Table of Contents
Advertisement
Fig. 3
Block diagram of PA1002-A
AM Tuner
The AM tuner employs a 2-stage variable capaci¬
tor, one IC (HA1197) and one AM ceramic filter.
Its block diagram is given in Fig. 4.
HA1197 is an IC containing an RF amplifier,
converter, 2-stage IF amplifier, detector, and AGC
circuit,
and
features
excellent
AF
frequency
response and distortion. Qj of the output circuit
is a special AM muting circuit. This circuit is
operated until the AM tuner stabilizes immediately
after the FUNCTION AM switch has been set to
ON. The instant the AM switch is set to ON, +B
is supplied to Rr, thru C
9
, and the base of Q, is
forward biased. Consequently, Q, is turned ON,
and the AM output signal is shorted to ground
during the time constant of C
9
, Rg.
8.2 PHONO, MIC INPUT CIRCUIT
PHONO 1, PHONO 2/MIC input switching is
performed by the switch and the MIC jack.
When a microphone plug is inserted into the
MIC jack, the input is switched to MIC at both the
L and R channels. Since the IC (HA1457) is used
as both a microphone amp and equalizer amp, a
circuit having a reverse RIAA curve is provided at
the MIC input circuit to obtain a flat frequency
response.
8.3 FLAT AMPLIFIER
The power amplifier is an all-stage direc1>coupled
circuit. It amplifies the signal to the required level
and sends a low impedance signal to the tone
control circuit.
8.4 TONE CONTROL
The tone control circuit is an NFB type inserted
in front of the power amp.
Bass turnover (200Hz
400Hz) and treble
turnover (2.5kHz <—► 5kHz) switches and a tone
defeat circuit which bypasses the tone control
circuit to obtain a flat characteristic are provided,
in addition to variable resistor which adjusts the
rise and fall.
8.5 POWER AMPLIFIER
The power amplifier is an all-stage direct-coupled
pure complementary OCL circuit having a differen¬
tial amplifier at the first stage, current mirror cir¬
cuit at the predriver stage and a parallel push-pull
amplifier at the final stage (Fig.5).
The first stage (Q,) is a PNP type dual transistor
differential amplifier that amplifies the input signal
and stablizes the center voltage of the power stage.
Q
3
and Q
4
are driven by the opposite phases of the
output of Qi. The output of Q
4
is applied to the
current mirror circuit consisting of D
2
and Qs and
phase inverted. Consequently, Q
3
and Qs are in-
phase signals, and are operated as a push-pull pre¬
driver. Low distortion and improved rise charac¬
teristic at high amplitudes have been realized by
making the predriver stage push-pull.
Overcurrent is detected and destruction of the
power transistor prevented by Qg. The final stage
is a parallel connected power amplifier.
Advertisement
Table of Contents
