Reflex Lexicon - Analog and Converter Circuitry

input
stage
Separate
unbalanced
1/4"
phone jacks
(J1
0
and
J11
)
are
provided
for
left and
right
input signals.
A
single
input source will
be
routed
to both left
and
right
input
stages by applyingthe
signalto
eitherinput
jack,
thereby maintainingthe
proper operating
level.
Capacitors
(C66-69) at
the
inputs prevent
unwanted high
frequency interfer-
ence
from entering orleaving
the Reflex
through
the
input
cables. DC
isolation
is
accomplished
by
bipolar
capacitors
(C71
and
C76)
in
line
with
the
signal
path. The
left and
right
signals feed
a
dual
potentiometer(R74) which
controls
the input signal level
to the preamplifier
stage. A
dual
op
amp
(U37) acts as
a
preamp
by
providing
30dB
of
gain.
The
left
and
right
outputs
of the
preamp are
applied
to
the
stereo
Mix
control
(R64)
and are also terminated
by
resistors
(R68
and
R69)
which
sum the left
and right
signals.
This
summed
signal
is applied
to
a
passive
5-pole
16
kHz
low
pass
filter
(U36).
The
output of the filter
goes to a
pre-emphasis
op
amp
stage
whose
output drives
the sample
and hold circuit.
SAH
The
sample
and hold
circuitry
consists
of an
op
amp
(U30), an
SPOT
analog
switch
(U31
)
and
a
capacitor
(C51
) . The
SAH
is
controlled
by
the
SAMP signal
command from
the Lexichip
II, which
terminates
at
pin 1
1
of
U31
. When SAMP
is
high,
pins
1
3
and
1
4
of switch
U38
are
shorted, which
performs
the sample
function. At
this time,
the
output level from pin
1
of
U30
is
used
to
charge C51
.
The
hold function
takes placewhen
SAMP
goes
low,
shorting
pins 12
and
14
of switch
U31
. The
charge
across
C51
keeps
audio
level
constant
during
the
A/D
conversion.
SAH
output
is
passed through
an AC
coupling capacitor
(C48)
and
a
pulldown
resistor (R50)
to prevent
DC
offsets
from affecting
system performance.
DAC
The
analog
to
digital
conversion
is
accomplished
by
a
comparator
(U29),
the
Lexichip
II
(U14),
and
a 16-bit
digital-to-anaiog
converter (DAC,
U24).
The
SAH
output
is applied
to
pin
3
of the
comparator;
th
e output DAC
output
feeds
pin
2. During
the
analog-to-digital
conversion
cycle,
the
internal
successive
approximation register (SAR)
of
the Lexichip
II is
used
in
conjunction
with
a
DAC
to output
different
voltages
which
are
compared
with
the audio signal
level.
The
conversion is
complete when
the
two
voltages
are equal.
After
conversion,
the L+R signal
is processed
digitally.
Separate left and right signals
reappear afterthe
analog-to-digital
conversion
and
digital
signal processing. At
this
time, the Lexichip
II
sends 16-bit
data to
the
DAC,
which converts
it
into
analog
voltage levels
which
are applied
to
the
output
hold circuitry.