A-SERIES ULTRA LOW NOISE LIMITER FOR LOUDSPEAKER PROTECTION
This design is no longer in production.
LI06 is a PP A-series loudspeaker protection limiter for Modular crossover network type XO01.
The limiter is a high ratio, hard-knee, feedback type with a delay for attack and release to make it's operation almost inaudible.
The attack delay prevents the limiter from reacting on signal peaks, while still retaining a reasonable short attack time.
The release delay holds the limiter's gain reduction until the signal has fallen below the limiter threshold, thus preventing the low-frequency distortion normally seen in limiters with a short release time.
A versatile link system on the module enables the gains of several limiters to track each other either unidirectional or bidirectional.
The limiter's gain, threshold, time constants, link system and LED function are programmed by jumpers on the module.
A number of LI06 can be linked via the link signals "LINK 1" and "LINK 2" so their gainreductions will track. The two paralleled connectors "LKA" and "LKB" on the module allows the two link signals to be daisy-chained over several limiters. Jumpers on the LI06 select the link mode individually for LINK 1 and LINK 2.
Bidirectional link mode is used if it is essential to maintain the level balance between the outputs at all times like for example in studio monitors.
Unidirectional tracking can be useful in PA speaker systems to insure that if the low-frequency driver is driven into limitation, the high-frequency driver will follow to avoid a sudden increase in high-frequency output - a situation that can be very unpleasant for the audience. This can also be achieved by bidirectional link mode, but it is undesirable that fast transients from the high-frequency driver will modulate the low-frequency output via the link.
|Off||The limiter is disconnected from the link.|
|Track||The limiter will track the gainreduction determined by the link signal or, if its own gainreduction is larger than determined by the link signal, it will control the link signal.
Two or more limiters in this mode will always have the same gain.
|Master||The limiter's gain is not controlled by the link signal, however if its own gainreduction is larger than determined by the link-signal, it will control the link signal. If there are more limiters in this mode on the link, the one with the largest gainreduction will control the link signal.|
|Slave||The limiter will track the gainreduction determined by the link signal, but will not control the link signal.|
LI06's four time constants are defined as shown in the figure below.
Input signal is a 5 KHz burst signal with a burst level 6 dB above threshold and a pause level 2 dB below threshold. As the time constants are program dependent, the values are only valid at this drive level. Each time constant can be set to two different values independent of each other. The time constants are different for different versions of LI06. LI06A is optimized for high-power PA loudspeakers, where the slow settings are used for cone-drivers and the fast settings for compression-drivers.
The two most common causes of speaker damage are thermal overload and mechanical overload. A limiter can keep the power supplied to the speaker within safe levels and thereby protect the speaker from thermal overload, but to protect speakers from mechanical overload, they must be driven only within the frequency range they are designed to handle.
To protect speakers from thermal overload, the limiter's threshold must be selected to match the speaker's continuous power handling - normally half the power handling stated as continuous pink noise power handling - and the amplifier's voltage gain. A common practice is to set limiter threshold to a higher value than this to obtain a higher transient output from the system, but this is not necessary with the LI06, as it does not respond to transients due to its attack delay. This again means that LI06 does not protect the system against high-power transients, but most modern power amplifiers has a soft-clipping circuit so this is normally not a problem. 5 jumpers allows the LI06A's threshold to be set from -15 dBu to 16 dBu in 1 dB steps.
The limiter LED controlled by the LI06 can operate in fast or in slow mode. The system will generally play louder when slow mode is selected, as very few people are able to hear the LI06 until the LED turns on. The LED only responds to the limiter's audio input and not to control signals via the link system.
Limiter LED modes
|Fast||In this mode the LED will light up whenever the signal is above threshold or when the limiter's gain is reduced by 3 dB or more. The LED may flash without the the limiter is reducing the gain (due to the attack delay). This mode is used when it is essential to know when the limiter threshold is reached.|
|Slow||In this mode, the LED will be on when the limiter has reduced the gain by 3 dB or more.|
LI06 Technical specification
Specifications are for LI06A installed in XO01 in position K5.
LI06 settings: Input gain: 6 dB, Attack delay: slow, Attack time: slow, Release delay: slow, Release time: slow, Threshold: 6 dBu.
Specifications in [ ] are typical values.
THD+N meter bandwidth is > 500 KHz [80 KHz].
Unless otherwise noted, specifications are for 20 Hz - 20 KHz.
|Maximum input level||21 dBu|
|THD+N @ 0 dBu input|
| - 20 Hz||< 0.003 % [0.0006 %]|
| - 1 KHz||< 0.003 % [0.0006 %]|
| - 20 KHz||< 0.003 % [0.002 %]|
|THD+N @ 20 dBu input|
| - 20 Hz||< 0.2 %|
| - 1 KHz||< 0.2 %|
| - 20 KHz||< 0.3 %|
|Voltage gain @ 1 KHz||± 0.3 dB [± 0.1 dB]|
|Linearity below threshold||± 0.05 dB [± 0.03 dB]|
|Linearity above threshold||± 0.2 dB [± 0.1 dB]|
|Output noise, RMS||< -106 dBu [-107 dBu]|
|Output noise, CCIR 468-2 Q-peak||[-97 dBu]|
|Output noise, A-wgt RMS||[-110 dBu]|
|Threshold accuracy||± 0.5 dB|
|Ratio, 1 KHz||> 1:250|
|Attack delay (Slow / Fast)||120 mS / 15 mS (program dependent)|
|Attack time (Slow / Fast)||250 mS / 40 mS (program dependent)|
|Release delay (Slow / Fast)||150 mS / 15 mS (program dependent)|
|Release time (Slow / Fast)||200 mS / 20 mS (program dependent)|
|Link tracking||± 0.5 dB|
|Limiter LED turn on level||At threshold, or at gain reduction > 3 dB|
|Supply current (excl. LED)||< 33 mA|
|All specifications subject to change without prior notice.||PP 940831|
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