Nexo NUAR - Manual (Page 39)

BLOCK DIAGRAM DESCRIPTION
The NXAMP TDcontroller will limit the user gain adjustment to a group of channel in specific
case such as cardioids setups (for example on CD18 setup, gain is linked between front
and rear loudspeaker).
Protections
Each channel has its own simulation and protection process. Each audio channel contains a
combination of controlled gain stages (let's call them VCA’s as in our analogue circuitry).
These VCA's are embedded into complex composite structures in order to change their
basic operation into frequency selective attenuation. This operation is similar to that of a
voltage controlled dynamic equaliser (VCEQ).
Each VCEQ and VCA is controlled by the synthesis of several signals issued from the
various detection sections. That synthesis is in fact the envelope of those signals, with an
optimised release and attack time for each VCEQ and VCA (depending on its frequency
range and the cabinet selected).
One or several of the protections below can be used depending on the setup chosen.
Source signals for protection algorithms (25)
Signals coming from amplifier output voltage/ current, processor output, and status from
amplifier are all sources that will lead to protection system implementation.
Displacement control (7) (8) (9)
The amplifier output voltage sense input signal is sent to a shaping filter producing a signal
whose instantaneous amplitude is proportional to the voice coil excursion (this is Global
displacement block (7)). This signal, after rectification, is compared to a preset threshold
matching the maximum usable value, as determined from laboratory measurements. Any
part of the signal exceeding the threshold is sent to the VCEQ control buffer while the
VCEQ acts as an instantaneous limiter (very short attack time) to prevent displacement
from overriding the maximum permissible value.
Another set of VCEQ (this is first displacement block (8)) is used to protect the loudspeaker
from an excessive displacement in the next worst displacement frequency area (this
usually is 3 dB bellow the global displacement protection area). In case of band pass
cabinets, we need another set of VCEQ (this is first displacement band pass block (9)) to
protect from another peak of secondary displacement.
All these VCEQs have separate shaping filter, separate action filters for VCEQ, and separate
Ratio, Attack and Release time.
Mechanical stress control (10) (11)
Whereas some frequency areas are dangerous at high level due to excessive displacement
of the loudspeaker (see above), there is another area where the displacement of the
loudspeaker is minimum, but then the mechanical stress on the cone during large input
signals is maximum.
To protect from overstressing the loudspeaker, VCEQ process can also be used in these
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