The sound input port is stereo and requires an ‘auxiliary level’ signal in the 100 millivolt range. This means that no standard professional microphone can be connected to this input without the use of an active preamplifier to boost the signal level.

The first generation of Power Macintosh models, up through the G3, are equipped with a unique four-conductor 3.5 mm minijack that allows the computer to detect if the device that is plugged in is stereo or mono. The input settings in the Sound Control Panel will then be automatically configured for a stereo or mono device. This is a custom-made non-standard jack that is longer than a standard 3.5 mm connector. The Sleeve is used for ground, the first Ring is used for the left channel audio, the second Ring for right channel audio, and the Tip carries 5 volts of bias to the microphone.

The bias voltage from the sound input port is used to power a special preamplifier mounted inside the standard Apple microphone that comes with the computer. The preamplifier boosts the microphone signal up into the 100 millivolt range.

Left and right audio inputs.

Inside the Apple microphone.

Connecting Mics to a Mac
To interface a standard professional microphone to a Macintosh with a built in sound input port, a preamplifier must be used to boost the output level of the mic (typically less than 1 millivolt) to the level required by the sound card (about 100 millivolts). A standard 3.5 mm miniplug will fit into the sound input port in such a way that the bias voltage in the jack does not contact any of the conductors of the miniplug, and will not be fed to the microphone.

A mono, unbalanced, auxiliary level signal that is carried by a one conductor shielded cable can be connected to the Macintosh sound input port by routing the audio to the tip of a mono miniplug and connecting the shield to the sleeve. The computer detects the presence of a mono miniplug and will set the input for mono operation.

A mono, balanced, auxiliary level signal that is carried by a two conductor shielded cable can be connected to the Macintosh sound input port by connecting pin 2 of the microphone’s XLR connector to the Tip of a mono miniplug, and pins 3 and 1 to the Sleeve of the miniplug.

A stereo, unbalanced, auxiliary level signal that is carried by a two conductor shielded cable can also be connected to the Macintosh sound input port. In this case, simply connect the shield to the Sleeve of a stereo miniplug, the Left channel audio conductor to the Tip, and the Right channel audio conductor to the Ring.