Audio Developments AD144 - Manual (Page 18)

17
The discreet and discrete microphone amplifier is the one designed for AD146 and
has been tried, tested and proved in the field and in the studio. The transformer-
balanced, ultra-low-noise 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. (On request, the rotary switch may be
replaced with a potentiometer for continuously-variable 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 (16). 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 (16), input gain switch (3) at 35 and channel fader (15) 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.
φ1 is the normal position.
The high-pass filter (17) 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 (18) 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 (18) auditions the signal pre and post
equalisation without disturbing chosen settings.
There are four auxiliary sends from the module; each has its own source selector
combined with an on/off switch, and each has its own level control which acts purely
as an attenuator. The auxiliary sends are independent of the channel mute (routeing
switches) and, therefore, it is essential that the AUXILIARY OFF position is
immediately and positively identifiable. To this end, the order of functions on source
selectors (19) to (22) has been very carefully chosen. When the level controls (7) to
(10) are set at 0 and sources selected to post fader, auxiliary outputs will have the
same level as the module output(s), and will track with them.
The overload LED (14) illuminates 3dB before clipping at the input to the channel
fader (equaliser output).