dbx 463X - Circuit Theory of Operation; Audio Signal Path Details

Circuit Descriptions
(Theory of
Operation)
Audio Signal
Path
Preamp
(Input
Buffer)
The Incoming
audio signal
Is
first
buffered
by the
LF353
type op-amp,
0A1A.
Ac
coupling
Is
through
C2
to
the
FET Input
stage
of 0A1A.
The Input
Impedance
of
391k
ohms Is
determined
by
the sum of
R1
and
R2.
Cl
provides
some
RF
suppression,
shunting
to
ground
high
frequency
signals
greater,
than
about 1MHz.
When
the front
panel
H1-Z Input
Is
used, R3
Is
connected
to
ground
through
the
sleeve
of
the
Input plug
activating
the
gain
trim,
R4.
The gain
can
be adjusted from unity
(OdB) to
a maximum
of
10
(20dB).
When
the
rear
panel
input
Is used,
the
signal
travels
via
shielded
cable
through
the
tip-shunt
of
the front panel
Jack
and
then
to 0A1A.
One
end
of
R3
Is
now
left
open
and
the circuit
will
have unity
gain
regardless
of the
setting
of
R4.
Correct
operation
of this
circuit
can
be
checked
at TP1.
R5,R6
and
zener
diodes
CR1 and
CR2
provide
soft
clipping
on
Input
signals
greater
than
+18dBv.
VCA
The
output
of
0A1A
Is then
ac
coupled through
C5
and
R7 to
the Input
of
the
Voltage
Controlled
Amplifier
(VCA),
IC1.
The ratio
of the
Input
current
(at pin
1)
to
the
output current
(pin
8)
Is controlled
by
the
voltage
applied
to
pins
2
and
4.
Positive
voltages
here
produce
attenuation
and
negative
voltages
produce
gain. The
actual scaling
Is
-6mV
per
dB.
If,
for
example,
the
voltage
applied
to
pins
2
and
4
Is
*36mV,
then
the
current
gain
will
be
-6dB
and
the
output current
will
be
half
as
big
as
the
Input
current.
Since
the
output
signal
at
pin
8 is
a
current,
it
cannot
be
viewed
easily
on
an
oscilloscope
and Is best
checked
after It
has
been
converted
Into
a
voltage
by the
Output
Buffer
described
below.
Rll
Is
used
to
adjust
the
VCA
symmetry
and is
set to
minimize
distortion
and
offset.
For
a complete
description
of
the
VCA
design and
operation
see
dbx
application
note
AN106
Output
Buffer
The
output
current
of the
VCA
Is fed
to
the
summing
Junction
(pin
2)
of
the
Output
Buffer, 0A2,
and
converted
Into a voltage
by
R14.
This
signal
Is
then
fed
to the
Output
Jack
through R17.
R17
provides
some
protection
to
0A2 from
Improper
output
connections
and
sets the
output
impedance
at
22
ohms. The
other
22
ohm resistor, R16,
Is inside
the
feedback
loop
and
does
not
contribute
to
the
output
Impedance.
C6
and
C7
roll
off
the
gain at very high
frequencies,
necessary for
the
closed-loop
stability
of
the
op-amp. This
5534
op-amp
will drive
a
600 ohm
load
to
«
18 dBv.