Nexo NXAMP - Manuel (Page 25)

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25
AMPLIFIERS (GAIN, POWER)
Amplifiers (Gain, Power)
Power
NEXO recommends high power amplifiers in all cases. Budget constraints are the only reason to select
lower power amplifiers. If an incident occurs on an installation without protection the fact that amps only
generating half their rated output power (-3dB) are used will not change anything in respect of possible
damage. This is due to the fact that the RMS power handling of the weakest component in the system is
always 6 to 10 dB lower than the amps' ratings.
Current rating
It is very important that the amplifier behaves correctly under low load conditions. A speaker system is
reactive by nature, on transient signals like music it will require much higher instantaneous current than
its nominal impedance would indicate (four to ten times more). Amplifiers are always specified by
continuous RMS power into resistive loads (which is irrelevant); the only useful information in that respect
is the specification into a 2 ohms load. It is possible to make an amplifier listening test by loading them
with twice the number of cabinets considered for the application (2 speakers per channel instead of one,
4 instead of 2…) and modulating at high level (onset of clipping). If the signal does not noticeably
deteriorate the amplifier is well adapted (overheating after approximately ten minutes is normal but
thermal protection must not operate too quickly after starting this test).
Amplifier gains
As you already read in the Quick Start section Information on the amplifiers used is MANDATORY. This
value is the key of a correct protection setting. It is very important to know the gain and power of all
amplifiers present in your set-up. The NX242 Digital TDcontroller provides tools to help you in this task
(although the process in not totally automated for safety reason).
Some amplifier brands have an identical input sensitivity for models of different power rating (this means
DIFFERENT GAIN for each model). This problematic practice, inherited from non-professional
applications, is easily detected when the manufacturer specifies the same input sensitivity for all its range
(like 775mV/0dBm or 1.55V/+6dBm). This translates to very high gain values on higher power models.
Other brands do offer constant gain but only within a given product range (like higher gain on all semi-
professional amps). Even if a manufacturer is conscious of this problem and applies the constant gain
rule to all its models, the value he chooses is not necessarily the same as other manufacturers.
How to set correct GAIN and POWER information in the NX242
Menu 2.6 Amp Gain
The first line of the display shows the value entered by the user
(hereafter referred to as “user gain”, while the second line
displays the value read by the NX242 directly from the sense
lines (hereafter referred to as “read gain”. The enter WX button
allows the user to swap between channels.
USER MANUAL LOAD2_22
DATE: 03/12/2004