User`s guide

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From: Rubywand
003- My IIgs has a stereo cable plugged right into the
Sound Output jack. Isn't that two-channel sound?
Nope; and, it is not "stereo" either. The jack is a standard stereo jack;
but, the "Left" and "Right" outputs are connected to a single 'composite'
source. Possibly, Apple originally intended to supply two-channel sound;
perhaps, the idea was just to simplify connections to stereo systems. (Stereo
devotees get upset when you give them just one 'channel' to plug in.) Whatever,
the fact remains: for multi-channel output you need to add a "stereo board".
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004- Why should I add a stereo board to my GS?
Some programs offer true stereo-- effects and music are lifted from stereo
sources or 'recorded' using two mics-- others deliver simulated stereo. Many
products output some effects (like a bow twang) through one channel and other
effects (the THUNK! of an arrow hit) through the second channel. Stereo, of
course, produces spacious, '3-D sound'; but, even the 'separated channels'
approach can spread out the action and add realism.
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005- Can I build my own GS stereo board?
Yes. Apple includes a rough outline for a stereo board design in the GS
Hardware Reference Manual. The "TDX Stereo Board" is a real-world realization of
the Apple description.
Note: See the FAQs Resource file R004STEREO.GIF for the TDX diagram.
From input to output, the TDX design is straight-forward and simple. First,
IIgs audio enters the 14052 where the "Left" and "Right" channels are separated
using the C0 input to turn ON the appropriate section (output X or Y) when its
channel is valid. (This happens at supersonic speeds so that the user doesn't
notice that each channel is ON half of the time.) Then, each output goes to a
pair of op amps where it's filtered and amplified.
Design objectives were low noise, low distortion, and low power
consumption. From the start I expected that on-board power amp IC's would be too
puny to drive our speakers to desired volume levels and maintain low distortion;
so, the board includes no power amp IC's and is not intended to directly drive
low impedance loads such as speakers. Like a tuner, CD deck, or other hi-fi
source, it connects to a stereo amplifier's AUX inputs (or "Tuner", "Tape", etc.
inputs) or to the inputs of speaker units wi
th built-in amplifiers. TDX has plenty of juice to drive any decent stereo
system at 'blow out the windows' volume levels.
As shown in the diagram, nearly all connections between TDX and the
computer, including ground, are made over the J-25 lines. Power (+5 Volts and -5