The ICOM IC-SM5 is a medium gain amplification microphone made to match some second generation formers ICOM RIG up to the ICOM IC-251A with the same MIC pinout, meaning +9V and audio IN on PIN 1 ( IC-251, 290, 451, 505, 720, 730, 740 ). In this case medium gain means high gain.
This MIC is an electret base station microphone with amplifier, a two stage transistorized amplifier. The first stage being a N- FET transistor needed to match the very low current of the electret insert. Insiders informations said that the insert was of high quality and having a high voltage output.
The MIC has a PTT lock facility, is very well built, nice footprint, full metal jacket, labels are high quality. This MIC had a modulation level regler below the mic and give a very large dynamic range from about 0 to 446mV (given@94dBSPL).
My exemplar gives a little less as the minimum -34dB given (199mV @ 94dBSPL), I got 120mV @ 94dBSPL, but if you have a second generation ICOM RIG and your IC-SM5 is a little bit out of specification and too low on voltage, you just have to try to speak a little louder as I got with my specimen 500mV@110dBSPL before distortion appears. Some internet sites give a make year earlier as 1978 but I can’t find any information from before 1978.
Year : 1978
Type of microphone : Electret
Microphone characteristic : uni directional
Sensitivity : -27dB -+6dB @1000Hz 0dB=1µbar
Nominal output : 446mV
Measured output : 120mV (potentiometer @ max)
Max output : 500mV @110dBSPL before compression (measured)
Output at mid range : 60mV
Frequency response : 400Hz – 10KHz
Output impedance : 1300 Ohm (measured max level)
Amplifier voltage gain : not given
Compression : No
Meter : No
Echo : No
Lock switch : Yes
Tx LED : No
Roger Beep : No
Battery : No (9V from RIG apply to MIC PIN 1)
Remarks : All measurements were made with 1300 Ohm load @ 94dBSPL. -27dB (0dB=1Volt/microbar) is a quite old reference system used mostly by non european manufacturers. -27dB(0dB=1Volt/microbar) is equivalent to -7dB(0dB=1Volt/Pa) the now used reference. This means that if you speak loud (not cry) in your microphone from about 2 inches a distance you should get 446mV at the output of the microphone (at 1KHz and 1300Ohm load).
I make a simulation of this MIC in LTspice IV and it works great, with the 40mV measured at the FET gate and used for the voltage source I got 400mV at the output under 1.3KOhm, very close to the official value:
I replaced in the simulation, the electret capsel with a voltage source (no electret in my componenet data base). Seems that the FET is a MPF102 (seems to be the replacement one of choice) for the first stage, impedance transformer. I use a BC550B for the second stage, the one I had in my data base that matched the 2SD661.
Original ICOM schematic:
My opinion, A good microphone with a pleasant modulation on all mode, silent pedal switch, solid full metal jacket. Built to highest standart and built to last. This MIC can match many ICOM RIG from second to last generation (pay attention on MIC pinout with third ICOM RIG generation) due to his output voltage versatility.
You can really cry in this mic before getting distorsion and the output level will be very high.
You can also use this MIC with non second ICOM but PAY ATTENTION TO THE NECESSARY CAP (1µ works fine), see information below:
For use with NON ICOM RIG from second generation you will have to add the 3 components on the right:
The 1K resistor (as small as you want, current is about 5mA) between your 9V source (6LR61 for instance or taked from your RIG if available) and IC-SM5 MIC PIN 1.
The 0.001µF non polarized ceramic cap between IC-SM5 PIN 1 and ground from the IC-SM5 and your RIG’s MIC ground.
The 1µF electrolytic polarized cap between IC-SM5 PIN 1 and your RIG MIC audio input (this cap will block the DC component to your RIG.
Also, another word of caution, don’t use a dynamic microphone (unless e clear and specific information from MICs makers such as HEIL, that the dynamic MIC is fully compatible to ICOM standart) with ICOM second generation RIG. Your dynamic insert will be destroyed due to the DC component on RIG MIC PIN 1.
Seen price: 30€ (used conds) to +/-75€ (very good conds).
My evaluation : 3.5/5 (only 3.5 due to the mod to be done to use this MIC with some others RIGs).