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19.005 Can I build my own GS stereo board?




Description

This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.

19.005 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 Volts) comes from the slot into which the board is plugged. The outputs go to
"RCA-type" hi-fi jacks mounted on the rear of the board for easy access via
standard audio cables. When placing the jacks, be sure to allow space (between
the jacks and rear of the computer) for the cable plugs or arrange for the
jacks to line up with an opening.

     The J-25 connection is via a 7-pin mini-molex ribbon cable. It can go to
J-25 (located near the memory expansion slot); or, if J-25 is being used by the
Hyperstudio A/D input board, it plugs in there. (On both J-25 and its extension
on the A/D board, pin #1 is nearest the front of the computer, pin #2 is next,
etc..)  All of the parts, including the Apple-compatible circuit board, are
commonly available.

     I built the TDX stereo board near the start of the IIgs era just as games
like "Tower of Myraglen" were beginning to appear. It sounded great then and it
sounds great today running "Dungeon Master", "Instant Music", "Jam Session",
and Hyperbole MIDI-synth pieces. If your IIgs is still in mono mode, why not
make this the year you 'go stereo'. Add a commercial unit or build the TDX.
Either way, when you spread out the sound you open up the fun!

By:  Bryan Ogawa
    

 

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