Some Thoughts on Mics

[Steve & Anne Ray]

I have had my K2 since 2000 and really enjoyed it, when I first built the
KSB2 I was using a Kenwood MC50 mic.   When I built the KSB2 it had the
original crystals supplied, back in 2000, I have since changed out crystals
to the matched set.   The audio drive with the MC50 mic was insufficient, to
drive the K2 to the correct output power.   Upon recommendation of the
Elecraft mailing list, I changed R-14 from 1K to 10K, while this gave me
plenty of drive, I got reports of the audio being very bassy.  Discussions
with several hams and Bob Heil of Heil Audio, all told the same thing.  The
MC50 was designed for hams in Japan and tended to boost the bass
frequencies.  I changed the element in the MC50 to a Heil HC-5 element and
started getting great audio reports.  For an experiment I changed R-14 from
10K back to 1K and found that it was lacking the necessary audio drive.
Going back to 10K at R14 provided plenty of audio drive.

Recently I decided to try some other mics, the following is the results.

Computer electret mic, biased with a 1K resistor and 5V.   Good audio
reports and very cheap to implement.  I have only tried this with R-14 at
10K.  

D-104 mic with the circuit proposed by N9WB to transform the impedance from
high to low.   http://www.qsl.net/hcara/D-104.htm   Again good audio
reports.  When I first built this it worked fine on QRP, but anything over
25W resulted in rf feed back.   I built a simple pi network on the output of
the design by N9WB and solved the problem of rf feedback.   The pi network
circuit was a couple of .002 microF caps with a 1mH choke.
 
Output Mic Amp _______1mH_______  Input to K2
                |           |
             .002mF       .002mF
                |           |
                G           G

Heil HC-5 with the amp suggested by N9WR
http://www.ac6rm.net/mailarchive/html/elecraft-list/1999-11/msg00700.html
N9WR amp works well, but I found it was not needed if I left R-14 at 10K.
I also found I had to again put a pi network filter on the output to
eliminate rf feedback.  I used the same pi network as I used for the D-104
circuit and solved the rf problem.

Well on the air reports are great, but they are subjective and prone to the
bias of the person who reports to you.   I found a easy way to tell how the
audio sounds is to use my psk31 interface with  the KSB2 in the bal mode and
record into the "Sound Recorder" on my PC, then play it back.  I found this
works much better then trying to monitor my audio while I am speaking into
the mic.   This way I get to hear how the audio sounds to me.

Your results may vary, but I have decided to remain with the HC-5 element in
the MC-50 mic, with leaving R-14 at 10K and not using a preamp.  

I have no connection with Heil, or the D-104 or any other mics.

72,
Steve Ray K4JPN ex K1VKW
EM82ep Warner Robins GA
Elecraft K2 1422 & KPA-100
Heath Fan HW-101, HW-8
http://www.thewinstonator.com/K4JPN.htm

 



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