JBL SG520 - Overall Circuit

Section 3
CIRCUIT DESCRIPTION
3.1 OVERALL CIRCUIT.
The SG520 Graphic Controller is a transistor stereo preamplifier designed for maximum operator
convenience and versatility. The circuits are somewhat complex because of the linear-actuated
potentiometers, illuminated pushbutton switches, and front panel screwdriver potentiometers
which provide this versatility. The stereo amplifiers are also unique, use large amounts of negative
. feedback in each transistor stage for consistent gain performance. A special! circuit is the 1000-
CPS test oscillator which is used for aural balancing. The power supply section provides —-24
volts for the amplifiers and oscillator and +24 volts for the F22 accessory. Each of the circuits
is described in. greater detail. For ease in referencing, all components have been numbered
by series: 100 series —- Channel A Amplifier
200 series — Channel B Amplifier
300 series — Front Panel
400 series —- 1000-Cycle Oscillator
500 series — Power Supply
Figure 3-1 is a block diagram of the complete unit. See pageze.
3.2 PREAMPLIFIER DETAILS.
Each preamplifier is contained on an individual etched-circuit board. The left channel is desig-
nated A; the right channel is designated B. Since both circuits are the same, only channel A
will be described here. The description also applies to channel B when the component numbers
are increased by 100 (for example, R105 becomes R205, in channel 8).
3.2.1 Phono Preamplifier—The phono preamplifier consists of three PNP transistor stages, Q101,
Q102, and Q103. The preamplifier is used only with the low-level inputs for PHONO 1 (J101),
PHONO 2 (J102) or MIC (J103) as selected by the SOURCE pushbutton bank.
In the TAPE PLAY, AUX FRNT, AUX REAR, TUN 1, or TUN switch positions, the preamplifier is
bypassed and Q104 accepts the output of high-level sources. In the low-level (PH1, PH2, or MIC)
switch position a signal of 4 millivolts will produce full amplifier output. At maximum VOLUME :
setting, the input sensitivity is 4 millivolts; at reduced settings, the preamplifier will not be i
overloaded with levels up to 250 millivolts. In both Q101 and Q102, the amount of DC degener- .
ation is greater than AC degeneration, because the emitter load resistance is split. For example,
Q102 has a total DC emitter load resistance of R115 + R116 = 2.4k. But its AC load is only
240 ohms, through AC bypass capacitor C114.
NOTE
The split-resistance emitter load is used in a number of preamplifier stages. When
troubleshooting stages, an open bypass capacitor produces a common symptom: the
ac gain has dropped to the dc gain value.
The output of the phono preamplifier is taken from the emitter of emitter-follower Q103 (PNP)
and ac coupled through C117, through one of the low-level (PH1, PH2, or MIC) selector switches
SW301, and through the TAPE MONITOR switch SW30 (OFF position) to the next amplifier
stage, Q104. The output at the emitter of Q103 is also applied as negative feedback back to
the emitter of Q101 through the networks for RIAA, NARTB, or flat response. This network has
a fixed dc resistance (which sets the overall preamplifier gain), but has lower reactance at high
frequencies. The resultant low-boost and high-cut response of the phono preamplifier compen-
sates for the typical magnetic pickup response curve (figure 3-2) in either the RIAA or NARTB
recording equalization method. The flat response network is used only for the microphone input.
Actually, the ‘‘flat’’ network has a small amount of high-frequency de-emphasis (C112) which
insures amplifier stability and suppresses high-frequency response beyond the audio spectrum.
3.2.2 Emitter Follower Stage Q104.—Transistor Q104 (PNP) receives its input signal from either of
two sources through SW301: (1) direct from the TAPE PLAY, TUNER, or AUX jacks, or (2) out-
put of the phono preamplifier, Q103. In either case, the nominal input level is 250 millivolts.
The Q104 emitter output has a dc load is (R130), the ac load is BALANCE potentiometer, R304.
The emitter follower effectively isolates the BALANCE potentiometer from the input.