View Single Post
  #9   Report Post  
Posted to rec.audio.pro
Scott Dorsey
 
Posts: n/a
Default How to measure a dynamic mic's output impedance?

Loren Amelang wrote:
On 4 Mar 2006 08:43:29 -0800, "Mike Rivers"
wrote:
And once you know the impedance, find a preamp that makes the mic sound
good. There's little correlation between the numbers and what works
best for a given mic. If you measure the mic and find that it's 80
ohms, it might sound better wtih a preamp that has a 2500 ohm input
impedance, or a 1200 ohm impedance, or you may even like what loading
it with 300 ohms does (though it's probably not technically "best").


Is this a widely accepted phenomenon, or something you've discovered
through your own experience? Do you believe it is actually due to the
load impedance, such that changing the input impedance of a less-good
sounding preamp to match that of the best-sounding preamp would
optimize the performance of the less-good preamp? Or might it be some
complex, non-linear effect that has little to do with the input
impedance?


No, it's pretty much the way the mikes are designed. They are designed
to work into a load that is a little higher than the actual measured
output impedance. How much higher depends on the mike.

The difference between an SM-57 with a 500 ohm load and a 2K load is
considerable. Much more than you'd ever expect.

There are nonlinear effects involved in loading too; if you damp the
diaphragm down, the coil moves less and covers a smaller section of
the field... this may improve linearity on some mikes.

Also note that some mikes want to see a slightly inductive load for
best square wave response. The SM-57 is one of those.

I ask because I've built myself an audio switcher that allows me to
remotely adjust the input impedance of my sound system amplifier from
my listening position. I find not only that each source device sounds
best working into a different input impedance, but that my preferred
input impedance for any particular source varies with the recorded
material being played through it.


What you are hearing is probably not related to the actual input impedance
issue. Electronics in general shouldn't care much about the load as long
as it's substantially higher than the output impedance. Dynamic mikes
are a very different thing; they are actual mechanical systems and the
impedance affects the mechanics.

This seems less explainable than your experiences with microphones,
that seem more likely to be affected by load impedances. Question - do
you find microphones with transformers or with electronics inside
behave differently from those with their physical element connected
directly?


Of course.

While I worry that I might be believing in an imaginary phenomenon,
the dramatic (to me, at least) result when the setting is just right
has kept me using it for many years now. And I do notice if I have
accidentally left the remote adjustment disabled and am tweaking a
disconnected slider...


The other problem with this is that you are probably changing level too,
and a tiny level change can mask all sorts of other things.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."