Orban 424A Operating Manual page 36

These
will
be described
in
order.
I)
Input
Buffer
The
signal
enters the
424A
in
balanced form. Cl,
C2
shunt
RF
from the
input
leads
to
the
chassis.
These
capacitors
are
not effective at
VHF
and
higher
frequencies; therefore,
ferrite
beads have been placed around
the
input
and output
leads
to
suppress such high frequency
RF.
It
should be noted that
this
degree
of
RF-proofing
is
moderate
but adequate for a vast majority
of installations.
However,
installation next to a
high-power
transmitter
may
still
cause problems.
Additional
RF
suppression, careful
examination
of the grounding
scheme, and other
considerations familiar
to
the broadcast engineer
may
have
to
be
used
in
conjunction with the 424A's
built-in
RF
suppression.
The
filtered
signal
is
applied to IC2,
a very
low-noise
opamp
configured
as a
differential amplifier
with a gain
of
I.
When
both non-inverting and
inverting inputs
are driven by a source
impedance which
is
small with respect to I00K (such as 600
ohms
or
less),
the
amplifier
is
essentially insensitive
to signal
components
that
appear equally on the non-inverting and inverting inputs (such as hum), and responds
with
full
gain
to
the difference
between
the non-inverting
and
inverting inputs.
Thus
it
serves
as
an
"active transformer". Ordinarily, best results are
obtained
for
unbalanced
signals
if
the non-inverting input
is
grounded and
the inverting input
is
driven.
The
GAIN
control
is
located after IC2. Therefore, IC2
will
overload
if its
differential input
exceeds approximately +20dBm.
2)
Voltage-Controlled Amplifier
(VCA)
Operation
Current-Controlled
Gain:
The
current-controlled gain block used
in
the
424A
is
a
proprietary Class-A
VCA
which
operates
as a
two-quadrant analog
divider
with
gain
inversely proportional
to
a current injected
into
a
first
gain-control
port,
cascaded
with a
two-quadrant analog
multiplier with gain directly proportional to a current
injected into a
second gain-control
port.
For most
gains,
levels,
and
frequencies,
THD
is
well under 0.1%. Overload-to-noise
ratio (noise
measured
in
a 20-20,
000Hz
band)
is
typically 90dB,
and
is
constant with respect
to
gain
and
level.
A
specially-graded
Orban
1C contains
two matched
non-linear gain-control blocks
with
differential inputs
and current outputs.
If
used
alone,
one
such gain-control
block
would
introduce
considerable distortion. Therefore, the
first
of
the
two
matched
blocks
ICI3A
is
used
as the
feedback element
in
a high-quality operational
amplifier,
1C
1
4.
The second
of
the
matched
blocks
ICI3B
is
then driven by the
predistorted output
of
1C
14.
To
provide
more
detail:
The output
of 1C 14
is
first
attenuated by R20, R2I, C9, and then applied
to
the
input of the
feedback element
1C
1
3
A
.
The
output
of 1C 14
is
predistorted as
necessary to force the current output
of
ICI3A
to precisely
and
linearly
cancel the audio
input into
the
"virtual
ground"
summing
junction
of
1C
14.
This
same
predistorted voltage
is
also
connected
to
the
input of ICI3B. Thus the output
of
ICI3B
is
an undistorted current. This current
is
converted
to
a voltage
in
current-to-voltage converter IC9B, R22, CIO.
The
output
of
IC9B
is
the output
of the
VCA.
Because ICI3A
is
in
the feedback loop of
1C
14,
the gain
of
the
VCA
is
inversely
proportional
to
the gain
of
ICI3A. Thus
if
the control current
is
applied to the
control
port of
ICI3A
(from
Ql),
then the
VCA
behaves
like
a
two-quadrant analog
divider
.
The
gain-control current injected
into this control port
is
developed by the
compressor/limiter
control circuitry.
32