Brief Description And Features Of Circuits.11; Protection Circuit - Pioneer SA-9900 Service Manual

5. BRIEF DESCRIPTION AND FEATURES OF CIRCUITS
Equalizer Amplifier Circuit
The circuit diagram of the equalizer amplifier
circuit used in the SA-9900 is shown in Fig. 1.
The first stage of this circuit is a differential
amplifier consisting of two field-effect transis¬
tors (FETs), and the output stage is a single-
ended push-pull (SEPP) amplifier which pro¬
vides a maximum output signal voltage of
approximately 29V with a distortion of 1%.
(1) Selection of input impedance
If the resistance R1 in Fig. 1 is infinite,
the input impedance of the equalizer
amplifier circuit is designed so as to be
100kS2.
If the FUNCTION selector switch on the
front panel is set to PHONO 1, the re¬
sistance of Rl is 100kf2, thus the input
impedance of the PHONO 1 terminals
becomes 50kS2.
If the FUNCTION selector switch is set to
PH0N02/MIC, the resistance of Rl can
be selected as 53.8k£l, lOOkft, 233kS7, or
infinity with the PHONO 2 IMPEDANCE
switch on the right-hand side panel.
The input impedance of the PHONO 2
terminals, therefore, can be set to 35, 50,
70 or lOOkfi.
(2) Changing of input sensitivity
The amount of AC signal negative feed¬
back in the equalizer amplifier circuit is
determined by the values of the equalizer
RC network and resistor R2. If the FUNC¬
TION selector switch is set to PHONO 1
the resistance of R2 is 1.54ki2. This was
established so that the rated output can
be obtained with input signal of 2.5mV at
1kHz. If the FUNCTION selector switch
is set to PHONO 2/MIC, the resistance of
R2 can be varied in the range 1.54 - 3.24
kfi with the PHONO 2/MIC LEVEL con¬
trol on the right-hand side panel. This
permits the input sensitivity to be varied
between 2.5 and 5mV. By turning the
PHONO 2/MIC LEVEL control, the vari¬
able resistor VR indicated by a dotted line
in Fig. 1 also moves. The input sensitivity
of the PHONO 2 terminals can therefore
be varied throughout the range 2.5 - lOmV.
Protection Circuit
Fig. 2 is a block diagram of the protection
circuit used in the SA-9900. The protection
circuit functions to detect DC voltages in the
power amplifier output circuit and power
transistor overloads and overcurrents, to mute
click noise produced by turning the POWER
switch ON or OFF, and to prevent abnormal
over-heating of the power transistor heat sink.
When any of these is detected, the protection
circuit operates the relay connected to the
power amplifier output circuit to interrupt
the speakers. Particularly, for the muting pre¬
venting click noise, a fieldeffect transistor
muting gate is placed on the input side of the
power amplifier so that the power amplifier
input signal can be interrupted at its input side.
This makes sure that the relay contacts turn on
or off without sparks because there is no
output signal, thus preventing deterioration of
the relay contacts and improving the reliability
of relay operation.
Fig. 1 Circuit Diagram of Equalizer Amplifier
11