Heil HC5 and ICOM Radios

Did you ever try a Heil HC5 with a 746PRO or a 756PRO2 or PRO3 ?  Typically, they sound very bassy unless heavy EQ is used. Another problem is the low output of the microphone element. This can cause problems with VOX as well.
    The problem is that the input impedance of the ICOM microphone circuit is fairly low. The Heil element can be thought of as a tuned circuit. It is actually an acoustic transmission line that is resonant in the mid range. When the element is improperly loaded, the Q of that circuit is changed drastically, and so is the resultant frequency response. The first picture is what the frequency response of the Heil HC5 and the DSP combined look like when the DSP is set flat and wide.

Heil HC5 and ICOM DSP total response


 Note the bass hump and general shape of the response curve. From this it can be seen why it sounds so bad. I made a simple preamplifier that primarily serves as a buffer for the Heil element. The idea is to show the mic element a fairly high impedance so as to reduce the loading. The next picture shows the result of that addition.

Heil HC5 with Buffer and ICOM DSP


 Quite a difference! And the audio now sounds pretty good. Again the DSP is set for flat and wide. If you do build a buffer preamplifier for your Heil elements, I recommend a voltage gain of about 3. This places it in the same ballpark as the ICOM Handheld microphone that comes with the radio. Below is an overlay for direct comparison.

Overlay of Both Curves



The Test Circuit

  Below is the mic preamp circuit used to produce the buffered response curve. As you can see, it is very simple. The 8 volt supply and 1k resistor on the right represent the ICOM 746PRO input circuit. The signal generator, resistor and capacitor on the left represent the microphone.




  Below is the frequency response of the buffer amplifier. The high frequency roll off is due to RF bypassing (C4,R9).
The -3db points are approximately 17hz and 27.7Khz.



James E. Lanier

Gas Dynamics Lab