8.8 OUTPUT ROUTING AND POWER AMPLIFIERS
Analog
switches
U402
and
U403
are used to route the
attenuated,
and possibly range extended,
signal to the left and right
power
amplifiers.
The
CPU reads
the
status of
the
front
panel
left/right switch and controls the analog switches accordingly.
The
use of
two power amplifiers
(U400,
U401)
as
well as
the
ability to
turn
them off (Pin 1) minimized
the
potential of
cross-channel
leakage.
The
amplifiers,
which
can
deliver
approximately 1 watt,
can be turned off for minimal current draw.
This is a very important feature for use under battery power.
A
series
130 ohm resistor at their output serves as an
attenuator
for both signal and noise when the 60 ohm phones are connected.
8.9 POWER SUPPLY
The on-board power supplies consist of a
+5
Volt regulator, -5
Volt regulator and a voltage converter.
The input supply,
whether it be from a battery or
a battery eliminator, is
approximately
+7
Vdc.
This positive voltage is applied to the
U102,
+5
Volt regulator.
The positive 5 Volts supplies both the
analog and digital circuits.
The unregulated
+7
Volts is also
applied to converter
U101
where,
through its switched capacitor
circuitry,
is inverted to -7 Volts.
The unregulated negative
voltage is then applied to the -5 Volt regulator
U100.
Note that
the voltage converter is disabled (shutdown) whenever the sleep
mode is entered.
This is accomplished via controls switched
through
U202.
8.10 OSCILLATOR
The
oscillator is
comprised of the following
IC's
and
their
functions:
U203
-
level shifter
U202
-
antialiasing filter
U201
-
programmable switched capacitor filter
U200
-
Low Pass/Band Pass selector
U203
takes
the 0 to 5 Volt sqaure wave (from
U511.10)
at
the
front panel selected frequency and converts it to a
+5
V
signal.
This
signal is
then applied to the
antialiasing
filter. An
antialiasing
filter
"rounds"
the edges of
the sine
wave to
eliminate high freqeuncy harmonics.
The
output of this filter is
applied to
the input of switched capacitor
filter
U201.
The
switched capacitor filter is programmed via clock frequencies and
the
state of the
50/100
line.
U201
(from
U507
controls can be
found at the end of the I/O port designation listing (see
U507).
The switched capacitor filter is set to a Low Pass configuration
for
all frequencies except 8 KHz where it is set for Band
Pass.
This filter reduces harmonics and any other noises resulting in
a clean sine wave product.
67
To Next Page
Loading...
