9.11.6.2 Power limiter
Given the low efficiency of electromechanical transducers,
almost 50% of power reaching the voice coil is transformed
into heat.
The power limiter is intended to avoid melting the voice coils
of drivers while at the same time exploiting their maximum
performance, therefore the power limiter should not be
engaged at normal working levels. The power limiter acts by
decreasing the amplifier’s gain in order to reduce the power
delivered to the load.
A correct power limiting is not an easy task and is
multifaceted, based on a number of variable, like the
knowledge of the component heat dissipation and the goals
that must be achieved. Therefore may be difficult and a little
bit empirical decide thresholds and constants time. Power
limiters behaviour base their operations on a mix based on
threshold, dynamic behaviour of the output readings (voltage
and current) and the type of output readings monitored.
Check the gain reduction: in order to obtain the optimal
sound it should not be greater than 2-4 dB even for the
loudest piece of music. Please note that a common musical
signal has very high peaks, but a rather small average level
(high crest factor). A stationary tone has a much higher
average power (e.g. a stationary sine wave has 3 dB crest
factor) even if it “sounds” less loud to the human ear.
There are three main operating modes for the Bias Series
power limiters.
• TruePower™: the amplifier’s active output power
is estimated by measuring the load current. The
TruePower limiter is a Void patent technology useful
to avoid overheating of the voice coil; it can however
also be used to avoid power compression. The DSP
provides the measurement of the real power delivered
(and then dissipated) to the coil, ignoring the apparent
power handled by the line.
Voice coil size
(inches)
Threshold
(W)
Attack time
(ms)
Release
time (ms)
1” tweeter 10-20 100 300
1.5” tweeter 20-30 150 300
2” comp. driver 20-40 200 400
3” comp. driver 30-50 300 500
4” com. driver 40-60 500 3000
2” midange 30-100 500 3000
3” midbass 50-150 1000 5000
4” woofer 100-200 2000 5000
4” woofer 150-250 4000 8000
6” woofer 250-500 6000 10000
TABLE 9: Filters parameters.
Empirical observation yields the following equation
• Power vs voltage @ 8 ohm: the amplifier’s output
power is estimated by measuring the RMS value
of the output voltage, assuming an 8 ohm load.
This mode allows to create settings that work well for
any number of speakers connected in parallel. For
example, if a “power @ 8 ohm” limiter is set to limit
the output power to 150 W, a single cabinet will be
delivered a maximum of 150 W with 8 ohm load. Two
speaker cabinets connected in parallel will be delivered
a maximum of 300 W with 4 ohm load (8 ohm loads in
parallel) and so on.
This limiter is a pure RMS limiter whose functioning is based
solely on the voltage module measured at the amplifier
output. Differently from the TruePower limiter, this limiter
Thresh.(Vpk)
ok
fast
- +
169 Vpk
CH1
GR= 0.0dB I= 11.7dBu
FIGURE 30: Peak limiter: threshold
voltage and gain reduction.
P
max
=
3
P
AES
where P
AES
is the declared AES power and P
max
is the maximum
power the speaker can dissipate “in real life”.
If the P
AES
is not available, the average or continuous power,
known as P
rms
can be used as well; however, it is important
to proceed with caution in evaluating how the P
rms
value is
obtained. If no other values are declared, this rule of the
thumb can be used: the P
AES
can be estimated as 6 dB below
the peak power (¼ of the peak power).
It is very important to note that, contrary to what happens
with the peak limiter, setting the TruePower limiter parameters
must take into account the number of speakers connected
to the amplifier. This is due to the fact that the real power is
calculated not only with the output voltage (which is identical
for all speakers connected in parallel) but also with the
output current (which changes according to the number of
parallel speakers).
Determining the ideal time parameters for TruePower limiters
is a very empirical process. As a guide, consider this simple
rule: larger the coil, larger the thermal inertia, larger the time
constant (ref. TABLE 9).
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