ATI Audio SDA200 - Manual (Page 3)

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channel
’s gain over a 4dB range above and below the nominal stage gain of
22dB.
Board mounted DIP switch S1 provides four normally open switch sections to
independently direct the left and right input signals to either or both output
amplifiers. Resistive mixing allows both left and right inputs to be summed (L+R)
into either or both outputs.
The output stages consist of a bridged pair of high current output ICs
(NE5532AN) that provide the first 20ma. of output current directly. Above that
level, the complimentary class B output booster transistors take over. This simple
but unique, wide bandwidth, high slew rate design provides effective class AB
operation with minimal crossover distortion from a power output stage operating
in true class B with zero quiescent power dissipation. The outputs provide 3.5dB
of stage gain, 6dB additional due to the balanced configuration and 3.5dB of
output loss through the build-out resistors when loaded by 600 ohms.
The HI and LO amplifier outputs are split into four isolated balanced outputs
through 150 ohm build-out resistors. The build-out resistors are individually
bypassed to prevent RF pickup on an output line from affecting the operation of
the DA. The build-out resistors totally isolate disturbances on one output line
from all other outputs. All outputs will tolerate short circuits across the outputs, or
to ground, without damage. However since all outputs are driven by active
stages, DO NOT CONNECT EITHER HI or LO OUTPUT TERMINALS TO
GROUND or to the grounded side of an unbalanced load to avoid excessive
power supply loading and loss of headroom. Instead, connect the unbalanced
load between the HI output and DC GROUND (rather than HI and LO) or use a
transformer output equipped DA module such as an ATI MIDA100-1 or isolate
the unbalanced load through a balanced-to-unbalanced converter such as an ATI
L1000-1 Line Amplifier or an isolation transformer.
The SMDA200 includes a bargraph peak-reading meter. Front panel switches S2
and S3 select input or output and either Left, Right or the L+R sum signal. A4A
sums and amplifies the switch output with a 3dB gain reduction allowance for the
L+R rms sum. The meter circuit is designed for equal input and output levels and
as shipped is calibrated by R71 for +4dBm at 0VU. A4B and A4C form an active
full wave meter rectifier driving the LED bargraph with a range of 12dB above
and 15dB below 0VU in 3dB increments. The display and circuit time constants
approximate PPM ballistics. The meter driver, A5, is a National LM3915 and A6
is a General Instrument MV50164 ten segment LED display.
Unregulated input power at ±18VDC nominal is filtered and limited to 16VDC
maximum by zener diodes CR2 and CR3. Power indicator LED CR5 detects the
loss of either power supply voltage. On-board fuses, PI and F2, (3/8 A) protect
the main power buss from module faults. Zener diode CR1 provides 11VDC
maximum to the meter display.