances is not necessary any longer in relation to the mic output and preamp input (load impedance). This was the case back in the days of early telecom technology as matching the impedances allowed a higher power transfer, but nowadays we go by a simple rule of thumb - one that will help you decipher going forward what impedance to set your preamp at for your mic choice: 

The load impedance of the preamp input should be at least 5x that of the output impedance of your microphone

Knowing this, we can demonstrate in a couple of examples which we tested in the video above. The first example, the BB29, has an output impedance of 150 Ohm, the second example, the HH1, has an output impedance of 300 Ohm. The preamp choice was pretty simple to demonstrate this properly as the ISA One from Focusrite has a simple one button on the front to change between the various load impedance options, thus making it ideal for a quick test. 

The ISA One has 4 settings: 

  • Low - 600 Ohm
  • ISA 110 (we referred to this as Mid for the video) - 1400 Ohm
  • High - 2400 Ohm
  • Very High - 6800 Ohm. 

A quick bit of math concludes that the BB29 and the HH1 will both suffer at the lowest impedance. The BB29 in particular was rather obvious on the top end and the bottom end, but the high frequencies were much more apparent due to it being a rather bright mic anyway. The HH1 was a little less obvious in the top end, but the lower midrange content to our ears was blatantly clear when we switch to the second impedance option (110 ISA, rated 1400 Ohm).

We made sure to match up the capsules to ensure the proper phase between both mics, despite varying melodies being played. We also replaced the HH1 capsule cover before recording as well. 

From here, the mic's sounded as they should but we noticed something as we proceeded up the load impedance options that many other current blogs and videos don't often mention: the dynamic range is drastically different the higher your load impedance is. As you can see above in the video, from the lowest to highest load impedance options (left to right within the project), the transient information is vastly different from the lowest option, 600 Ohm, to the highest option of 6800 Ohm. 

Let's conclude

So in conclusion, and as we've always recommended, the highest impedance is the best for any job on basically any microphone. Not only will the mic perform as intended, but it'll also result in a higher signal overall, better dynamic range, and no degradation to the microphones frequency response. 

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