Nexo RS - Manual (Page 18)

Page 18/58 A MPLIFIER SELECTION FOR USE WITH RS15
3 A
MPLIFIER SELECTION FOR USE WITH
RS15
NEXO recommends high power amplifiers in all cases. Budget constraints are the only reason to select
lower power amplifiers. A lower power amplifier will not reduce the chances of driver damage due to
over-excursion, and may actually increase the risk of thermal damage due to sustained clipping. 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.
3.1 RS15 recommended amplification
RS15 is rated for very high power handling and has a 2x8 Ohms nominal impedance.
These impedance valuse allow connection of up to 4 cabinets in parallel for each amplifier channel.
Nexo recommends amplifiers in agreement with table below:
Recommended
Amplifier# OMNI MODE DIRECTIONAL MODE
1 x RS15 2 x 700 Watts to 1200 Watts / 8 Ohms or
1 x 1400 Watts to 2400 Watts / 4 Ohms (*)
2 x 700 to 1200 Watts / 8 Ohms
2 x RS15 2 x 1400 Watts to 2400 Watts / 4 Ohms or
1 x 2800 Watts to 4800 Watts / 2 Ohms (*)
2 x 1400 Watts to 2400 Watts / 4 Ohms
4 x RS15 2 x 2800 Watts to 48000 Watts / 2 Ohms 2 x 2800 Watts to 4800 Watts / 2 Ohms
(*) driving both drivers in parallel requires dedicated speaker cable
3.1.1 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 four to ten times more instantaneous
current than its nominal impedance would indicate. Amplifiers are generally specified by continuous
RMS power into resistive loads; however the only useful information about current capacity is the
specification into a 2 Ohm load. It is possible to perform an amplifier listening test by loading the amps
with twice the number of cabinets considered for the application (2 speakers per channel instead of one,
4 instead of 2) and running the amps up to the 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).
3.1.2 Amplifier settings
Gain value
Gain is the key to correct alignment of the system. It is especially important to know the gain of all
amplifiers used in your set-up. The tolerance should 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, all 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 fixed input sensitivity only 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. Others
may have gain switches fitted internally where it is impossible for the user to verify the actual
setting without opening the amplifier casing.