[ProAudio] Microphones question

[Scott Dorsey]

> Scott Dorsey writes=20
> > 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.=20

This is all true, unfortunately.  It is far from a perfect world.

I'm waiting for a noiseless transformer that gives me free voltage gain and
I'm not expecting to see one any time soon.  But once I get one I'll be
able to get the largest possible signal into an input in order to swamp the
noise.

I'll point out that specifying minimum load impedance is sufficient for
a condenser microphone but that a dyanmic microphone should also be specified
for maximum load impedance.  In some cases with weak coupling where the 
electrical load does not make a large proportion of the total damping that
maximum might be infinite, but not all microphones are like that.

> 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=E2=80=99t 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=E2=80=99t 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.=20

This is all true, and it's why some preamps perform better with some 
microphones while other preamps perform better with other microphones.

But nobody sells an "optimized for condenser microphones" or "optimized
for moving coil dynamic microphones" preamp although there are a couple of 
"optimized for ribbon microphones" preamps out there.
--scott