Audio Developments AD146 - Manual (Page 18)

17
The transformer-balanced, ultra-low-noise microphone amplifier does not require an
input attenuator as the available 50dB gain change is achieved with a 6-position rotary
switch (3) acting on the feedback alone - in 10dB steps.
This has the advantage of
not degrading signal-to-noise performance as the gain is reduced. (The rotary switch
may be replaced with a potentiometer for continuously-variable gain control.)
A DC voltage is available for phantom powering, either at 48v or 12v, condenser
microphones (1). If the T (tonader) powering option has been included in the mixer, it
will be selected by switch (1) to 12v.
The line input is isolated from the microphone power and no damage can be done to
external equipment if microphone power remains selected when the channel is
switched to accept a line input (12). Such is the isolation between signal paths, a
microphone input (Conn 1) and a line input (Conn 2) may simultaneously be
connected to a channel.
With line selected (12), input gain switch (3) at 35 and channel fader (17) at 0dB, the
mixer has unity gain, with 10dB gain in reserve. (All output faders must be set at 0dB.)
Phase change (2) follows the mic/line selector and is therefore active on both
inputs.
1 is the normal position.
The high-pass filter (13) is also active on both inputs and precedes the input
transformer. In this position, the filter protects the transformer against saturation and
overload caused by excessive low-frequency content in the input signal. The high-
pass filter has a slope of 12dB/octave and is -3dB at 150Hz in position 2 and -3dB at
90Hz in position 3. The filter is independent of the equaliser.
The EQ selector switch (14) routes the signal through the equaliser section. The
high-frequency control (4) has an amplitude of
10dB at 10kHz, with variable slope.
The mid-frequency control (5) has an amplitude of
15dB at 2.8kHz, with a
proportional-Q of 1.2. The low-frequency control (6) has an amplitude of
10dB at
100Hz, with variable turnover. The EQ switch auditions the signal pre and post
equalisation without disturbing chosen settings.
If the balanced, post-fader channel output (clean feed) option has been included, the
signal will appear on a stereo jack in the centre of (XLR) CONN 2. Alternatively,
it may be chosen to replace LINE input on CONN 2.
The components associated
with the clean feed circuitry appear on the sub-board. The DIL switch allows the
module to drive unbalanced loads - without a reduction in signal level.
Refer to TECHNICAL LIBRARY.
The overload LED (11) illuminates 3dB before clipping at the input to the channel
fader (equaliser output).