Owner's Manual

AMPLIFIER SELECTION FOR USE WITH GEO S12S Page 15/112
• 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:
1) Unplug any loudspeakers from the amplifier outputs
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
3) Measure the voltage at the output of the amplifier
4) Calculate the gain using the formula Gain = 20 * LOG
10
(Vout/Vin).
Some examples:
Vin / Gain 20dB 26dB 32dB 37dB (1.4V sensitivity / 1350Wrms)
0.1 V 1 V 2 V 4 V 7.1 V
0.5 V 5 V 10 V 20 V 35.4 V
1 V 10 V 20 V 40 V 70.8 V
Remember that constant sensitivity settings will give a different gain value when the amplifier power is
different.
NEXO recommends low gain amplifiers: +26dB is recommended, as it is at the same time adequately
low and quite common amongst amplifier manufacturers. This gain setting improves signal to noise ratio
and allows all preceding electronic equipment, including the NX242 TDcontroller or GEO S12
TDController, to operate at optimum level. Remember that using a high gain amplifier will raise the
noise floor proportionally.
Operating Mode
Most two channel amplifiers available on the pro-audio market have the following operating modes:
• Stereo:
two fully independent channels deliver identical power into identical loads
• NEXO recommends Stereo Mode for all amplifier channels feeding GEO S12’s.
• Bridge-Mono:
the second signal channel processes the same input as the first channel, but with
reversed phase. The (single) load is connected between the two positive channel outputs using a
suitable connection. While the total output of the amplifier remains the same, the available output
voltage, the minimum impedance that can be connected and the voltage gain are doubled as
compared with stereo operation. Typically, only channel 1 input is active. Positive and negative
output connections vary depending on amplifier manufacturers.
• NEXO does not recommend Bridge Mono Mode unless amplifier power is clearly not sufficient.