Main Amplifier - Sharp OPTONICA SM-3000H Service Manual

MAIN AMPLIFIER
The main amplifier consists of all-stage direct-coupled pure complementary output capacitorless circuit.
FEATURE OF PURE COMPLEMENTARY OCL CIRCUIT .
Since this circuit is not using output capacitor, the frequency characteristic is kept uniform even at very low frequency band
and the output impedance is low in any of frequency bands resulting in that the value of damping factor is made larger so that
the braking efficiency of speaker is increased. With this circuit, since a 100 percent NF is assured when the frequency of
signal is zero and the value of NF is determined at only one place when the frequency of signal is at low band, the function
of circuit is stabilized.
CIRCUIT DESCRIPTION
The main amplifier is OCL circuit in which the class "A" drive circuit consists of 2 stage differential amplifier circuit.
The signal coming from the filter circuit is amplified by differential amplifier (Q502, Q503) via R502 and C504.
Since this differential amplifier consists of PNP type low noise transistor (2SA836D), a high value of S/N ratio is assured. And
this amplifier functions to keep zero the center voltage (speaker terminal voltage) as well as to amplify signals. The zero adjustment
of potential is to be made by using semi-variable resistor (VR501). Signal thus amplified by the differential amplifier is further
amplified by differential amplifier (Q504 and Q505). Moreover, the signal is amplified for the half cycle at the driver amplifier
stage consisting of Q510 (NPN type transistor) and Q511 (PNP type transistor). Then, the signal is further amplified for the half
cycle at Q901 (NPN type transistor) and Q902 (PNP type transistor) to be supplied to the speaker. Q501 and Q506 is constant-
current circuit and its amperage is determined by D501. Q501 functions to protect the differential amplifier (Q502 and Q503)
against fluctuations of temperature and voltage resulting in that the center voltage (speaker terminal voltage) is kept constant.
Q506 is constant-current circuit to supply constant current so that the load applied to the class ''A" driver Q505 will be reduced
thus the gain being increased. As a result of the gain of Q505 being increased by Q506, plenty of NF is produced and so the dis-
tortion is lessened. NF factor of NF circuit is determined by R520 and R517, and the higher NF factor, the higher is the gain.
NF factor at the low frequency band is determined by C511 and R517. Q507 and D506 are to cause the bias of class '"'B'' drive
Stage and to produce idling current of 30 ~ 80 mA so that cross-over distortion due to class ""B'' operation is eliminated. The
idling current is to be adjusted by semi-variable resistor (VR502). Q508, Q509, D502 and D503 are short circuit, etc. at the output
section and they detect voltage which will be caused at R528 and R531 (emitter resistor) if current runs excessibely — if the —
detected voltage is higher than as rated, the collector-emitter of Q508(Q509) becomes conductive. D502(D503) and Q508(Q509)
are being located between the base of Q510(Q511) and the center (speaker output) and serve as constant-current circuit, and this
results in that the power transistor (Q901, Q902) is assured of a rating current.
SLAD (Spike-less Amplifier Design) CIRCUIT
This circuit is a unique circuit out of those in this unit and it actually consists of D504, D505, R527 and R529.
In the case of power transistor of class "B'' amplifier operation, the most part of input signal is cut off for the half cycle and there
arises charge or discharge at the junction according to its capacitance. Therefore, spike-like voltage is generated or the switching
time is disordered against the high frequency signal resulting in that cross-distortion of high frequency band is enlarged.
SLAD circuit is for the purpose to prevent such phenomenon as above.
TO S038-4 TO $038-5
100/!10V
de 507. | Rice
VR R502 C504 2502 roe
'@ DY
aA
res R5O4A
-342v
Figure 7