Table of Contents
Advertisement
A
current
developed
from
the
de-esser
control
loop
is
injected
into
the
control
current
port
of
ICI3B.
The
gain of
ICI3B
is
directly
proportional
to
its
control
current.
The
de-esser function
results
from
varying
this
control
current.
The
de-
esser function
is
functionally
in
series
with
the output of the compressor/limiter
and
does
not
affect
its
operation,
since
the
feedback
signal
to
activate
the
compressor
limiter
is
taken
from
the output
of
1C
14,
which
is
unaffected by the
gain of
ICI3B.
Second-harmonic
distortion
is
introduced
by
differential offsets
in
either
ICI3A
or
ICI3B.
This
distortion
is
cancelled
by applying a
nulling
voltage
directly
to the
input of
ICI3B
by
means
of resistor
network R24, R25, R26.
If
the
VCA
is
not perfectly balanced,
"thumps" due
to control
current feedthrough
can
appear
at
the output.
These
are equivalent
to
multiplying the control current
by
DC.
If
a correct
DC
offset
is
applied to the
VCA
input,
then
this
equivalent
DC
multiplication can
be
nulled
to
zero
and
the
"thumps"
eliminated.
Such
an
adjustable
DC
offset
is
provided by
RI7, RI9.
C8,
RI8
are
frequency-compensation components
to
prevent
the
VCA
from
oscillating supersonically.
"Thumps"
can
also
appear due
to
de-esser control
current feedthrough
at
ICI3B's
output.
Nulling
compressor
feedthrough
by
means
of
RI9
may
make
de-esser
feedthrough
worse. Therefore, a separate de-esser feedthrough
null
circuit
consisting
of
IC8B-E, R27,
R28
is
included.
The
de-esser
control
current flows
from
the collector
of
IC8A.
A
current
identical
to the de-esser
control
current flows
from
the collector
of
IC8B, since
IC8A
and
IC8B
are
matched
transistors.
The
current
output
of
IC8B
is
inverted
in
current
mirror IC8C-E, and
injected
into
the junction
of
R27
and R28,
where
it
develops
a
voltage.
The
voltage
is
amplified
in
both
inverting
and
non-inverting
modes
by
IC9B.
When
the
wiper
of
R28
is
at
50%
rotation,
then IC9B's
inverting
and
non-
inverting
gains
are
equal,
and
no
effect
is
produced
at
IC9B's
output
due
to
cancellation.
Moving
the
wiper
of
R28
to
either
side
of
this
null
point
permits
introduction
of
sufficient
control
voltage
into
IC9B's
output
to
cancel
any
feedthrough
in
ICI3B, regardless
of
the feedthrough's
polarity.
Exponential Converter:
The
basic
current-controlled gain
in
the compressor/limiter
is
inversely proportional
to
the control current.
We
wish
to
transform
this
into
a
gain
which
is
proportional
to
a
control
voltage
in
dB.
This
is
done
in
the
exponential
current
converter
consisting
of
1CI2B and
associated
components.
ICI2B, IC7A, IC7B,
IC7C and
associated
components form
a log/antilog multiplier.
This
multiplier
multiplies
the
current
flowing
in
R63
by
the exponential
of
the
voltage on
the base
of
IC7B.
The
current gain of the
multiplier increases as
the
voltage on the base
of
IC7B becomes
more
negative.
Because
the voltage on
the
base
of
IC7B
is
in
log
(i.e.,
dB-linear)
form,
various
control
voltages can be
summed
into
this
base,
and
they
will
add
in
a dB-linear
manner.
These
control
voltages include the
main
gain-control
output
of
the timing
module
(through R62), a
gain trim (through R6I),
and
the user-adjustable
OUTPUT
TRIM
(through
R65) which
adjusts
not
only
the
gain
of
the
VCA,
but
the
compression
threshold as well, as
will
be described below.
The
current
output
of
the
log/antilog multiplier
appears on the
collector of
IC7A.
It
is
the
wrong
polarity
and
level
to
correctly drive the control-current
port
of
1C
1
3
A.
It
is
therefore applied
to
a current
inverter
ICI2A, Ql, R57, R58, C2I.
This
circuit
has a gain of
6
.
66x,
and
operates
as follows:
33
Hide quick links:
Advertisement
Table of Contents
