3 current rating, 4 amplifier settings – Nexo GEO T User Manual

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GEO

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UP

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NSTRUCTIONS

2.3 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).

2.4 Amplifier

settings

2.4.1 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.

• 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 * LOG10(Vout/Vin).

Some examples:

GainVin

20dB

26dB

32dB

37dB (1.4V sensitivity / 1350Wrms)

0.1V 1V 2V 4V

7.1V

0.5V 5V 10V 20V

35.4V

1V 10V 20V 40V

70.8V

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, to operate at optimum
level. Remember that using a high gain amplifier will raise the noise floor proportionally.

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