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Andy
 
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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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Don Pearce
 
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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
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Bob Quintal
 
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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.
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Scott Dorsey
 
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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."
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Mike Rivers
 
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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").



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Andy
 
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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


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Paul Stamler
 
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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


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