Auxiliar De Enfermeria

Cobra HG M75 Power Mic fix/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 maxvolume) is not much different from stock/dynamic mic this article might help. It will increase the 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 muchhigher 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) lowaudio. 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 volumecontrol. In fact other than the CA71B using SMT (Surface-Mount) components and the HG M75 using standard components the circuit appears to be exactly the same. Approach: Draw a schematic and do a side byside comparison 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...