Using Ceramic / Crystal Mics on New Radios

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WA2SQQ:
So I’ve used a JFET Source follower to match my D-104 to my Flex Radio – works fine. I recently found my old Turner 454X which I believe had an impedance of ~ 50K. Got two questions:

1.   What is the easiest may to actually measure the impedance of the mic?
2.   Can I use the same JFET Source follower and just drop the input resistor to, say 50K to 100K?

N1BCG:
This is a very popular question, particularly with the number of newer rigs being used with older Hi-Z microphones.

It has to be said that no microphone should be tested with a resistance meter (VOM) because it won't produce a meaningful reading and the microphone element may get damaged by introducing a current, even a small one.

The best way is to locate the manufacturer's data for the recommended load impedance, which is often a range. Another way which is a bit trickier is to vary the load resistance to match the design frequency response and output voltage. Clearly the latter method is quite involved and requires specialized equipment.

Your idea of using a matching resistor is reasonable.

W2NBC:
The "X" on your 454 identifies it as a crystal element. (very high impedance). The source follower will work FB!

MikeKE0ZUinkcmo:
I would use a simple test setup and establish a 1KC audio source set to a level of 94db SPL.   Replace the SPL meter with the mic element  placing the face of the diaphragm at the same location as the SPL meter  was.   Measure the element's output with a 10 meg Ohm meter, then substitute load resistors, starting at 10 meg Ohms, and reduce resistance until the element's output drops 6db.  That's the elements source resistance.  Don't forget the meter's 10 meg Ohm input is in parallel with your added load resistor.

All that said, I'd just do as Clark suggested, try the circuit you used for the D-104, it'll probably work fine.



Click pic for more..

Mike

WA2SQQ:
Neat idea, that I can certainly do. Thanks

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