Nexo Alpha System - Manual (Page 21)

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P 21
USER MANUAL
VERSION :1.01
DATE : 06/10/10
4. Amplifiers
4.1. R ECOMMENDED P OWER
For best results, NEXO specifies a range of amplifier powers relative to the capacity of the cabinets (see
« Technical Specifications »). The use of amplifiers with lower power rating has no justification other than
budgetary. On the contrary, high power amplifiers (within reasonable limit) present no real technical
problem (bearing in mind that the servo-control system limits the delivered power when needed), but may
simply represent an unnecessary expense.
4.2. C
URRENT
C
APABILITY
When evaluating an amplifier, it is important to take account of its behaviour under load conditions (current
capacity) : a speaker system is highly reactive, with transient signals like music it can require a lot more
current than the nominal impedance would indicate. Apart from the manufacturer’ specifications, it is
possible to test the amplifier with two times the intended number of cabinets (two cabinets per channel in
place of one, four in place of two, etc...), and drive the system to the onset of clipping. If there is no
noticeable (audio) signal degradation, the amplifier is well suited (do not take much notice of amplifier
heating after several minutes but amplifier protection should not appear too quickly).
4.3. V
OLTAGE
G
AIN
If V
in
is the voltage applied to the input of the amplifier (small, so that the amplifier does not clip, typically
100 mV@1kHz between pin 2 and pin 3 of the XLR connector), and V
out
the voltage measured at the output
of the amplifier (between black and red barrier strip, in Volts), then the voltage gain is :
L
G
= 20 log
10
(V
out
/ V
in
)
It is very important that all the amplifiers within an installation have closely matched gains ; the variation
allowed must be less than +/- 0.5 dB. This precaution is very important for reliability in the case where only
one TD Controller is being used for several amplifiers. It is also recommended to use the same amplifiers
throughout and to check their gain periodically. If some different amplifiers must be mixed (e.g. in rental
situations), at least check their gain and adjust as necessary.
NEXO recommends the use of low gain amplifiers whenever possible, 26 dB being the preferred value as it
is quite common. The use of high-gain amplifiers has a negative effect in terms of signal to noise ratios :
the delivered noise is increased whilst the maximum voltage level remains the same (4.5 V peak at the
input is enough to drive a 26 dB gain, 500 W / 8 Ohm power amplifier to the onset of clipping).
Some amplifiers use non-linear compression circuits to smooth signal before clipping: for correct use of the
TD Controllers, NEXO recommends that these circuits are switched off in order to maintain the amplifier
linearity.