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REVISION C
BMX SERIES III CONSOLE
82
PACIFIC RESEARCH & ENGINEERING
CONSOLE MICROPHONE/AMPLIFIER
The console microphone is an electret type, and is powered by a fi11 volt DC supply derived
by CR1, R1, R2 and C1. The signal from the microphone is connected to preamplifier U1A, which
operates with a midband voltage gain of 40 dB. The output of the preamplifier is routed through front
panel CONSOLE MIC level control R148, and then to amplifier U1B, which operates with 31 dB of
midband gain. The reactive components around the microphone preamplifier stages have been chosen
for a response characteristic favoring speech frequencies.
PRODUCER MICROPHONE INPUT
The Producer microphone input is a conventional design intended to be driven by a 150 ohm
microphone, which is coupled to the preamplifier using input transformer T1. The first stage of the
amplifier uses U2A operating with a midband gain of 32 dB. The output of this stage is routed through
the front panel PRODUCER MIC level control to amplifier U2B, which operates with 21 dB of midband
gain. As with the console microphone preamplifier, the reactive components around these circuits have
been chosen to favor speech frequencies. The output of U2B is connected to Producer mic relay K1B.
TEST OSCILLATOR
The built-in multifrequency test oscillator is of the Wein-bridge configuration, with an AGC loop to
keep the operating level precisely defined. U10 and the associated circuitry form the basic oscillator.
Positive feedback around this amplifier is via the frequency-determining network. The output of U10
is applied to the top end of the network, and the midpoint of the network is fed back to the non-inverting
input of U10. The resistor/capacitor network has maximum transmission and minimum phase shift at
only one frequency, which is the frequency of oscillation.
The frequency-determining network uses a pair of fixed value capacitors and a set of 5 pairs of resistors,
which are selected by the front panel FREQUENCY switch assembly. Additional pairs of fixed value
capacitors are switched into the network to provide three ranges of frequencies.
Negative feedback for U10 is provided with R86 in parallel with opto-coupler U9, R85, R84, and
feedback adjustment control R83. The oscillator’s AGC circuit operates by adjusting the amount of
negative feedback around the circuit. The output of U10 is connected to full-wave, absolute value,
rectifier system U7A and U7B. The output signal of U7B is a precise full wave rectified version of the
oscillator output signal. It is connected to the non-inverting input of comparator U8A, and the reference
voltage connected to the inverting input. The reference voltage is +5.9 volts, as set by voltage divider
R79 and R81.
Should the oscillator output peak amplitude rise above +5.9 volts, the comparator will deliver a more
positive output. This is applied to the input of voltage-to-current converter U8B, to drive the LED in
opto-coupler U9 harder. This lowers the resistance of the output side of the coupler to increase the
negative feedback around U10, and thus reduces the output amplitude of the oscillator to its design
value.