Whirlwind DA-2 - Manual (Page 2)

Block Diagram: The DA-2 can be configured as a 2 by 4 or a I by 8 distribution amp through the
combine circuit.
THEORY OF OPERATION
The U.S. Audio DA-2 is a dual channel line-level
distribution amplifier with four outputs per channel. All
inputs and outputs are active electr onically balanced
circuits accessed through XLR connectors. The input
section and all critical gain sectio ns of the DA-2 use the
same IC chips as top professional mixing boards. The
inputs are instrumentation grade differential amplifiers
tuned for a high amount of Common Mode Rejection
(CMR). The CMR of the DA-2 is greater than 90dB, 20 to
20KHz. The DA-2 has tunable input impedance, factory-set
by a resistor at 1 Meg ohms, which allows many units to be
connected together with their inputs paralleled. If lower
input impedance is desired it can be easily accomplish ed by
an internal resistor change. Output volume pots control the
level of the rear panel output XLRs over a gain range of -60
to +18dB. This feature allows the DA-2 outputs to easily
compensate for variations in input levels. Unity gain on the
DA-2 is approximately at the 9 O’clock position and 14dB
of gain is approximately at the 3 O’clock position. 60dB of
attenuation in the “off’ position allows the DA-2 to
adequately “turn off” line level sig nals.
Maximum signal level through the DA-2 at clipping is
+28dBm balanced and +22dBm unbalanced. The same
figures apply to both the input and the output, and
individual clip LEDs are provided which illuminate at 3dB
below actual clipping. Green LED s are provided for each
channel, which verify input signal levels above -14dB
balanced and -20dB unbalanced. With THD+n typically
less than .004% throughout the entire audio spectrum, and a
unity gain signal to noise ratio greater than 85dB with
22dB of headroom, the DA-2 is well suited for use in all
audio distribution applications. Along with great audio
quality, the DA-2 features a high degree of isolation
between channels, as well as i mmunity to RF and
electromagnetic radiation.
The DA-2 headphone monitoring section consists of a
channel select switch, a volume control and a 1/4” TRS
headphone jack. The headphone circuit is a mono type
which drives both left and right stereo earpieces in any
headphones with 20 ohms or greater impedance. The
volume control has a gain range of -60 to +18dB to
accommodate a wide range of aud io levels. The select
switch allows listening to either the channel 1 or channel 2
inputs.
The combine switch is a unique feature th at serves two
functions. First, it allows the DA-2 to operate as two
independent 1 in to 4 out distribution amps or as a single 1
in to 8 out amp when in combine mode. Secondly, in
combine mode the two inputs are actively mono summed,
allowing the DA-2 to provide a mono feed from left and
right stereo signals. Either input may be used to feed the
eight outputs when the combine switch is activated.
The DA-2 utilizes a dual primary power transformer that is
configured with internal jumpers for 120 VAC 60Hz or 230
VAC 50Hz operations. There is an internal fuse on the hot
side of the AC cord and the power switch makes and breaks
both the hot and neutral legs of the AC cord. A ground lift
switch is provided which disconnects the audio circuit
ground from the AC chassis ground when engaged.