Reflex Lexicon - Deglitch and Output Circuitry

Theory
of Operation
Lexicon
Deglitch
(Output
Hold)
Separate
output
hold
circuits are provided
for
reconstruction
of the left
and
right
audio signals.
This
circuitry includes
a
dual op amp
(U32)
and two
analog
switches
(U31).
DEGO
and
DEG1
commands
from
the
Lexichip
terminate
respectively
at
pins
1
0
and
9
of
U31
,
providing
separate control of the right
and
left
output hold switches.
During
the digital-to-analog
conversion
cycle,
either
the DEGO or DEG1
signal
goes
high when
the
DAC
is
ready
to output
the
right
or left analog
voltage.
This
permits capacitors
C56 or C62
to
charge
to
the
corresponding
voltage
level for
the
audio
waveform.
When
the
DEGO
or
DEG1 signal
goes
low,
the
related capacitor
will remain
at the current
signal
level
until
the
next
conversion
cycle.
Output Circuitry
The left
and
right
signals from
the output hold
circuitry are applied
to separate
5-pole
passive
low
pass
filters
{U33
and
U34).
De-emphasis
circuitry
is
incorporated
into
the
signal
path
to
compensate forthe
pre-emphasis
function
performed
before
the
conversion
process.
A
dual
op
amp
(U35)
with
its
associated
circuitry
provides
the de-emphasis function.
The
outputs
of
U35
are
applied
to
one end of the stereo
Mix
control,
R64.
The
other
end of this
potentiometer is
connected to
the
outputs
of
the input
preamplifier. By
adjusting
the
wiper
of
the
Mix control,
the ratio
of
wet
(processed)
to dry
(unprocessed)
signal
can
be
controlled.
The wipers
of
R64
are
connected
to one end of the
Output
level
control,
R49
through bipolar
AC
coupling
capacitors
(C64 and
C65) to
prevent
DC offset
voltages
from
appearing
at
the next
stage.
R49
adjusts the
signal
level
being
applied
to
the
output ampifier,
U27.
This
dual
op
amp provides
unity gain
and
buffers
the left
and right
signals
separately.
Two
analog
switches
(U28)
provide
output
muting
during
power
up
or
power
down
conditions.
The
RSTMUTE/
signal
that controls
these
switches
is
generated from
the
reset
circuitry and is
terminated
at
pins
9
and 10 of
U28.
When
the RSTMUTE/
is
low, pins
4 and
5
of
U28
are
connected for
the
left
output
and
pins
15 and
2 are connected for the right
output, creating
a
low
impedance
signal
path
to
ground. These
switches
are placed
in parallel
with
the
output level
pot,
before
the
output stage.
The
2KQ
series resistors
provide
a
minimum
25dB
of
attenuation when
the switch is on
(RSTMUTE/
is
low).
An
impedance
of 75Q
is developed
by
R39
and
R40
for
the left and
right
outputs. These
resistors
also
provide
current
limiting
protection. The
output
1/4“
unbalanced
phone
jacks (J8 and
J9)
are configured
in
such
a
way that
the left
and right
output
signals are summed
together if
only
the
right
jack
(
J9)
is connected.
The
left
jack
(J8)
can
support stereo headphones
if
the right
jack
(J9)
is not
connected.
The
left channel
appears
on
the
tip, the
right
channel
on the
ring
of
a
stereo
phone plug
through
this 3-conductor
1/4"
phone
jack.
The
output
amps
can
drive
high-impedance
(>100
Q)
headphones,
but
are
not designed for low
impedance headphones. For
best results,
use a
headphone amp.
Finally,
capacitors
C41
and
C42 prevent unwanted
high
freqency
interference from
entering or
leaving the
Reflex
through the
output
cables.
4-2