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Old 30th May 2012, 09:26 PM   #1
jbowman is offline jbowman  United States
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Default Help! Microphone impedance matching?

I have a dynamic microphone with 5 Ohm impedance and a radio designed to use an external electret microphone with 150 Ohm impedance. I attempted to use the two together after matching the impedances with a transformer (pretty close to a 1:30 impedance ratio), but I could barely hear anything when transmitting to another radio. ???

The transformer has an primary impedance of 16 Ohms and a secondary of 500 ohms, which is a close ratio to the microphone/radio impedances (5/150).

So does a transformer used for impedance matching only need to have the right turns ratio, or do the impedances of the actual windings need to match the source/load impedances as well?

Any way to do this without winding a custom transformer?

Oh yeah, here's the crux: It has to be passive...
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Old 30th May 2012, 10:26 PM   #2
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Well, I never came across a dynamic mic with a 5 Ohm impedance. Maybee if you could furnish some part#s we could help. E
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Old 30th May 2012, 10:29 PM   #3
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Those are winding resistances, I suppose. They are effectively in series with the source and load, respectively. Not too bad for the load side, but at the source side you have 5 ohms vs. 16 ohms. IOW you're already losing about 12 dB there, as only 5 out of 21 ohms are actually contributing signal.

Then if you really have a 1:30 turns ratio, this means 21 ohms (5 + 16) get stepped up to 19 kOhms... and connected to a 1 kOhm input. Poof, another 26 dB gone at the output side.

And who knows what kind of low-frequency response the respective inductances are good for.

In short, you need a beefier transformer. Maybe you can use one for 70V / 100V systems. Primary winding resistance <<5 ohms (like 1 ohm or smaller), inductance probably in the hundreds of mH or higher. Ideal turns ratio maybe 6..10.

Oh, and the 150 ohms of that electret mic probably was the output impedance of the built-in source follower. BTW, electrets will always be more sensitive than dynamics... easily 10..20 dB.
Quote:
Originally Posted by mickeymoose View Post
Well, I never came across a dynamic mic with a 5 Ohm impedance.
Neither did I, but Google says they seem to be common in aviation.

Last edited by sgrossklass; 30th May 2012 at 10:35 PM.
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Old 30th May 2012, 11:30 PM   #4
jbowman is offline jbowman  United States
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wow, I'm impressed, sgrossklass. you did your homework, the microphone is for an aviation headset, which I am trying to integrate with a portable radio. I can't change the microphone (or it will no longer work with the helicopter intercom system) and definitely can't modify the radio, but I can do whatever I want in-between.

I measured the DC resistance on the transformer; 2.2 Ohm primary, 45 Ohm secondary. supposedly the transformer has an primary impedance of 500 Ohms, and secondary of 16 Ohms, for am impedance ratio of about 1:30. So the turns ratio should be about 5.3:1, right?

Is the relationship between turns and DC resistance proportional? if so, the transformer is definitely not what it says it is (45 Ohms/2.2 Ohms = 20:1, not 30:1).

So do I understand correctly that I either need a transformer with fewer windings on both sides (lower inductance?) or a transformer wound with heavier gauge wire to reduce the DC resistance?
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Old 31st May 2012, 12:48 AM   #5
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An electret microphone is a very different critter than a dynamic, much higher output and requiring DC for its buffer. You'll probably end up using one.

All good fortune,
Chris
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Old 31st May 2012, 02:55 AM   #6
jbowman is offline jbowman  United States
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I know that's possible to do this effectively. I have an adapter on my desk designed to solve exactly this problem. Unfortunately, the adapter is extremely expensive and is not able to be opened up and examined. I tried, whatever was in there was fully potted in plastic and got destroyed in the process. I tried testing both sides of the adapter with a multimeter and I know that there is some sort of inductive component as well as a capacitor in there.

I only have 1 other of these adapters, and can't afford to destroy it. But I REALLY want to know what's in there...

I was convinced it was just a transformer similar to the one I tried, but thats not the case...

Thoughts?
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Old 31st May 2012, 10:17 PM   #7
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Does the commercial adapter have a power supply or batteries?
Do you have a link to this product?
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Old 31st May 2012, 10:56 PM   #8
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Quote:
Originally Posted by jbowman View Post
I measured the DC resistance on the transformer; 2.2 Ohm primary, 45 Ohm secondary. supposedly the transformer has an primary impedance of 500 Ohms, and secondary of 16 Ohms, for am impedance ratio of about 1:30. So the turns ratio should be about 5.3:1, right?
Yup.

This thing is about a factor of 3 away from what I'd like impedance-wise, but it should still work passably (though it may be a little lean in the lower registers). Hmm.

Wait a microsecond... you have used a suitably sized coupling capacitor between xfmr and mic input, haven't you? We don't want to drive the poor xfmr into saturation, though I can't quite imagine a bias supply for electrets would manage that so easily.

I also assume you had the xmfr wired up the right way around (lower impedance side at the mic).

Do you have any sensitivity spec for the mic (mv/Pa or dBV/Pa)? Run-of-the-mill small electret capsules tend to be in the -50 dBV/Pa territory, better ones a little over -40.
OK, I found something for an M-87/AIC:
Quote:
34dB to 40.98 re. 1mV AT 1KHz WITH AN INPUT OF 28 DYNES/CM²
Grr, I hate unit conversions. So that's like -34 dBV/Pa, it appears. Odd. I'd try that one with just a coupling cap.

Aircraft audio seems to be a nice mess. Oh my.
Quote:
Originally Posted by jbowman View Post
Is the relationship between turns and DC resistance proportional?
Not necessarily - that depends on wire gauge.
Quote:
Originally Posted by jbowman View Post
So do I understand correctly that I either need a transformer with fewer windings on both sides (lower inductance?) or a transformer wound with heavier gauge wire to reduce the DC resistance?
Actually you'd need higher inductance and lower resistance, i.e. more turns with thicker wire or a more substantial core.
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Old 1st June 2012, 04:16 AM   #9
jbowman is offline jbowman  United States
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The commercial adapter does not have batteries, it is entirely passive. There are actually many different versions of the same thing available online. It seems to be a common problem.

Im sure I have wired the transformer correctly (I did try it both ways actually). I also tried 2 of them in series, but then it really didn't work.

I'm sorry, a what? Coupling capacitor?

I don't have any more specs on the transformer, only what's on the online data sheet (it's part # 42tl026):
http://www.mouser.com/catalog/specsheets/XC-600127.pdf

I don't know if this helps, but the microphone is almost impossible to hear unless I literally put it in my mouth and scream. Then it sounds okay...

You guys are awesome. This is way more fun/educational than hassling the manufacturers...
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Old 1st June 2012, 05:17 AM   #10
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The commercial adaptor may have an active buffer stage inside, powered from the radio. An electret mic does, typically a JFET with open drain connected to the output signal line. The radio provides maybe 5 volts DC through a 2K2-ish resistor, all down the same wire.

Can you measure DC voltage at the radio's input jack (with transformer removed, of course)?

All good fortune,
Chris
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