I have an RCI-2950DX. When the radio first arrived, I couldn't wait to get a few signal reports after putting up an antenna and hooking up the radio. Unfortunately, I found out pretty quickly that the stock microphone really does a poor job in modulating the radio. It was kinda disappointing, and I had wondered if I had made an expensive mistake. When I ran AM, I'd get reports that said my modulation didn't match my carrier... audio too low. The radio has a mic gain control, but I had it wide open and it didn't matter. It wasn't because the radio was out of adjustment, I checked that also.
Reports were lackluster on SSB also. Man... what did I do... buy an expensive (to me) "dog"?
Well, I found out... "everybody knows" the stock mics are always junk.
Great...
I just bought an expensive radio and now I need to spend more for another (power) mic.
O.K. now what? Not wishing to spend a lot more on another mic, I had an idea. I'd build my own power mic! This one would be different. I rejected using a simple amplifier circuit to boost mic volume. That just leads to splatter as you're not controlling the peak audio power. What I really needed was a speech compressor. You increase the overall talk power without raising the peak audio amplitude.
Yes! O.k. Now which circuit? After a lot of reading, I settled on a one IC circuit using a single SSM2166. It combines everything needed in one chip to accomplish the task of speech compression. I found a number of schematics online to use.
The stock mic on the radio has a 6-pin circuit to provide for channel changing at the mic. It is a dynamic mic, and has a little "speaker" in it that acts as the microphone. That speaker and the case it sits in takes up most of the space in the mic. There's no room to insert any other circuit in the mic.
O.K., now what? I remembered I had an old cobra 4-pin mic I wasn't using. I looked inside it and it had a tiny electret mic element inside with plenty of room for another circuit.
Hmmmm.
First things first... By luck, I had bought a couple of the SSM2166 compressor IC's a number of years back. Good! I decided to build my own compressor circuit, as small as I could, and mount that circuit inside of the cobra mic. I used a piece of copper clad circuit board as the mount for the IC as well as for all the other components the chip needed. I mounted the IC on the copper upside down, "dead bug" style and fixed it in place with super glue. Following the schematic, I placed all the components around the IC that are needed to make the IC a complete circuit. I was careful to minimize all lead lengths so that the completed circuit board would be as small as possible. I also added some rf choke beads on input-output-power leads as a precaution.
After building the compressor board, I took both mics apart. I lined the cobra mics covers with copper foil to act as an rf shield. I took the original 4-pin mic cord out of the cobra housing and set it aside. I took the 6-pin cord out of the stock mic and placed it in the cobra housing. I placed the compressor board in the cobra mic... it just fit. All that remained was wiring the electret mic element to the input of the compressor circuit, and the output to the mic cord to the radio.
You may wonder what powers the compressor circuit. It needs 5 volts. Fortunately, I found that the original mic wiring provides 8 volts into the microphone via a black wire. I used the 8 volts provided. I added a 5.1 volt zener diode circuit to drop the 8 volts down to 5.1 for both the compressor circuitry and the electret mic cartridge power.
Yes, there was a bit of troubleshooting needed to adjust some of the circuitry surrounding the IC, and I also had to attenuate the audio from the electret mic... it was too much at first for the circuit. I also found I had to really back down on the mic gain at the radio also... no problem.
The bottom line. I get GREAT audio reports, both on AM and SSB. The mic really has brought the radio back to life. I'm not splattering, yet the radio sounds "louder" than before. I can see the difference on an in-line power meter. The average ssb power stays a lot higher on the meter than it did with the stock mic. This project is a "win" for me.
The new 6-pin Cobra power microphone:
The label stuck into the cover acts as an insulator between the copper foil and the circuitry.
Reports were lackluster on SSB also. Man... what did I do... buy an expensive (to me) "dog"?
Well, I found out... "everybody knows" the stock mics are always junk.
Great...
I just bought an expensive radio and now I need to spend more for another (power) mic.
O.K. now what? Not wishing to spend a lot more on another mic, I had an idea. I'd build my own power mic! This one would be different. I rejected using a simple amplifier circuit to boost mic volume. That just leads to splatter as you're not controlling the peak audio power. What I really needed was a speech compressor. You increase the overall talk power without raising the peak audio amplitude.
Yes! O.k. Now which circuit? After a lot of reading, I settled on a one IC circuit using a single SSM2166. It combines everything needed in one chip to accomplish the task of speech compression. I found a number of schematics online to use.
The stock mic on the radio has a 6-pin circuit to provide for channel changing at the mic. It is a dynamic mic, and has a little "speaker" in it that acts as the microphone. That speaker and the case it sits in takes up most of the space in the mic. There's no room to insert any other circuit in the mic.
O.K., now what? I remembered I had an old cobra 4-pin mic I wasn't using. I looked inside it and it had a tiny electret mic element inside with plenty of room for another circuit.
Hmmmm.
First things first... By luck, I had bought a couple of the SSM2166 compressor IC's a number of years back. Good! I decided to build my own compressor circuit, as small as I could, and mount that circuit inside of the cobra mic. I used a piece of copper clad circuit board as the mount for the IC as well as for all the other components the chip needed. I mounted the IC on the copper upside down, "dead bug" style and fixed it in place with super glue. Following the schematic, I placed all the components around the IC that are needed to make the IC a complete circuit. I was careful to minimize all lead lengths so that the completed circuit board would be as small as possible. I also added some rf choke beads on input-output-power leads as a precaution.
After building the compressor board, I took both mics apart. I lined the cobra mics covers with copper foil to act as an rf shield. I took the original 4-pin mic cord out of the cobra housing and set it aside. I took the 6-pin cord out of the stock mic and placed it in the cobra housing. I placed the compressor board in the cobra mic... it just fit. All that remained was wiring the electret mic element to the input of the compressor circuit, and the output to the mic cord to the radio.
You may wonder what powers the compressor circuit. It needs 5 volts. Fortunately, I found that the original mic wiring provides 8 volts into the microphone via a black wire. I used the 8 volts provided. I added a 5.1 volt zener diode circuit to drop the 8 volts down to 5.1 for both the compressor circuitry and the electret mic cartridge power.
Yes, there was a bit of troubleshooting needed to adjust some of the circuitry surrounding the IC, and I also had to attenuate the audio from the electret mic... it was too much at first for the circuit. I also found I had to really back down on the mic gain at the radio also... no problem.
The bottom line. I get GREAT audio reports, both on AM and SSB. The mic really has brought the radio back to life. I'm not splattering, yet the radio sounds "louder" than before. I can see the difference on an in-line power meter. The average ssb power stays a lot higher on the meter than it did with the stock mic. This project is a "win" for me.
The new 6-pin Cobra power microphone:
The label stuck into the cover acts as an insulator between the copper foil and the circuitry.
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