Sony TA-2000F - Detailed Circuit Analysis

1-2. DETAILED CIRCUIT ANALYSIS
The following text describes the function or
operation of all stages and controls. The text se-
quence follows signal paths. Stages are listed by
transistor reference designation at the left margin;
major components are also listed in a similar manner.
Refer to the block diagram on page 9 and the
schematic diagram on pages 35 to 36.
Stage/Control Function
PHONO-! Equalizer/Head Amp
Head Amplifier Amplifies extremely small input
Q101 signals (as from a moving-coil
type cartridge) to the level re-
quired at following equalizer am-
plifier.
A common-gate configuration is
suited to this job because it has
low input impedance and high
voltage gairi. Input signal ap-
plied to the PHONO-1 terminal
is routed to this amplifier only
when IMPEDANCE SELECTOR
switch $10 is set to the 30 ohm
or 10 ohm position.
IMPEDANCE $10 changes the PHONO-1 input
SELECTOR impedance to meet the cartridge
switch S10 manufacturer’s recommended
load impedance because of its
effect upon frequency response.
Equalizer Ampli- This newly developed direct-
fier coupled four stage amplifier am-
Q102, Q103 plifies the phono cartridge signals
Q104, Q105 to the level required at the input
Q106 of the following tone-amplifier.
Q102 forms a conventional FET
amplifier while Q103 and Q104
act as buffer amplifier which has
a high input impedance.
This FET-PNP combination am-
plifier forms a modified source-
follower circuit in which Q104
acts not only as constant-current
source, but also as a drive am-
plifier for the negative-going half
cycle. This has the advantage of
low harmonic distortion and
wide dynamic range. In addi-
tion, the FET generates less
noise than a conventional silicon
transistor.
Stage/Control
lot voltage
number
Fig. 1-1.
Bias circuit
Equalization
circuit
‘TA-2000F
Function
The FET’s used in TA-2000F are
selected according to their Idss
rank, and care should be taken
to use replacement FET’s with
the exact same Idss. Idss is
indicated by the identification
number, as illustrated in Fig. 1-1.
Note that Q105 and Q104 are
newly developed high-voltage
transistors which make the wide
linearity (dynamic range) pos-
sible. For this purpose, a 150-
volt power supply is employed.
lot ldss rank
Idss rank number
Example of Idss rank
De bias voltage for Q102 is
determined by the current flow
in source resistor R112, and the
dc negative feedback voltage ap-
plied to the gate of Q102 from
the emitter circuit of Q105
through R110, R107 and R109.
This. dc negative feedback tech-
nique provides stable operation.
De bias voltage of Q103 is de-
termined by the drain voltage
of Q102 and the current flow
in the Q103 (which is restricted
by its Idss). Current flow in
Qi03 also determines the bias
voltage applied to the Q104 and
Q105 as they are directly cou-
pled.
RIAA equalization is achieved
by the negative-feedback loop
containing R117, R118, R119,
C110 and C109. Be sure to use
replacement components with
the exact same values. The
equalizer amplifier’s output is
fed to the FUNCTION-2 switch
through R120 (1 k{&2) to prevent
interaction between the left and
right channels when the MODE
switch is set to L + R.