R/E/P > Klaus Heyne's Mic Lab Stickies

Mic/Pre impedance "matching"- What Are The Rules?


Much attention has been given to finding the optimum impedance match between a microphone and preamp, especially with the rise of impedance-selectable pres.  

Working with a wide range of both mics and pres, I am wondering how one determines - from a technical perspective - which pre would best suit a specific microphone?  

The base research that I have done yields guidelines, such as: "the input impedance of the pre should be 10x the source impedance of the mic..."

Sonic factors notwithstanding, what are the "rules" for impedance matching?

Technically speaking, the loading of the mic is supposed to be at a minimum when interfacing to a pre; especially earlier models with an o/p circuit less hefty than modern ones exhibit an increase of distortion as they are loaded down more. A practical relation of 1:5 (mic:pre) is specified as minimum load impedance, absolute values being around 200 ohms R.source (mic) and 1K or higher for R.load (pre). Some vintage pre's have an R.load of only 600 ohms, others do go up to 2K (e.g. V72) or 4K (e.g. V72a).

For modern transformerless high output condenser mics with an R.source as low as 50 ohms, a variation of R.load will basically make no difference, since these mics will perform equally well into both, high and low loads.

Transformer-based condensers can exhibit a divergence in frequency response, due to different interaction with the frequency-dependent impedance components of the transformer, resulting in increase of resonance peaks or damping thereof at either the lower or upper end of the audio band -- all depending on the construction of the individual transformer. This, in turn, may affect the sound to some degree.

Dynamic mics have a source impedance that varies with frequency, so a change of R.load will definitely make a difference here.

But, what it all comes down to in the end is the judgement by your ears, isn't it? Even if freq. response and/or distortion characteristic should not measure right...  

Klaus Heyne:
Thanks for your clarity of expression, Volker, very helpful!

I will make this a "sticky".

what about older pre's with much lower input impedance some as low as 37.5?


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