Sony STR-VX4 - Page 34

‘SONY STR-VX Series Circuit Operation
STR-VX5, -~-VX6 Power
Amplifier
The left-channel power amplifier circuit
of the STR-VX5 is shown in Figure 9.
In the first stage, a dual PNP transis-
tor (Q702) is connected to a dual NPN
transistor (Q701) as a current mirror
for high voltage gain. The DC offset
voltage can be zeroed by RV701 if
slight DC imbalances occur. The
second stage, which consists of Q703
and Q704, supplies more voltage gain.
The base-emitter voltage of Q705 is
regulated by D701, D704, and R717.
Q705 acts as a regulated current source
for Q703 and Q704. D702, which is a
varistor consisting of four diodes con-
nected in series, biases the output
stage in class AB mode to reduce
C707 R707
erossover distortion. The bias cur-
rent is adjustable by RV702. The
complementary output stage consists
of Q706, Q707, Q708 and Q709.
The power amplifier of the STR-VX6 is
nearly identical to that of the STR-VX5,
except for the absence of an input ca-
pacitor, and a dual FET (Q701), which
configures a DC amplifier. Q703 and
Q704 are connected between Q701 and
the current mirror Q702 to reduce high-
frequency distortion caused by the gate-
drain capacitance of Q701. Q705, D701,
D702 and R710 are a regulated current
source for Q701, and the DC offset volt-
age is adjustable by RV701. Current
gain is supplied by Q710, Q711, Q712,
Q713, Q714, and Q715, which are con-
nected in a three-stage Darlington
arrangement.
Fig. 9. STR-VX5 Power Amplifier (Left Channel).