Crown FM150, User Manual

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4.2 Audio Processor/Stereo Generator Circuit Board
The audio board provides the control functions of audio processing-compression, limiting,
and expansion, as well as stereo phase-error detection, pre-emphasis and generation of the
composite stereo signal.
Illustration 6-4 and accompanying schematic may be useful to you during this discussion.
The overall schematic for the audio board is divided into two sheets; one each for the proc-
essor and stereo generator sections of the board.
Reference numbers are for the left channel. Where there is a right-channel counterpart, ref-
erence number are in parenthesis.
Illustration 4–1 Audio Processor/Stereo Generator Board
4.2.1 Audio Processor Section
Audio input from the XLR connector on the rear panel of the transmitter goes to instrument
amplifier, U2 (U8). Two-bit binary data on the +6 dB and +12 dB control lines sets the gain
of U2 (U8) to one of four levels in 6-dB steps. Gain of U2 is determined by R5, R6, or R7
(R45, R46, or R47) as selected by analog switch U1.
U3 (U9) is a THAT2180 voltage-controlled amplifier with a control-voltage-to-gain constant
of 6.1 mV/dB. The 2180 is a current-in/current-out device, so signal
voltages
at the input
and output will be zero. R11 converts the audio voltage at the output of U2 (U8) to current at
the input of U3 (U9). U3 (U9) output current is converted to audio voltage by U4A (U10A).
U4B (U10B) is a unity-gain inverter. When the positive peaks at the output of U4A (U10A) or
U4B (U10B) exceeds the gain-reduction threshold, U15 generates a 0.25 Volts-per-dB DC
control bias, producing wide-band gain reduction for U3 (U9). The dB-linear allows a front-
panel display of gain control on a linear scale with even distribution of dB.
4-3 Principles of Operation