I believe that the information contained here is correct, but the possibility of errors always exists. Please note that in all cases, it is your responsibility to make sure that what you're doing is correct. Consult with your computer's documentation, or contact Apple if in doubt. I cannot be held responsible for any damage to your computer.
There have been two styles of microphone inputs for the Mac over the years, the older style Apple Omni-Directional Microphone and the new style PlainTalk Microphone. The two microphone styles are not directly interchangeable. In addition, some Macs (the AV models in particular) have RCA-style jacks, allowing the direct connection of line-level signals.
Both microphone styles use a plug with three conductors. Following the industry standard, these three conductors can be called the Tip (the end or point of the plug), the Ring (the middle conductor), and the Sleeve.
Both microphones use the 3.5 mm style plug, not the 1/8 inch style plug, which is only 3.175 mm in diameter, and will be too loose.
_ / \ TIP Monophonic microphone output \_/ | | | | RING +5V Power |_| | | | | SLEEVE Ground (common) | |
_ / \ TIP +5V Power \_/ | | | | | | RING Blended (L+R) Microphone output | | |_| | | | | SLEEVE Ground (common) | |
The older Mac models used the Omni-Directional Microphone, which produced low level audio signals, typically around 20 millivolts. The PlainTalk Microphone produces a line level audio output, typically 200 millivolts, up to 2 volts.
If you have a Mac which uses the older microphone style, and you want to run direct (line level) audio into your Mac, you need to attenuate the audio level. Most Macs came with a small adapter that plugged into the Mac, and gave you two RCA jacks which accepted line level audio. You should be able to build your own, using a suitable audio transformer (the Mac wants around 600 ohms, line-level is around 47,000 ohms). You may be able to get away with a simple resistor-divider also. Not ideal, but it may work.
As you can see, the power and audio signals are reversed between the two microphone styles. Also, note that the PlainTalk microphone is slightly (0.25 inch) longer than the normal 3.5 mm plug (0.75 inch vs 0.50 inch).
You should not just cut a PlainTalk microphone to use the plug. You generally don't need the extra plug length, since all it gives you is the +5V power, which is seldom needed. And, you run the risk of shorting out the +5V, which would be bad. Just use a standard 3.5 mm plug. You may want to run the audio to both the Tip and Ring, so it is fed into both the Left and Right channels.
If there is, you may have to experiment with the wiring. One possibility is to use transformer(s) to break the ground loops.
Another one, which I have used, is to just break the ground (common) between the audio input on the Mac, and the radio, relying on whatever other ground is present.
But don't let all of this intimidate you. It's really quite easy to connect your Mac to your radio. If you just want to get started quickly, you can always try setting your Mac's microphone over your radio's speaker. No, it won't work great, but it will let you get started exploring the fascinating world of digital communications with your Mac!
The following Macs use the new PlainTalk style microphones:
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