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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?
"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
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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 |
#4
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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
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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?
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
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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 |
#9
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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
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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. |
#11
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How to measure a dynamic mic's output impedance?
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#12
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How to measure a dynamic mic's output impedance?
On 4 Mar 2006 17:54:53 -0800, "Mike Rivers"
wrote: .... 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. I agree that as soon as transformers, or even dynamic mic coils are involved, things get really complex. 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. And in my line-level example, I'd personally prefer to have direct, DC-coupled connections between each source and my amp - but of course that would eliminate almost all commercially produced equipment. .... 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. My best guess is that I am hearing effects of the DC blocking capacitors in my source outputs - I don't know what else to suspect. Eliminating them (by adding active circuitry) or improving them gets me closer to the goal I'm seeking with the impedance adjustment. The input stages of my homemade amp are DC-coupled and capacitor free. 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? No transformers. Variable resistance across the amp inputs. Have you looked at the signal at the input of your switcher to see how it's changed when you change the impedance? I can't "see" anything testing with pure tones, but then what I hear is mainly changes in the "space" of the soundfield, rather than in the tonality of the music, so the differences are probably tiny phase shifts. 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. You're suspecting poorly designed output stages? Loren |
#13
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How to measure a dynamic mic's output impedance?
Loren Amelang wrote: I agree that as soon as transformers, or even dynamic mic coils are involved, things get really complex. And this encompasses at least half the mics in existence, so you can't ignore them as part of a general discussion. And in my line-level example, I'd personally prefer to have direct, DC-coupled connections between each source and my amp - but of course that would eliminate almost all commercially produced equipment. Yup. But you can always build your own capacitorless electronics for a condenser mic. My best guess is that I am hearing effects of the DC blocking capacitors in my source outputs - I don't know what else to suspect. That's possible. Capacitors can be pretty non-linear. But either you're awfully sensitive or you have a good imagination. Trouble is that you can't eliminate them without changing a lot of other things, so you can't compare with and without. No transformers. Variable resistance across the amp inputs. This could certainly affect the frequecy response of a mic with output capacitors. I can't "see" anything testing with pure tones, but then what I hear is mainly changes in the "space" of the soundfield, rather than in the tonality of the music, so the differences are probably tiny phase shifts. You can see and measure phase shift with test equipment. You just aren't trying hard enough. |
#14
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How to measure a dynamic mic's output impedance?
On 6 Mar 2006 13:14:18 -0800, "Mike Rivers"
wrote: That's possible. Capacitors can be pretty non-linear. But either you're awfully sensitive or you have a good imagination. Trouble is that you can't eliminate them without changing a lot of other things, so you can't compare with and without. That's one I have been able to do, at least to my satisfaction. On the few bits of gear I have bothered to modify to add direct DC outputs, I can switch the blocking capacitors back into the connection after the new output stage. Flip the switch and hear the sound space blur... I can't "see" anything testing with pure tones, but then what I hear is mainly changes in the "space" of the soundfield, rather than in the tonality of the music, so the differences are probably tiny phase shifts. You can see and measure phase shift with test equipment. You just aren't trying hard enough. Got me. "Someday" I hope to have access to good enough test equipment and sufficient time simultaneously... Loren |
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