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AMPLIFIERS
ALPHA SERIES USER MANUAL V1.0
DATE: 14/01/00 18:27
Amplifiers
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
amplifiers 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 amplifier rating.
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 generally specified by continuous RMS power into resistive loads however the only
useful information in that respect is the specification into a 2-ohm 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
Technical knowledge on the amplifiers to be used with the system is essential. This data is the key
to the correct alignment of the system. It is especially important to know the gain of all amplifiers
used in your set-up. The tolerance shall be about ±0,5 dB. In practice this can be difficult to achieve
because:
•
Some amplifier brands have an identical input sensitivity for models of different power
rating (this infers a different voltage gain for each model). For example a range of
amplifiers with different power outputs having a published input sensitivity of 775mV/0dBm
or 1.55V/+6dBm will have a wide range of actual gains, the higher the power, the greater
the gain.
•
Various other brands may offer constant gain but only within a given product range for
example they may fit only fixed input sensitivity on their semi-professional amps.
•
Even if a manufacturer applies the constant gain rule to all models, the value selected will
not necessarily be the same as that chosen by other manufacturers.
•
Some products can exhibit manufacturing tolerances for the same model of ±1dB or more.
Some amplifiers may have been modified, possibly without any label indicating the new
values and some may have gain switches fitted internally where it is impossible for the
user to verify the actual setting without opening the amplifier casing. In cases where you
don't know the gain of your amplifier (or want to check it) please read the following
instructions.
1. Unplug any cabinet from the amp
2. With a signal generator feed a sine wave at 1000Hz at a known voltage (say
0.5V) to the input of the amplifier under test.
