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This is the original design for the audio input module. Through clever
use of a normalling jack, either balanced or unbalanced signals may be
input. Capacitive coupling is used to remove any residual DC offset from the
A trim stage, based on the Dallas Semiconductor DS1800 digital
potentiometer, provides audio trim capabilities. Unfortunately, when
this circuit was designed (and later fabricated), it was not known that
the DS1800 is not a bipolar device, thus it is inappropriate in this
situation. For the prototype, this functionality was sacrificed, but a stock
of National Semiconductor LM1972 bipolar digital volume controls was obtained.
At some point in the future, the circuit will be redesigned using this part.
Audio input module, line receiver, buffer, and gain adjust schematic
(2/8 of module).
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