Limiters: configuration procedure Foreword Principal of

Limiters: configuration procedure Foreword Principal of
Whitepaper - S
peaker protection: how to correctly configure the Apex
Intelli-X² limiter
Revision: June 2011
Foreword
Each output of the Intelli-X² System Manager is
provided with a two-stage limiter, intended to protect
loudspeakers against damage whilst preserving
sonic transparency at high sound pressure levels.
It is of paramount importance that all limiter
parameters are correctly configured in order to
minimise the risk of speaker damage. As such, it is
necessary to understand the causes of loudspeaker
damage and how the Intelli-X² limiters can help
prevent this.
Principal of operation
The first limiter stage in the Intelli-X² responds to the
RMS level of the signal, the second stage is a very
fast-acting limiter responding to signal peaks. The
thresholds of each may be set independently.
The RMS limiter is intended to protect the speaker
against thermal damage when excessive power is
applied for extended periods of time, resulting in the
voice coil overheating and finally burning.
The role of the peak limiter is to protect against
mechanical damage due to over-excursion when
the speaker voice coil is driven out of the magnetic
gap (where displacement exceeds Xmax). The
peak stage may also eventually be used to prevent
the amplifier from clipping.
Warnings and disclaimer
It is of primary importance that the amplifiers will
not be overdriven as they must be able to deliver
the necessary power without clipping. The required
power depends on the application. If the full
capability of the loudspeaker is needed to achieve
the desired sound pressure level then we recommend
that an amplifier capable of continuously delivering
at least twice the loudspeaker’s power handling be
used. If several loudspeakers are to be connected in
parallel to the same amplifier output channel, then
it should be able to deliver the loudspeaker's power
handling multiplied by the number of loudspeakers
connected, multiplied by two. Some amplifiers have
in-built clip prevention circuits and if this is the case
then we recommend that they be enabled.
Note: Always follow the loudspeaker manufacturer’s
recommendations regarding crossover filters in
order to avoid driving speakers outside of their
operational frequency range.
Due to the high degree of variance between
loudspeaker
and
amplifier
manufacturers’
specifications, Apex can not guarantee total
protection against loudspeaker damage or failure.
Furthermore, even when the limiters are correctly
configured, improper system operations such
as sustained microphone feedback, excessive
equalisation, clipping anywhere in the signal chain,
turntable rumble etc., may lead to loudspeaker
damage.
Apex therefore does not accept
any liability or responsibility for any damage to
equipment.
Limiters: configuration
procedure
The limiter settings in the Intelli-X² fundamentally
depend on the amplifier voltage gain (otherwise
known as input sensitivity) and loudspeaker power
handling capabilities. The voltage reaching the
loudspeaker (VL) has to be kept under control and
within an acceptable range in order to avoid any
damage. This voltage is simply equal to the Intelli-X²
output voltage (Vout
max
) augmented by amplifier
gain (GA).
1.Loudspeaker maximum driving voltage
First, look for the nominal impedance (ZL) and power
handling (PL) of the cabinet (or each driver in the
case of a multi-way system). This information can be
found in the unit’s datasheet or user manual. The
nominal impedance can be found easily, however
1
things are not that straight-forward for power
handling. You will need to look for the continuous
power, otherwise referred to as “average power”,
“Watt AES” or “Watt RMS”. Do not use program or
peak power for limiter settings!
Using the impedance and power handling, we can
compute the maximum RMS voltage (VL) in dBu that
may be applied to the loudspeaker:
Caution: w
e strongly recommend the use of
identically rated amplifiers, all with the
same voltage gain settings within one
system. We also recommend that all level
control knobs are set to maximum. Any
gain mismatch between amplifier channels
will not only impact limiter settings, but will
also require adjustments to be made to
the overall balance of the system.
If the voltage gain is specified in dB disregard the
next two sections and move directly to Section 3.
2.1. Linear voltage gain conversion to dB
With PL, the power handling in Watts
If the gain of the amplifier is specified as a multiplier
we need to convert it to dB using the following
formula:
ZL , the nominal impedance in Ohms
As we are working with voltage, you may connect
several identical cabinets (or drivers) in parallel
without making any changes to your limiter settings;
in fact, the same voltage will be applied to all of
them. Nevertheless, you need to ensure that your
power amplifier is capable of driving the reduced
load impedance. The total load as seen by the
amplifier will be:
WithZL, the impedance of one cabinet or component in Ohms
N, the number of cabinets in parallel
2. Power amplifier voltage gain
Look for the voltage gain specifications of the
amplifier (GA) in its documentation. Some amplifier
manufacturers specify the voltage gain in dB (e.g.,
26 dB) or as a multiplier (e.g., x 40). Some other
manufacturers specify input sensitivity as volts (or
dBu) for full rated power (e.g., 0.775v or 1.4v).
