24
LOOP Circuitry
Output Circuitry
Display Board
The left and right LOOP inputs are processed directly by differential amplifiers Al and A2 The
output of Al and A2 feed analog switch Al 4. The outputs of analog switches Al 5 (controlling
mix feed to the output) and A14 (controlling LOOP feed to the output) are paralleled and coupled
to Al 3 the selectable gain program amplifier. This amplifier, A13, provides phase inversion and
gain adjustment for +4 or +8dBu output. Jumpers near A13 are used to set the gain of A13 to
unity or 4dB.
Analog switches Al 4 and Al 5 receive complementary logic from the logic board -when one is on
the other is off Al 4 and Al 5 form the switching necessary for operation of the LOOP mode.
The mix bus amplifier, A16, directly drives the inputs of the LOOP output amplifiers. The SSM
2142 ICs produce balanced output that behaves more a transformer than typical active balanced
outputs. LOOP output is fixed at +4dBu.
The MAIN and SUB output amplifiers are driven by A13, the adjustable gain program amplifier.
In this case, however, pairs of 21 42s are driven for each output. This parallel arrangement can
p30dBm with very low distortion. Output source impedance is 25
Ω
and is ideal for
driving studio wiring and telco lines.
See schematic page 36, Display Board.
The display board, mounted behind the front panel, holds the three mode switches, the six select
switches and the audio bar-graph displays. Switching is accomplished by contact closure to
ground. For each function, front and rear panel switches are wired in parallel. All switches are de-
bounced on the logic board.
Front panel status lights are diode isolated from the status lines on the individual rear panel logic
connectors. Status lines for remote control use, those on remote connectors ‘A” and “B,” are
isolated from all other status lines.
Left and right audio is sampled from the mixing amplifiers and feeds two Sanyo LB1412 display
drivers. Multi turn trimmer V1 calibrates the right channel display and V2 the left.
The color coded 12-segment bar displays are driven by the open collector outputs on the LB1412.
The display drivers and the LEDs are powered from the unregulated 12 volt output of the low
voltage supply on the audio board.
