ATI Audio DA208 - Manual (Page 2)

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DESCRIPTION
Your DA208 or DA416 provides two (DA208) or four (DA416) independent one-in by
four-out circuit groups. A four-output feed is sufficient for many applications; however,
if you need more outputs, you can parallel channels by bridging several inputs across the
same line for 1X8, 2X8 or 1X16 operation. Since each input circuit has 30k ohms
balanced input impedance, you can simply terminate your source line with a single 620
ohm resistor (if necessary) and then parallel as many inputs as necessary across it.
Each input can bridge line levels up to +24dBm without clipping. Common mode hum
signals are attenuated by 70dB and the input resistor network is split and heavily
bypassed for good protection from RF signals riding on the input lines. The input
differential amplifier operates at slightly less than unity gain to optimize input headroom
and to prevent any signal clipping from occurring prior to the gain controls.
Each output consists of an adjustable gain stage driving the HI output terminal along with
a unity gain inverter stage. The inverter drives the LO output in opposite phase to provide
an active balanced output. The adjustable gain stage utilizes a unique circuit arrangement
that allows us to provide a smooth, full range logarithmic gain control for each output
using a linear cermet potentiometer. This circuit provides 20dB gain at full clockwise
rotation, unity gain in the 12 o'clock position and tapers smoothly to full off at the full
CCW position. Since we actually vary the gain of the output stage rather than taking the
more conventional approach of adjusting the input level to a fixed gain stage, you can use
the DA at low output levels with very little noise penalty. You can easily match the DA
outputs to console medium level inputs (-20dBm) or drive semi-pro RCA inputs (single
ended at .10Vrms) without requiring outboard attenuator pads.
Each balanced output stage is isolated from the output terminals by DC blocking
capacitors and 400 ohms of build-out resistance. The build-out resistors are split and
bypassed to prevent any RF signals that might be picked up on an output line from
feeding back into the DA.
Your DA has clipping indicators to warn you of overdriven and overloaded outputs.
These indicators individually sense nonlinear operation of any output channel by
measuring the error voltage at the feedback terminal of the HI side output amplifier. As
long as the output stage operates linearly, the error voltage at its negative input (for
example WA pin 9) will remain very small. The onset of clipping, current limiting or
even slew rate limiting immediately causes a large increase in this error signal, which is
then fed through R25 to the error amplifier U5A at pin 13. U5A, in turn, drives the
clipping LED through Q1. Feedback voltages also feed the error amplifier from three
other output channels. To avoid any crosstalk through the summing network, CR3 across
R13 prevents nonlinear clipping of U5A and therefore maintains an extremely small
common point voltage at U5A pin 13. If any single output or any combinations of outputs
are driven too high, they will light the clipping LED. Each clipping indicator monitors
only four outputs, making it easy to locate the "hot" output.