Control Amplifier Circuit; Low Frequency Control; High Frequency Control - Pioneer QX-949A Series Service Manual

6.5
CONTROL AMPLIFIER CIRCUIT
The control amplifier
circuit of the
QX-949A is
the NFB type, using a FET (fietd effect transistor)
in the first
stage.
The FET
amplifier being
a
controlable
voltage
type, which holds"the input
impedance constant,
even
if
the level
of
the NFB
changes,
and
has
additional
advantage as
a
coupled
circuit,
as the
input
impedance can be raised
Low
Frequency Control
The low
frequency
control circuit
is
shown in
Fig. 16,
and the equivalent circuit,
when boosting
low frequency,
is shown
in Fig.
17.
As the
parallel impedance
of
VR1
and C29, in
Fig. 17, is high at low frequency, the volume of
the NFB
decreases
and the gain
in
the
low
frequency
range increases.
The
equivalent
circuit, when cutting
out
low
frequencies, is shown
in
Fig. 18. In this
case, the
input
signal
is
applied
to
Q9, through the parallel
impedance
of
VR1 and
C33, which is high
in
the
low
frequency range and
suppresses
the
lower
frequency
signals.
High Frequency Control
The high
frequency
control circuit is
shown in
Fig.
19, and the equivalent circuit,
when boosting
high frequencies,
is shown
in
Fig.
20.
In
this circuit, the input
signal
is
applied
to
Q9
through the parallel
impedance circuit. This
imped-
ance
is
small
in
the high
frequency
range
and
produces a
signal
with
an
enhanced
high
range.
Fig. 21
shows the equivalent circuit
when cutting
out
high
frequencies.
As the
impedance
of
R53,
R41 and
C4L
of
the circuit
becomes small, the
level
of
the NFB
increases
and
the gain
of
the
circuit
decreases.
BX.949A
Fis.
16
Fig.
17
Fis.
18
F
is.
19
7
rN
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Fig. 20 Fig.21
23