Furthermore, some amplifiers are fitted with switches
(usually located on the rear panel) that adjust gain/
sensitivity.
With MA: the amplifier gain multiplier
2.2. Input sensitivity conversion to voltage gain in dB
If only input sensitivity is specified then calculating
voltage gain in dB will be a little more complicated.
The sensitivity of an amplifier depends of the
impedance of the load. Amplifiers have a lower
input sensitivity at lower impedances because the
voltage gain remains the same.
In the amplifier’s technical specifications, you will
find that sensitivity is specified for a given load
impedance. To calculate the voltage gain we will
therefore need: a) input sensitivity, b) the impedance
at which the sensitivity is given and c) the full rated
power for this sensitivity (and impedance). You
may then compute the voltage gain in dB using the
following formula:
With PA: the amplifier’s full rated power in Watts for the given sensitivity
ZA: the load impedance in Ohms at which the sensitivity is rated
2
SA: the amplifier input sensitivity in Volts
If the sensitivity is expressed in dBu instead of volts,
you may convert it using this formula:
3. Calculating the RMS limiter threshold
Now that the maximum RMS voltage (VL) that may
be applied to the speaker and the amplifier gain
(GA) are known, we can compute the maximum
RMS voltage at the output of the Intelli-X² (Vrms_max):
Note: M
anufacturers determine power handling of
a loudspeaker by the applying a test signal
(usually pink noise) over a period of several
hours. After this test, the driver should not
show any appreciable damage, which is
expressed as a percentage of change in the
specification. As our goal is to protect the
loudspeaker against any damage, we need to
ensure that the power is kept below the given
rated power handling figure.
Furthermore, all previous formulas are correct for
purely resistive loads and use of a steady sinusoidal
tone. Loudspeakers are not pure resistors (and you
probably won't just play sine waves), but these
assumptions are sufficient provided that some
margins are taken into consideration.
We therefore recommend using a margin of at
least 2 or 3 dB on the previously calculated Intelli-X²
maximum output voltage. The formula to obtain the
RMS limiter threshold is amended as follows:
4. Calculating the peak limiter threshold
Returning to the way manufacturers test their
speakers; the applied test signal usually has a
crest factor of 6 dB. You may also have noticed
that loudspeaker peak program power is mostly
specified as being 4 times the continuous power.
Quadrupling the power into a resistor is equivalent to
doubling the applied voltage, which means adding
6 dB of gain. So we could deduce that setting the
Peak limiter 6 dB above the RMS stage would be
ideal. However, loudspeaker drivers can usually
withstand momentary power peaks well in excess of
those they are subjected to during power handling
tests. Furthermore music has a far higher crest factor
than pink noise. We recommend the Peak limiter
threshold should be set no higher than 9dB above
RMS limiter threshold. This should ensure sufficient
protection against driver over-excursion.
We now have to consider the clipping point of the
amplifier. Driving an amplifier in clip may have
dramatic and disastrous consequences on your
loudspeakers. If the amplifier is not powerful enough
or its voltage gain is very high, there is a risk that the
amplifier will clip before the limiter peak threshold is
reached.
4.1. Preventing amplifier clip
To prevent clipping, the peak limiter threshold (Vpeak_
) of the Intelli-X² must match or be set just below
the amplifier maximum input level. The maximum
input level, sometimes referred as input clipping may
usually be found in the technical documentation
in the form of a gain value in dBu (e.g. +15dBu or
+21dBu) or tension value in volt (e.g 16v). If the
clipping point is written in dBu, no further investigation
is required and this value may directly be use to set
the peak limiter threshold. A small margin may be
required.
max
Some amplifier manufacturers do not specify the
maximum input level but a maximum output voltage.
The maximum input level may be calculated as
follows:
WithVA_peak_max:the amplifier’s maximum output
level in Volts
Vpeak_max: the peak limiter threshold in dBu
3
5. Calculating attack and
release time constants
The Intelli-X2’’s attack and release time constants
are mainly related to the RMS limiter stage. The Peak
limiter has a virtually instantaneous attack time and
a release time logarithmically proportional to the
release time set in the RMS stage.
The primary purpose of the time constants is for sonic
enhancement, although the faster the attack time,
the more efficient the protection. However, if the
attack time is too short, transient suppression would
result (which typically is unwanted). Similarly, the
release time has no real protection purpose, but
correctly calculating the release time is necessary
in order to prevent the limiter from modulating the
programme material, which may sound unpleasant.
As a rule of thumb the Attack (Tattack) and Release
(Trelease) time constants should be set depending on
the frequency of the crossover’s high-pass filter:
4
With Tattack:limiter attack
milliseconds
time
constant
in
time
constant
in
Trelease:limiter release
milliseconds
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