Audio Technologies Inc. | 154 Cooper Road #902 | West Berlin, NJ 08091 | Voice 856-719-9900 | Fax 856-719-9903 | www.
audio.com
DESCRIPTION
Your DA208/416 provides two (DA208) or four (DA416) independent one in by
four out circuit groups. A four way 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 1 x 8, 2 x 8 or 1 x 16 operation. Since each input
circuit has 30k ohm 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, we 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 to 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 a 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 it's 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.
