Nexo GEO S8 - Manual (Page 9)

GENERAL SET-UP INSTRUCTIONS P.9
Amplifier Selection
Power
GEO S8 Series array elements are rated for 500 Watts power handling. Although each array element has a
16 Ohm nominal impedance, NEXO recommends that you connect no more than six S8 Series array
elements to a single amplifier channel. The amplifiers used for this application should be capable of delivering
1500 to 3000 Watts into a low impedance (typically specified as 2 Ohm) load. Budget constraints are the only
reason to select lower output power amplifiers. A lower power amplifier will not reduce the chances of driver
damage due to overexcursion, and may actually increase the risk of thermal damage due to sustained
clipping.
The CD12 requires two amplifier channels delivering separately processed signals to produce its
hypercardioid pattern. The amplifier model should be the same as that used for the GEO S8 Series array
elements. Two CD12s can be connected in parallel: take care that both front woofers and both rear woofers
are connected in parallel.
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).
Amplifier gain settings
Technical knowledge of 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 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. In cases where you don't know
the gain of your amplifier (or want to check it) please follow this procedure:
Unplug any loudspeakers from the amplifier outputs
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
Measure the voltage at the output of the amplifier
Calculate the gain using the formula Gain = 20 * LOG10(Vout/Vin).