Lab.gruppen fP 3400 - Manual (Page 7)

Three of the switches in the DIP switch selects the maximum gain of the channel to be either 20, 23, 26,
29, 32, 35, 38 or 41dB. (See page
6 )
9. AC power cable
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Before turning the amplifier on please read the following sections to make sure that the unit is set up
properly for your specific application.
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The gain switch located on the rear panel (the central DIP switch) changes the input sensitivity of the
amplifier. This can be handy when using low or high nominal input signals. E.g. most professional mixing
consoles operate at a nominal level of +4 to +6dBu; therefore you may use the 32dB position to provide
you with plenty of fader movement. On the other hand, for a disco mixer operating at a nominal level of
0dBu or less use the higher gain positions.
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There are separate sections for the two channels, so different gain can
be selected for channel A or B. The selection of maximum gain in the
amplifier is always a trade between noise and headroom. A low gain
amplifier amplifies less of the noise of the preceding equipment
(mixer, crossover, equalizer etc.). On the other hand a higher level is
needed to get full power, so the headroom will be decreased on mixer
output and/or crossover units. In an actively divided system there is
very often a so-called loudspeaker processor or controller involved, it
being unique for the loudspeaker system. In most cases the
manufacturers of these processors and/or loudspeakers recommend a
specific system gain for the amplifiers.
There are eight positions with different gain, from 20dB to 41dB in 3dB steps. See the different settings
for the DIP switches in the table below. The three switches to the very left are for channel B, while the
three to the very right are for channel A.
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Sensitivity is defined as how many volts (rms) or dBu (referred to 0.775Vrms)
are required to get full output power. As the output power varies with the load
impedance, usually 4 ohms is the reference. But in case of an MLS-switch
equipped amplifier there are enough choices for full output levels into
different load impedances for a sensitivity table to fill several pages.
Hence we recommend calculating the sensitivity if this is necessary. Our
”Audio calculator” can do this, which is an Excel file with many useful
formulas. It contains help for setting up digital loudspeaker processors and can
be found on our website
www.labgruppen.com : check in the ”Downloads”
area.
The sensitivity calculator is found in the box labeled “Amplifier gain conversions”. The values to enter
are in red; desired output power (see MLS table), load impedance and the selected maximum gain. The
sensitivity is in the box labeled “Input level for clip” in Vrms or dBu.
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