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BMX SERIES III CONSOLE
regulators are used to set the output voltage and improve the noise and transient response of the
regulators.
4.15 VU METER BUFFER AMPLIFIER
4.15.1 Function
The console VU meters are driven by balanced isolation buffer amplifiers, which are mounted on their
respective meters. The assemblies fully conform to American National Standard C16.5-1954.
4.15.2 Circuitry
The VU meter buffer amplifier consists of dual integrated circuit amplifier U1 configured for a balanced
symmetrical input, and operating with cross-connected feedback. The gain of the amplifiers is adjusted
by trim potentiometer R8. The output of the two amplifiers is coupled to the VU meter through the
recommended source resistance of 3600 ohms. Light emitting diodes DS1 and DS2 provide overdrive
protection to the meter movement.
4.16 POWER SUPPLY ASSEMBLY
4.16.1 Function
The fully regulated power supply provides four separate voltage outputs, and is protected with magnetic
circuit breakers, as well as electronic safeguards against excessive current.
4.16.2 Circuitry
The power supply has three discrete power modules: a bipolar 22 volt audio supply, a 12 volt logic
supply and a 48 volt "phantom" supply. Each module has its own magnetic circuit breaker, power
transformer and regulator circuitry. Color-coded voltage test points are located on the front panel for
ease of routine measurement.
The bipolar 22 volt, 5 amp, audio supply is configured by two identical +22 volt monopolar supplies
which are powered from two separate secondary windings of power transformer T1. The first secondary
of T1, terminals 6 and 7, is rectified by diode bridge CR1, filtered by capacitor C1, and regulated by
series regulator U1. The various small components around the regulator are used to set the output
voltage and improve the noise and transient response of the regulator. When the supply is initially
turned ON, Q2 is turned ON by Q1 to provide start-up current through R7. After a short time, Q1 and
Q2 turn OFF, leaving regulator U1 to provide all of the console current.
The second secondary of T1, terminals 8 and 9, is rectified by diode bridge CR2, filtered by capacitor
C2, and regulated by series regulator U2. The high current, low noise regulators are a plug-in integrated
circuit design and are mounted on the large heat sink located on the right side of the power supply (as
viewed from the front). The outputs of these two supplies are coupled in series for the required bipolar
PACIFIC RESEARCH & ENGINEERING
REVISION C
87
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