KX3 Microphone - Reverse engineering

I need a favor/favour from a fellow owner of a MH3 Microphone that is compatible with the Elecraft KX3.

The problem is simple, the electret microphone has a RC filter network inbetween the insert and the audio input of the radio. This is probably a nice commercial trick, but that is down to opinion. This alters the frequency response of the microphone.


Note the unmarked elements.

Feeding it with a standard non-filtered e.element produces not so good audio - as you would expect. This doesn’t need discussing, and yes EQ has been implemented extensively.

I could calculate \ guess the values from the response, but this is easier. Elecraft deliberately do not state the values ( I wonder why …).

If someone could PM or state below the values that would save me further work. It only takes the undoing of three screws. The motive is not having to purchase the £55 cost of the MH3 vs the cost of a £1.50 e.element …

I am sure this will be of use to others …


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My mike doesn’t agree with your schematic.
What I have is the following.

20150316_161334 by G4ISJ, on Flickr


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Hi Jonathan,

I wonder if you have seen M6NJX’s YouTube video Elecraft MH3 Mic into Oscilloscope?

Relevant part is in the 5th minute.

His mic has the 'horizontal capacitor and resistor at the top, and the values are stated as
0.33 microFarad and 2 kOhm respectively. His mic apparently does not have the
other resistor and two capacitors fitted.

Hope this is of use

Kind regards

Dave G0ELJ

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Looks like I doubled with you Pete!

Kind regards

Dave G0ELJ

We need QSK :smile:

I think we do Pete, but I have just noticed that whilst we agree the values,
you have the .33 microFarad across the output, M6NJX has it in parallel with the
2k Resistor.
I haven’t got a clue!

Kind regards

Dave G0ELJ

Thanks Guys,

A great response from you both :smile:. I did Youtube it - didn’t see that come up though.

I found that schematic here : - http://www.wn8u.net/. Maybe Elecraft use different configurations ?

Pete - This seems to be a little strange, that is a low pass filter. I am struggling for top response at the moment, I don’t think that configuration will work.

I am thinking …


Dave you’re probably right! I was trying to shine a light through the PCB to see the track layout with limited success.
It would be more logical if it was in series as the .33µF across the mike element doesn’t make much sense.
I would have disassembled more and buzzed the connections out, but I’m stretched for time right now (Packing for GI).
I expect it’s a series capacitor as you say.


See above. I believe Dave is correct, the R and C are in parallel.


OK he uses a series shunt R//C this would make more sense.

Little bit of work with LTSpice and Bing !

£55 saved …



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