[ICOM] Icom Microphones

[Adam Farson]

Hi Bob,

You may be able to adapt the headset microphones to your IC-728 if they meet
these conditions:

Source impedance 200 to 600 ohms

Output voltage 2 to 3 mV on voice peaks

You can get an approximate idea of the source impedance by looking at the
circuitry that the mic drives (if possible). Typically, an external mic for
a cellphone will be low-impedance (600 ohms or less). You can check the
output with an oscilloscope, if you have access to one.

A telephone headset mic has a very high output voltage, of the order of 500
mV to 1V on voice peaks. It has to be compatible with a carbon mic in a
telephone set. You will need to build an attenuator to reduce the output to
3 mV or less.

A 0.47 to 1 uF non-polarised capacitor must be connected between the mic or
attenuator output and the mic input of the radio (Pin 1), to prevent
shorting the electret polarising voltage. 

Incidentally, any of the Icom mics with the 8-pin Foster plug except some of
the very early ones will fit any Icom transceiver fitted with the standard
8-pin Foster socket. This includes the SM-6, SM-8, SM-10, SM-20, HM-12,
HM-14 and HM-36. There are also modular/Foster adapters (both genders).

Here is the mic socket pinout:

http://www.qsl.net/ab4oj/icom/heilsound/icom_mic.html


You may be able to power the external mic amplifier (as needed) from the
radio's +8V supply (Pin 2. Caution! 10mA max.) The Plantronics has an
amplifier, and may require a higher voltage than 8V. I am not sure whether
the cellphone headsets require external power or not. If the headset element
is an electret, you will be able to power it from Pin 2 via a 4.7K or 10K
ohm resistor between Pin 2 and the output lead (with the capacitor between
the output lead and Pin 2), or by connecting its output to Pin 2 without the
capacitor.

Hope this helps. If you would like some diagrams, I can send them to you
off-list.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of KE5CTY Bob
Sent: 31 July 2005 12:29
To: Icom Reflector
Subject: [ICOM] Icom Microphones

In talking to others, I understand that Icom Microphones are special
creatures for the Icom line of radios.

Sometimes I have noticed - you can't even swap them between Icom Models
themselves.

So I am asking this question to avoid messing something up on my rig.

I have an IC-728 and a hand microphone, I would like to be able to build a
headset microphone for it for higher quality audio output. 

I have some old Plantronics amplified headsets that were made for using on a
regular telephone and some headsets from old cell phones.

One from the cell phones (a Belkin) I especially like because of the quality
of the earpiece.

I am already using it plugged into the Headphone jack in the front of the
rig for listening.

Since I have Audio Output at the microphone jack I would like to be able to
wire this headset into the mic input for both functions of Mic and Headset.
What considerations do I need to look at to make this possible?

I don't have the money to buy a Heil or anything like that - so please let's
stay off that subject as it is frustrating for me already.

73 fer nw,
Bob
KE5CTY 


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