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Friday, March 16, 2007

Free Design of Cheap & Nice Microphone for Amateur Radio, CB, Broadcasting, TV Station, etc.

First, for low power transmitter:

low tx pwr mic schema

This is partly my design. Original design was not like this. With years of modification, while I did many other things, I came to this design. This microphone, if you follow my instructions, will work satisfying from side of money and quality. This mic is designed to be used together with around 10 Watts maximum transmitter output power. If you use more than 10W, erratic and RF feedback most probably would come to your microphone. This feedback causes your modulation sounds distorted. But if you follow my instructions, comes out voice would be crystal clear, high quality audio. So, in big powered Broadcasting and TV Station, this mic only useful in studio, before relayed to big powered transmitter station far from studio. But if you have maximum around 10W Broadcasting or TV Station transmitter close to this mic, this mic hopefully good one.

Transistor used: 2SC828, could be not available in your country. You could change with the equivalent of 2SC828: BC167A, BC167 or MPS6566, or whatever you could get from Transistor Data & Equivalent Book.

470n capacitor is mylar one. Mylar is capacitor with plastic cover. Type of 470n mylar capacitor I use is the one with transparent yellow plastic cover. The other type of mylar has dark green colour. While both 270pF capacitors are ceramic. While capacitors with + & - sign are both electrolytic capacitors. These specifications to provide best result. But if too difficult to get, you could try with other capacitors as long as same values (with slight difference on result of course).

Mylar capacitor used to provide high quality audio, while both 270p ceramics used to absorb as many as RF feedback, so RF feedback doesn't come to circuit.

For best result, this circuit should be put in metal box, also to prevent RF feedback. Best shape for this mic design is desk microphone. With desk mic position, you could reduce fatigue. From the output of this circuit to your transmitter, should use coaxial cable, also to prevent RF feedback. Length of your coaxial should be as short as possible. DC supply needed by this circuit, could be provided by your power supply. DC supply cable should be as short as possible also. All compulsories above to prevent RF feedback enters circuit and enters your transmitter. Best cable between condenser mic to this circuit also coaxial, as short as possible.

If you've finished with assembly, now it's time to set variable resistor (VR) at output and your transmitter audio level. Best indicator to do this uses another transceiver to monitor. Try to speak while set VR and transmitter audio level. Usually, if VR and audio level too opened, this mic will be overmodulated. Lower both down, until out voice becomes clear. Another good indicator to set VR and audio level use transceiver output power indicator, especially if work with SSB. High quality audio resulted when your indicator needle runs lightly and no tremble. If VR and audio level set too high, needle movement would be heavy, and tremble.

According to my experience, your antenna should be match with your transmitter (SWR 1:1), because if antenna doesn't match, there would be great amount of RF goes back to microphone causes RF feedback. Another tips: antenna cable (coaxial) should be placed far from microphone also to avoid RF enters microphone system.

Now, for high power transmitter (up to 100W), 2 designs:

Design nr. 1:
This is completely my design, from calculated all transistor bias voltages, etc.. With this design, high fidelity audio output is hopefully. This design uses 600 ohm dynamic microphone, most dynamic mic in market & in use. To surpress RF enters this mic, DC supply provided by 3V dry batteries, because if DC supply provided by ordinary power supply, strong RF feedback from high power transmitter would come to this mic. Photo below shows design process used breadboard:

high tx pwr mic design process

More under is schematic diagram of the design:
high tx pwr mic schema

Transistor used is 2SC828. Equivalents are: BC167, BC167A, MPS6566, or whatever you could get from Transistor Data Book. 220nF capacitor is mylar one. I use mylar with transparent yellow cover. Both 270pF capacitors are ceramic. These capacitors to ground RF feedback before enters circuit. While capacitors with + and - sign are both electrolytic capacitors. Dry batteries used could be 2x1.5V small size or big size, but I suggest big ones, so they would last longer.

Use shortest coaxial cable possible from mic to transceiver to avoid RF feedback. When you ready with the circuit, try to set Variable Resistor (VR) in this design and audio level in your transmitter, while seeing your transmitter power indicator. Set both till needle movement is light and no tremble (stable). You could use another receiver to help you monitor the result. I design this mic in the form of desk microphone so could reduce user’s fatigues. I hope the result satisfies you.

Design nr. 2:
For high power transmitter up to 100W, this design is quite simple (just for emergency. I suggest design nr. 1). I’ve experimented with single dynamic microphone. Dynamic microphone is microphone has moving coil at the back of front membrane. Around moving coil, there is magnet. Moving of coil causes various magnetic field to coil wire. This causes coil wire has various AC voltages. You could search for new or old dynamic microphone probably ex karaoke or given together when you buy recording means. Disassemble the dynamic microphone. Most of dynamic microphone have 600 ohm impedance. Ideally, mic and transmitter have same impedance. Unmatch between both impedance will cause unsufficient modulation pulled to transmitter. In my experiment, my 600 ohm mic gives sufficient audio voltage to transmitter.
After that, dynamic microphone tone could be set by adding tube in front of mic that could be set on its length. Use another radio to monitor result. Set tube length till out voice becomes as you like.

I design this mic in form of desk microphone. This would ease communication, and reduces fatigues.

Sounds cheap but nice, right? Find out yourself. Good DX.

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