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Connecting a stereo to an early 1990s Macintosh

THE SITUATION

Apple did a Good Thing when they put a microphone input jack in their 1989-1993 models.

Then they did a Bad Thing by making that jack perfectly incompatible with everybody else's plugs.

You want to plug in your Official Mac Mic? Fine.

You want to plug in some other brand of mic, or stereo or CD? Unh-uh!

Current Mac models that use the PlainTalk microphone (most Quadras and PowerMacs) don't have this problem. They can accept a standard line-level signal on a standard plug.

THE PROBLEM

[If you don't care what's causing the problem, jump to "THE SOLUTION"]

First of all, that Apple input circuit is designed for 4/1000 volts. That's okay for some third-party microphones (but see "second of all"), but useless for the kind of signals that come out of a stereo. Try to record from a stereo, and you'll hear mostly hum and distortion.

[Technical explanation: You get distortion because even the soft parts of your stereo's output are so loud they turn on most of Mac's digitizing bits. When it hears a loud part, there aren't any bits left.]

Second of all, Apple's mic input connector isn't wired like anyone else's on the planet. So even if a microphone or patchcord looks like it should plug in, it probably won't connect properly.

Apple's jack matches "stereo mini-plug": these have two end contacts (the metal tip, and an isolated ring next to the tip) and a long metal barrel. Everybody else uses the tip and ring for stereo signals, and the barrel for the signal ground. But Apple? Noooo.

Apple puts 8 volts on the ring connector, to power an impedance matching circuit in their microphone. (Don't worry why the 8 volts is necessary; it just is.) These volts are allegedly limited, so they can't damage non-Apple equipment. But Apple makes no guarantees... and the volts can most definitely mess up the output circuit of a standard-wired line signal.

The plug also conveniently fits a "two conductor mini-plug", found on many third-party microphones. Unfortunately, the barrel of these standard plugs happen to short out the 8-volt supply. (If you've ever stuck a screwdriver into a light socket, you know that shorts are things that power supplies frown upon.) Again, the Mac supply is limited so a standard mic shouldn't damage your computer. But, again, Apple makes no guarantees.

THE SOLUTION

A)
Get Apple's official Line Input Adapter Cable (there was probably one packed with your computer). It looks like this:

Plug either one or two "RCA" or "Phono" cords from your stereo into the jacks on that large egg-shaped thing on the left. Plug the other end into the MIC input jack on the back of your Mac.

Don't use a standard y-connector, even though they look alike. Apple's Cable includes an isolator and attenuator.

If you don't have the official Apple puppy, see the next section.

If you do, you can jump right to "Recording Tips".

B)
Build a circuit like this (or bribe a handy high-school student with a soldering iron to make it for you):

You can get parts at any electronics store or radio repair shop. Radio Shack numbers are given, simply because their stores are in every shopping mall in the known universe. Of course, you can replace the phono plug with anything that'll fit your hifi. Use any kind of shielded wire, and keep the wires as short as possible for best result (longer wires pick up hum and noise).

Use two resistors and one phono plug for a mono signal, or build the second half of the circuit to make it easier to hook up stereo signals -- the Mac will record them in mono anyway.

This circuit should work fine with the headphone output of battery-powered tape players, too (you'll have to experiment with the volume control on your tape player to find the best recording level). Don't connect it to the speaker outputs of anything: you can damage your computer that way.

The two (or four) resistors lower a line-level signal sufficiently for the Mac input. Leaving the ring connector (the middle one in our diagram) disconnected isolates the 8-volt supply.

RECORDING TIPS

If the sound source has a volume control, adjust levels there... not with any on-screen control or "automatic level adjustment" in the Mac's software.

If you hear hum when recording from a stereo, try the circuit with a battery-operated radio or tape or CD player. Power lines can introduce noise.

You can use the same attenuator circuit with a 1:1 isolation transformer and capacitor to record from your phone. Check the document "Mac2Tel", coming to this web page soon (and currently on various ftp sites).

DISCLAIMERS
The circuit follows standard engineering practice and works. But Apple and other companies can mess with specifications, anybody can burn themselves with a soldering iron, and actual mileage may vary. So the author makes no guarantees.

Apple, Mac, Radio Shack, and Lord-knows-what-else are all registered trademarks of their respective companies. ("Lord" is probably a registered trademark of Microsoft.)

This document is (c) 1993,95 Jay Rose.


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