[ProAudio] Microphones question

[David Josephson]

Scott Dorsey writes 

> In a perfect world the lowest noise would be when the input impedance of the
> preamp matches the output impedance of the microphone, ....

Um, no. Your perfect world would need to redefine a lot of other parameters for that to be so. A typical emitter-follower microphone output may have an output impedance of 100 ohms but if you load it with that value the distortion will be significant, the output level severely limited. Noise performance depends as Bill mentioned on the operating point where current and voltage noise of the input stage is optimal for the source impedance of the microphone. This is why IEC 60268-4 requires specification of both output impedance and minimum load impedance, which is typically 10-20x the output impedance. 

Dan is pointing out that one needs to know the output impedance of the microphone to design an optimal preamp input stage, and this information is seldom supplied. There are preamp designs that don’t play well with very low output impedance mics, for instance, because of their negative feedback structure (applying NFB to the input stage in parallel with the input signal.) There are many mics that don’t do well when loaded with anything less than about 1500 ohms, and if you parallel a few consoles at a venue without considering this, performance may be impacted. Performance may also be different depending on impedance from each side of the input to ground, which may be related to the differential output impedance, or not. 

David Josephson
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