Cobra HG M75 Power Mic fi
x/upgrade (+20dB gain)
by Henry Lukacik
hen040@hotmail.com
Description of Mic:
Cobra HG M75 4 Pin Power CB Microphone
[HGM75] (avg
$16.16)
Product Features
?
9' High-Flex cord
?
Heavy duty drop-proof ABS shell
?
Metal mesh grille
?
Left-side PTT button
?
Powered 2 transistor amp with gain control
Improvements:
If factory audio using this power mic (at max vol
ume) is not much different from stock/dynamic
mic this article might help. It will increase th
e audio dramatically matching or surpassing most
power mics on the market without distortion.
Findings:
After purchasing microphone, noticed very low audio compared to the older 5-pin cobra power
mic CA71B. Rewired the 5 pin cobra mic to 4 pin so that comparison can be made, and found
audio was much higher with the CA71B than the HG M75 (at max volume). In fact the HG M75
volume was closer to a non-power stock mic with a dynamic 600 ohm element, known for a
telephone sounding (300Hz to 3KHz) low audio. Very unusual considering my new power mic
HG M75 uses a 9V battery and the CA71 uses a 6V battery.


Here?s a picture of the comparison mic:
Cobra CA71B 5 Pin Power CB Microphone
[CA71B] (avg
$14.99)
Features volume control on back of mic.
Requires one 6V battery. (Eveready A544
or equal)
Technical:
Upon opening both mics, noticed extreme similarities
in circuitry. A (sensitive) electret-type mic
element feeding a basic 2 stage audio amplifier using 2 general NPN transistors with common
passive components (resistors and caps) for coupling, rf reject, biasing and volume control. In
fact other than the CA71B using SMT (Surface-Mount) components and the HG M75 using
standard components the circuit a
ppears to be exactly the same.
Approach:
Draw a schematic and do a side by side compar
ison of both mics. Simulate through CAD circuit
analysis. Although they?re might be beta differences
in the transistors, generic NPN?s are fine in
circuit simulation as this is in the audio
frequency range. End result is to have
noticible
gain.
Note: Component designations on boards and in
simulation are different (example: C1 in
simulation is not C1 on board)


Results:
Cobra CA71B sim (there are 2 more caps and a r
esistor that are not included here as they have
not made an impact in the sim):
Nice allmost 40db gain...at higher audio frequencies


Cobra HG M75 sim (new from factory):
Less than 20dB gain at peak...that?s poor.


The Improvement:
R6 (1K) should not be there, it should have been a cap. Even the silk-screening of components on
the board indicates that there should be a cap (elect
rolytic). Being that C1 is a low enough value,
no cap needs to be added (series caps lower
in value). Just short R6 making it 0 ohms.
R7 is too high of a value, it should be in the 100?s of ohms. 220ohms works really nice.
About 40dB at 1KHz..
High?s modulate stronger than lows, but if
more low range is desired increase C1 to .47uF


Increased C1 to 0.47uF
Now, that?s audio...over 40dB gain from 500Hz to 22kHz