Aphex 1788A Premium Eight Chan Mic Pre - Manual (Page 16)

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MICROPHONE PREAMPLIFIER
Page 17
Instruction Manual
4. FUNCTIONS IN DETAIL
NOTE: The gain does not move in 1dB steps. It ramps in small increments (1/4dB steps)
between 1dB “stops” to eliminate all gain adjustment glitches.
This brings us to making a comment about gain steps. While there are several remote
controlled preamps on the market, the problem is that they have stepped gain control
and the step size can be as large as 10dB. Even a 3dB step, up or down, in a microphone
preamp will cause an audible click or pop when it is changed. Only the Aphex Model
1788A overcomes this problem by the use of our patented LeNA™ digitally controlled
low noise amplifier technology. This method makes the step size very small, eliminating
the artifacts of sudden gain changes. This is especially appreciable when you have to roll
through a gain change of 6dB or more very quickly. The 1788A just sounds like a fader was
moved with no jumps, pops or zips. The 1788A rolls through the full span of 26 to 65dB
in 250 milliseconds. A single 1dB stop is completed in less than 15 milliseconds. In other
words, the 1788A’s smooth gain control is also fast and responsive.
Output Gain Control
The Main and Aux outputs have their own separate settings but work in the same way.
The gain is set for Main and Aux outputs depending on the position of the GAIN SELECT
button. The output gain programs an output stage attenuator to bring the 1788A’s
internal reference peak level down as necessary to match the maximum input levels of
external devices.
CAUTION: If you’re driving an unbalanced input from the Main Output, the actual output
reading on the channel status readout will be accurate up to +21dB as that is the
maximum output level when running Main Output unbalanced. Higher gain settings
than +21dBu will be inaccurate but will not cause any distortion. If you are using Aux
output and running a unbalanced input, the actual output will be 6dB lower than
what is indicated on the channel status readout.
There are many input stages that have odd maximum input levels (e.g. +18dBu). In order
to retain the function of the microphone limiter and maximize noise performance of the
entire audio system, the Model 1788A has output level trim of up to 27dBu balanced.
Once the maximum input level of the device following the Model 1788A is defined, the
output level of the Model 1788A can be trimmed to that level. The input gain of the
1788A is then adjusted to optimum performance. In addition to the 27dB of output
adjustment, each output may be muted.
In the event the input gain level is used to adjust the maximum output level, the benefit
of the MicLim™ function will be lost. The output level should be considered as a
“set-up and forget” function. Once it is adjusted properly, there should be no need
to make further adjustments. The displayed output level on the individual channel status
readout indicates the absolute full-scale output peak level of the output signal. This means
that no further considerations have to be taken regarding headroom when setting up the
system’s overall gain structure. For this reason, it is not recommended that the Main and
Aux output levels are adjusted remotely, even though it is possible. The test tones are quite
useful for set up purposes (described later in this section).
Mute
When the MUTE function is selected on an individual channel, both analog outputs are
muted. If MUTE is engaged when MULTIPLE SELECT is engaged then the entire group of