Pioneer SA-708 - Block Diagram; Circuit Descriptions; Phono Circuit; Tone Controls Circuit

3. BLOCK DIAGRAM
35.5dB
[renee FUNCTION
PHONO TAPE
=4kH:
16.54B
|
(f
MUTING voune|"
27.548
[retin O SPEAKER A
BALANCE
INTERFERENCE MONITOR MODE SUBSONIC
TONE POWER
PHONO
(©
6)
LS
(SX)
&)——&)
E
ae
&)
Fs
Fees
OT
oO
&
O
SPEAKER
B
L
RFI ;
ATT
FILTER
ATT.
‘a
'
art
|
PHONES
Se,
i,
1
a
®
Or[rve]
TUNER ©
6
LOUDNESS
eUaUeE
AUX (6
TAPE
DUPLICATE
protection
|
[\/
INDICATOR
>
TON
CIRCUIT
DRIVE
EREG
X9
CONTROL
/N
TAPE
1
H
H
1
i
Leray
©
|
+8
on
ie
||
|
|
FUNCTION
PREC &)
LpLay oe
'
i}
!
i}
(om
PHONO eu
1 oe
00
j AC LINE
TUNER
AUX
REC
TAPE 1
LPLAY
pREC
TAPE 2
‘pray
R
4. CIRCUIT DESCRIPTIONS
4.1 PHONO CIRCUIT
RF Interference Filter
Since the phono input circuit is extremely sensi-
tive, it is adversely affected by radio frequency
interference. This interference is reduced by insert-
ing a resistor in series with the phono input circuit
by setting the PHONO INTERFERENCE FILTER
switch on the rear panel to the ON position.
Equalizer Amplifier
The equalizer amplifier is a 3-stage direct-
coupled amplifier with emitter-to-emitter feed-
back. An S-N ratio of 86dB (at 2.5mV input,
IHF-A, PHONO INTERFERENCE FILTER switch
OFF)
has
been
achieved
by
using
a
ultra-low-noise
PNP transistor (2SA978) at the first stage, and
reducing the signal source resistance and equalizer
element impedance.
The 2nd stage load has been reduced, a large
F O SPEAKER A
R CHANNEL (SAME AS L CHANNEL )
O SPEAKER B
POWER AMP
INDICATOR DRIVE, FUNCTION INDICATOR
POWER AMP
EQ AMP
TONE AMP
output voltage obtained, and the output impe-
dance lowered by using an emitter follower at the
output stage.
Metal film resistors having a tolerance of +1%
and polypropylene film capacitors having a toler-
ance of +2% are used on the NFB circuit to obtain
an equalizer deviation of within +0.2dB over the
20Hz to 20000Hz range. The maximum allowable
input of this circuit is 200mV (at 1kHz, THD
0.01%).
4.2 TONE CONTROLS
The tone amplifier is a 3-stage direct-coupled
amplifier with emitter-to-emitter feedback.
Tone control (BASS, TREBLE) is accomplished
by providing the tone amplifier NFB circuit with a
frequency characteristic. The NFB circuit is
changed to a flat frequency characteristic when
the TONE switch is in the OFF position.