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#1
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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 |
#2
Posted to rec.audio.pro
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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 |
#3
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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. |
#4
Posted to rec.audio.pro
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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
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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
Posted to rec.audio.pro
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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
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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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