Operation Manual
I-Tech HD DriveCore Series Power Amplifiers
page 56
I-Tech HD DriveCore Series Power Amplifiers
Operation Manual
page 57
13 LevelMAX
TM
Limiter Suite
The I-Tech HD LevelMAX™ Limiter Suite features three types of limiting:
•
Peak
•
RMS
•
Transducer Thermal
and three modes of operation:
•
Auto
•
Advanced
•
Manual (OEM Factory Locked)
The standard i-Tech HD LevelMAX limiter panel (Advanced Mode) is shown
below:
LevelMAX has been designed to limit accurately to the specified voltage
thresholds. The addition of a thermal limiter allows thresholds for the
faster-acting RMS and peak limiters to be set higher, providing more output
and maintaining dynamic range before the onset of limiting, while protecting
the long term integrity of the transducers.
In Auto or Advanced modes, turning on the LevelMAX Limiter Suite will
enable all three limiters: Peak, RMS and Thermal.
The three LevelMAX limiter types and operating modes are outlined in the
following sections.
constant regardless of whether the LevelMAX Limiter suite is enabled
(ON) or bypassed (OFF).
14.6 LevelMAX RMS Limiter
LevelMAX RMS limiting accurately limits the output signal to the
specified RMS threshold voltage.
14.7 RMS Threshold (VRMS)
The RMS Threshold should be set by the user to correspond to the
short term (2 hour) power handling of the transducer or system.
Refer to the appropriate JBL Specification Sheet, Technical Manual or
Product Manual for two hour power ratings. To calculate the RMS
Threshold in VRMS, use the following equation:
where AES Power (Wrms, 2 hour) is the free air component power
rating.
If minimum impedance data is unavailable, use nominal impedance. If
2 hour AES power ratings are unavailable, use 2 hour IEC power
ratings. For most applications, it is recommended to operate LevelMAX
limiters in Auto mode so that peak thresholds are automatically
calculated based on the RMS threshold and selected LP frequency
(see 14.1).
14 LevelMAX Limiters
14.1 LevelMAX Peak Limiter
LevelMAX Peak limiting instantaneously limits the output voltage to
not exceed the defined peak threshold voltage.
14.2 Peak Limiter Threshold (V
pk
)
In Auto mode, the peak limiter threshold value is automatically
determined from the defined RMS threshold and the low pass
frequency defined in the XOVER block. For a low pass frequency less
than 100 Hz, the RMS to peak threshold ratio is 6 dB. From 100 Hz to
400 Hz, the RMS to peak ratio increases from 6 dB to 9 dB. Above 400
Hz, the ratio remains at 9 dB.
For Advanced and Manual modes, the peak limiter threshold can be set
by the user.
14.3 Peak Limiter Attack (sec)
The peak limiter attack time is instantaneous and cannot be modified.
The output voltage is limited to the value defined by the peak limiter
threshold (see 14.1).
14.4 Peak Limiter Release (sec)
In Auto mode, the peak limiter release time is determined by the high
pass frequency defined in the XOVER block.
For Advanced and Manual modes, the peak limiter release time can be
set by the user.
14.5 Peak Limiter Look-Ahead
Peak Limiter Look-Ahead inserts a delay into the peak limiter signal
path in order to ‘look’ at the input signal and anticipate when the peak
threshold will be exceeded. If it detects that the threshold will be
exceeded, the signal is gradually reduced prior to reaching the
threshold, while still limiting to the correct peak voltage. This approach
eliminates a sharp signal discontinuity which, in turn, could create
audible distortion.
The peak limiter look-ahead value is 0.5 ms. This value was determined
to be subjectively acceptable for most frequency ranges while not
adversely compromising overall latency. Overall latency remains
14.8 LevelMAX Transducer Thermal Limiter
LevelMAX Transducer Thermal limiting is designed to protect
transducers from long term thermal damage by gradually adjusting the
RMS threshold until the target long term thermal threshold voltage has
been reached. The thermal limiter functions only when the RMS limiter
is enabled and when the thermal voltage threshold is lower than the
RMS threshold. On the main limiter panel, the thermal protection LED
turns on once the thermal limiter is 0.75 dB into limit.
14.9 Thermal Voltage (V
RMS
)
The thermal voltage threshold should be set to correspond to the long
term (100 hour) power handling of the transducer or system.
Refer to the appropriate JBL Specification Sheet, Technical Manual or
Product Manual for 100 hour power ratings. To calculate the Thermal
Voltage Threshold in VRMS, use the following equation:
where Power (Wrms, 100 hour) is the 100 hour power rating.
If 100 hour power handling data is unavailable, set 100 hour power
rating 0.8 x 2 hour power rating.
14.10 Thermal Response Time (s)
The thermal response time is the amount of time the average RMS
signal is above the thermal threshold voltage before limiting begins.
Once the threshold has been exceeded for the amount of time defined
by the thermal response time, the RMS threshold is lowered until the
target thermal voltage has been reached. The thermal response time is
not the time it takes to reach the target voltage, although the longer the
thermal response time, the longer it takes to reach the target voltage. It
is also dependent on the amount of gain reduction and the ratio
between RMS and thermal thresholds.
Thermal response time is transducer-dependent and should be based
on the initial temperature rise of the transducer voice coil to protect it
from thermal overload.
If detailed transducer data is unavailable, the Thermal Response Time
should be left at the default value of 10 sec.
)
(
Impedance
Nominal
)
hour
100
,
(
Power
)
(
Threshold
Thermal
ohms
x
Wrms
Vrms
=
=
)
(
Impedance
Minimum
)
hour
2
,
(
Power
AES
)
(
Threshold
RMS
ohms
x
Wrms
Vrms
=
=
