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
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