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Mike Rivers
 
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Default How to measure a dynamic mic's output impedance?


Loren Amelang wrote:
On 4 Mar 2006 08:43:29 -0800, "Mike Rivers"


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?


It's something that's well known, based on wide 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?


A bad preamp is still a bad preamp even if it's properly matched to the
microphone. But a good preamp can be better if it's used with the
optimum microphone, assuming that the microphone is optimum for
whatever you're using the mic for.

I hate to be obtuse about this, but if there was only one ideal
combination, everybody would use it and that would be the end of the
story. If impedance were pure resistance, then you could optimally
terminate the microphone and hear the "pure" sound of the preamp. That
would allow you to hear what sort of coloration, if any, the preamp
had.

But one of the biggest differences between preamps is that some of them
have input transformers and others don't. And one of the biggest
differences (other than capsule and case construction) between
microphones is that some of them have output transformers and others
don't. Transformerless mics connected to transformerless preamps tend
to be less fussy about different loads than when there's a transformer
involved with either or both.

You might be inclined to simply choose transformerless mics and
preamps, but that cuts out some very good mic choices and peramp
choices. And other than a very few exceptions, these differences aren't
really like night and day, they'e just different shades of twilight.
But most people can hear the difference betwen a Shure SM57 connected
to a Mackie mixer and a Great River preamp, and of those who can hear
the difference, most experienced engineers will prefer the Great River
because of the smoothness. Many inexperienced semi-engineers will
prefer the Mackie because it sounds bright and crisp and makes
everything sound more cutting. That isn't usually a good thing. But
sometimes it is.

Or might it be some
complex, non-linear effect that has little to do with the input
impedance?


It's complex and non-linear and it has everything to do with input
impedance. Remember that impedance isn't just pure resistance, it's
inducance and capacitance as well, and those make up filters and phase
shift networks. There goes (in one sense) your linearity.

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.


That's because you have certain preferences and you're adjusting to get
closer to those preferences. How are you changing the impedance? Do you
have a tapped transformer? A variable resistor? Something else? Have
you looked at the signal at the input of your switcher to see how it's
changed when you change the impedance?

This seems less explainable than your experiences with microphones,
that seem more likely to be affected by load impedances.


Depending on your sources, it could be a lot easier to explain.

Question - do
you find microphones with transformers or with electronics inside
behave differently from those with their physical element connected
directly?


Yes. But understand that few microphones have the element connected
directly to the connector terminals. Most dynamic mics (and all ribbon
mics) have a transformer. All condenser mics have some electronics and
may or may not have a transformer following them. Even a crystal mic
(do they still make those) which has the element connected directly to
the output terminals and which requires a very high input impedance
looks to the input stage like a substantial capacitor. So a
microphone's source impedance is never simply resistive.
Transformerless mics are closer to resistive than mics with
transformers, but they aren't pure resistance.