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How to measure a dynamic mic's output impedance?



 
 
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  #1  
Old March 4th 06, 02:03 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?

Hello,
I'm a total beginner. How do I measure a balanced or unbalanced dynamic
mic's impedance? Can I do it with a multimeter and, if so, how?

Thank you,
Andy


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  #2  
Old March 4th 06, 02:28 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?

"Andy" > wrote in
:

> Hello,
> I'm a total beginner. How do I measure a balanced or
> unbalanced dynamic mic's impedance? Can I do it with a
> multimeter and, if so, how?
>
> Thank you,
> Andy
>

Applying a multimeter to a microphone's output may damage it.
Besides, the multimeter would give a DC resistance value, not a
true impedance, which is the resistance plus reactance at a
particular frequency, usually 1KHz.

The easiest way is to face the microphone to a speaker, play a
tone to the speaker, measure the open circuit output voltage,
then terminate the output with a resistance of approximately the
expected output impedance, remeasure the output of the
microphone and calculate the true impedance from the change in
voltage and resistor. When the termination is equal to the
source impedance, the voltage will be half.

The question is WHY? all microphones (except externally powered
carbon microphones) are designed to work into a higher impedance
than their own output impedance. The actual impedance value is
not really important as long as it's less than a tenth of the
preamplifier's input impedance.

Microphones manufactured today are all fairly low impedance,
about 100 to 200 ohms. Typical mixer inputs are 2.4 to 3.3
Kohms.

--
Bob Quintal

PA is y I've altered my email address.
  #3  
Old March 4th 06, 02:36 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?

On Sat, 4 Mar 2006 15:03:27 +0100, "Andy"
> wrote:

>Hello,
>I'm a total beginner. How do I measure a balanced or unbalanced dynamic
>mic's impedance? Can I do it with a multimeter and, if so, how?
>
>Thank you,
>Andy
>

Yes. Feed the mic into your computer, or whatever, and look at the
output level in real time. Have some sort of source of sound playing.
Now put a variable resistance across the mic output, and adjust it
until the level drops by 6dB. Measure the resistance of the variable
resistance on your multimeter and that, near enough, is the impedance
of the mic.

More accurately, feed the output of the sound card with a signal at
1kHz playing into the mic (it is quite safe) through the variable
resistance, which you should adjust to zero. Measure the voltage
across the mic capsule. Turn the resistance up until the voltage drops
by one half. The resistance of the resistor is now exactly equal to
the impedance of the mic.

Otherwise, go to the manufacturer's web site and look it up.

d

Pearce Consulting
http://www.pearce.uk.com
  #4  
Old March 4th 06, 03:02 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?

Andy > wrote:
>I'm a total beginner. How do I measure a balanced or unbalanced dynamic
>mic's impedance? Can I do it with a multimeter and, if so, how?


No. It's not an easy thing to do.

The QUICK AND DIRTY APPROXIMATION is to measure across the coil with an
ohmmeter, and assume the mike impedance is from two to ten times the DC
resistance. You can just check between pins 2 and 3 on the fly. This
gets you within an order of magnitude. This may not work on mikes with
transformer coupling inside, though (like the SM-57).

The CLOSE APPROXIMATION is to use reciprocal law and measure the _load_
impedance of the mike when used as a speaker. Apply 1 KHz to the mike
with a signal generator through a 100 ohm resistor. Measure the voltage
across the resistor and across the mike, and use ohm's law and the resistive
divider law to figure the impedance at 1 KHz. Note that the impedance at
other frequencies may be different. You don't need to use a fancy RMS
voltmeter here, because any errors in the meter will be the same on both
measurements and they null one another out. This gets you within a factor
of about two, I think.

The RIGHT WAY to do it is to apply a reference sound source to the
meter with a pistonphone, then measure the voltage developed open-circuit
off the mike, then through a 100 ohm resistor. The math is basically
the same, but now you have the problem of the a signal source. You could
probably build one with a speaker in a tube if you didn't really care about
knowing how the impedance changes with frequency.

For the most part, it's a lot easier just to look it up on the data sheet.

Note that the actual microphone output impedance is not the same as the
load impedance that the mike should be terminated with. It's usually a
good bit lower.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #5  
Old March 4th 06, 04:43 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?


Scott Dorsey wrote:

> For the most part, it's a lot easier just to look it up on the data sheet.


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").

  #6  
Old March 4th 06, 04:45 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?

Thanks everyone for the super-good info! BTW, the reason I asked is because
I see a lot of old microphones for sale/auction (Beyer, Sennheiser, etc.)
and I can't find data sheets anywhere on the Internet.

Andy


  #7  
Old March 4th 06, 06:27 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?


Mike Rivers (that's me) wrote:
> and if it has a phone plug, it's almost high.


Wish you could edit these things after posting. That's "almost always
high" but you probably knew that.

  #8  
Old March 4th 06, 08:01 PM posted to rec.audio.pro
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Default How to measure a dynamic mic's output impedance?

"Andy" > wrote in message
...
> Thanks everyone for the super-good info! BTW, the reason I asked is

because
> I see a lot of old microphones for sale/auction (Beyer, Sennheiser, etc.)
> and I can't find data sheets anywhere on the Internet.


If you assume they're 150-200 ohms you won't be far wrong, and the number's
really not important anyway.

Peace,
Paul


  #9  
Old March 5th 06, 12:59 AM posted to rec.audio.pro
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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" >
>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."
  #10  
Old March 5th 06, 01:54 AM posted to rec.audio.pro
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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.

 




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