JBL SG520 - Page 9

3.2.3 Balance, Loudness and Volume Controls. —The channel A BALANCE control R304A is a linear slid-
ing attenuator control which js ganged with the channel B BALANCE control R304B. When the
BALANCE setting is changed, the percentage resistance increases on one potentiometer and
decreases on the other. The wiper of R304A is connected to the VOLUME potentiometer R305.
Both the BALANCE and VOLUME potentiometers are dual-section. The two volume control sec-
tions are matched at the factory to within 2 db tracking.
Loudness compensation is achieved by tapping the VOLUME CONTROL and shunting higher
frequencies to ground, as shown in the simplified schematic, figure 3-4.
When the LOUDNESS switch is ON, capacitor C122 is placed in series with resistor R134.
This causes the higher frequencies to be bypassed to ground. As shown in the diagram, most
of the low frequencies are allowed to flow through the bottom terminal of the VOLUME control to
ground. So, the wiper of the VOLUME control picks off more lows when it is set below the tap
point. The curves in figure 3-3 show how the response varies for different VOLUME control
settings. Beyond the 50 percent setting, the response is flat. Notice that the amount of high fre-
quency de-emphasis levels off at the very high frequencies. This is the shelf level and is deter-
mined by resistor R134 which prevents the high frequencies from being completely shorted to
ground.
3:2: a Interstage Amplifier.—Transmitter Q105, (PNP) Q106 (NPN), and Q107 (PNP) are direct-coupled
ee DB.
amplifiers which raise the audio level to provide the high level voltage gain. Negative feedback
through C126 and R141 limits the ac gain of Q105, Q106, and Q107 to approximately 3. The-
R141 |.
/ R139 3. The
minimum ac gain is 3 because this is the ratio of the feedback voltage divider
collector output of Q107 is coupled through C127 to the tone control section.
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3.2.5 Bass and Treble Controls.—The BASS and TREBLE controls vary the frequency at which boost
or cut begins. For lowest possible distortion, tone control equalization is achieved by feedback
rather than passive networks. Figure 3-4 shows the equivalent circuit of the tone control net-
work and amplifier. When the tone controls are set in a flat position, the overall circuit gain
is one.