dbx 463X - Control Voltage Processing Circuits

Threshold
Circuit
The
output
of
the RMS
Detector
Is
then fed
to
the
OverEasy
threshold
circuit
Bade
up
of
0A4A,
0A5A,
0A5B,
and their
associated
components.
0A4A
Is an
Inverting
summer
that
sums
the
voltages
froa
the
RMS
Detector,
front
panel
Threshold
control
.R38,
and
the
temperature
compensating
diode,
CR4.
Also
added
here
Is
a small
offset
voltage
from
R69
to
calibrate
the
Threshold
control.
When
the
Inversion of the
sum
of
these
voltages
Is
less
than
zero,
CR5
Is
reverse
biased
and
the
output
at
TP12
Is
0
volts.
This
Is
the
case when
the
level
of
the
audio
signal
Is
greater
than
the
setting
of
the
Threshold
control
and
no
gating
Is
desired.
When
the
output
of 0A4A
Is
greater
than
zero,
as
Is
the
case when
the
audio
signal
Is
below
the
Threshold
setting,
CR5
will
begin
to
conduct,
causing
a control
voltage
to
be
generated.
This
control
voltage
Is
then
further
processed
by
the
Attenuation
Limit
and
Expansion
Processor
circuits
where
It
ultimately
goes
to
the
VCA
which
attenuates
the
audio
signal.
The
turn-on
characteristics
of
CR5
give
the
circuit
a
gentle
transition
Into
gating
for
the
first
6dB or
so
of
downward
expansion.
0A5A
acts
as
a
comparator
and
turns
on
the
yellow
Threshold
LED
whenever
the
audio
signal
level
Is
above
the
Threshold
setting.
The
minus
Input
to
this
comparator
comes
from
the
output
(pin
1)
of
0A4A
described
above.
The
plus
Input
comes
from
0A5B
which
subtracts
out the
temperature
compensating
voltage
that
was
added
In
at 0A4A.
This
voltage
Is normally
about
300mV
but
gets
pushed
to
7.5
V
when
the
Slave
switch
Is
activated.
This
Insures
that
the
LED
will
be off
during
Slave
operation.
Attenuation
Limit
Circuit
When
the
voltage
at TP4
Is
less
than
about
lOOmV,
0A4B
acts
as an
/*n^f
rter
a
9
a1n
14.6.
As
the
voltage
increases
above
this
level,
begins
to
conduct
and
will
ultimately
limit the
output
voltage
from
0A4B
to
1.4 V.
This
controls
the
maximum
amount
of
attenuation
at
the
VCA.
Resistors
R44
and
R68
form
a
voltage
divider
for the
feedback
through
the
diode,
allowing
the
output
to
go
to
1.4
volts
even
though
the
voltage
across
the
diode
Is
0.5 volts.
Expansion
Processor
The
amplitude
of
this
control
voltage
Is
adjusted
by the
front
panel
potentiometer
R67,
the
More
control.
This
Is where
the
actual
amount
of
gating
action
Is
set
by the
user.
The
gating
characteristics
are
further
modified
by
0A3B
and
CR3, which
create
an
additional
"OverEasy*
circuit.
When
the
control
voltage
levels
are
small,
CR3
is in
its
transition
region
of
conduction
and
the
gating
action
is
relatively
slow
and
gentle.
As
the
control
voltage
becomes
large,
as Is
the
case when
the
More
control
Is
set
to
Its
maximum
position,
CR3
Is
biased
full
on
and
the
resulting
gating
action
is
fast
and
severe.
The
temperature
compensating
voltage
from
CR4
is
summed
in
through
R40
along
with
a
small
offset
adjustment
voltage
from.
R
1
8
.