From Newsgroup: alt.ham-radio.hf
Hank queried:
Does anyone have a preamp circuit for the microphone or know how to get a proper mike for this radio?
Hank
Hi Hank,
At first blush, Stan's suggestion sounds very good. Heil (
http://www.heilsound.com/) makes top-notch microphones -- I bought a
Pro-Set Plus headset + footswitch for my Kenwood and love it. However, as described here, you may be out of luck with your 720 and Heil:
http://www.heilsound.com/amateur/icom_fix.htm. So much for the easy way
out!
The instruction manual for the IC-720A (which can be found at
http://www.ham.dmz.ro/icom/ic-720a.php) says the microphone input impedance
is 1300 ohms, with a typical 120mV input level. And, from paragraph 3-7 in
the instruction manual:
"The microphone supplied with the IC-720A is the IC-HM7 which contains a pre-amplifier. The optional electret condenser type stand microphone IC-SM5 can be used. Their circuit diagrams are shown. Should you wish to use a different microphone, make sure it has a proper pre-amplifier."
I saved their microphone schematics here:
IC-HM7 schematic:
http://tinyurl.com/y9skcu
IC-SM5 schematic:
http://tinyurl.com/yf5mm3
Because dynamic microphones are as common as dirt, and the HM7 uses fewer components, I recommend cloning that circuit. With only a handful of components needed, you can build it on a small scrap of circuit-board
material and mount it inside the rig (double-sided foam tape works well).
The 2SD661 transistor can be substituted with an NTE16 (available from
Mouser).
73,
Bryan
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