Icom IC-7300 Microphone Input Arrangement
In common with with their other rigs Icom have very kindly applied some DC to the microphone input pin (No 1) on the front panel microphone socket so that an electret condenser microphone can be used without any concerns about how to supply it ( Drain of the FET within the microphone capsule).
Here is the circuit of the microphone input arrangement within the IC-7300, it can be seen that the 1K resistor effectively becomes the drain load for the FET in the microphone capsule and the DC supply is very well de-coupled at DC, AF and RF thanks to the capacitors C1017,1018 and 1019. Any two connection electret insert can be connected directly to the microphone input (Pin No 1) and ground (Pin No 7) and the overall frequency response adjusted in the TX TONE menu to suit your own voice characteristics.
To enter the transmit mode you can either use the TRANSMIT button on the front of the rig or use a foot switch which is what I use and find very convenient. I use a foot switch which is intended to be used with a tattoo outfit and these are available on e bay for under £5 and they are of sturdy construction. I made up an adaptor lead and control mouse arrangement which suits this purpose and the details can be found elsewhere in this section.
Just as a matter of interest......................
Don't believe the utter nonsense put around by an American manufacturer of very expensive microphones and inserts that if you connect a dynamic microphone to the mic input (Pin No 1) of an IC-7300 the result will be in their words "Smoke City" because it is obvious that if you look at the rig's input circuit below that even if you connect a short circuit to the mic input the maximum current drawn will be only about 7mA, totally insignificant and will not harm the rig in any way. A coupling capacitor (NOT De-coupling Capacitor as the same manufacturer states) should however be used, the value of which should be between 0.47 and 10mfd, lower values of capacitor will reduce the LF frequency response.
Microphone Input Circuit within the IC-7300
This is the circuit within the stock microphone HM-219 and in order to maintain a similar frequency response and gain this circuit should be adopted if making your own microphone for the rig.
