Octave
band (Hz)
Attack time
(ms)
Release time
(ms)
Atk/Rel ratio
63
45
720
x16
125
16
256
x16
250
8
128
x8
500
4
32
x8
1000
2
8
x4
> 1000
1
2
x2
TABLE 8: Attack and release times per octave bands.
Where Re is the nominal impedance of only one driver, P
peak
is the peak power and V
peak
is the peak output voltage.
A peak limiter, used with a very short attack time (i.e., with a
very rapid onset), can also be useful in limiting the maximum
peak voltage in distributed constant voltage lines.
Void designed the Bias Series limiters as protective meas-
ures; therefore, they are not meant to “colour” the sounds
such as dynamic compressors can do. With this in mind,
time constants for these limiters should be selected so
parameters:
• Active: toggles the power limiter’s on/off status;
•
Threshold (V
pk
): the peak voltage threshold at which the
gain begins to be reduced;
•
Attack: the attack time, i.e. the response time of the
limiter intervention;
•
Release: the decay time, i.e. the time constant after
which the limiter’s action is released and the gain re-
stored to the nominal value.
In order to avoid choking the exceptional dynamic range of-
fered by Bias Series amplifiers, the peak limiter is designed
to ignore signal peaks lasting less than the attack time pa-
rameter. Moreover, the limiter has an additional lookahead
buffer (0.5 ms) to soften clipping and minimize distortion,
effectively yielding superior sonic performance.
When tweaking the peak limiter’s levels, it is preferable to
first setup the time parameters, and then adjust the thresh-
old voltage. When editing the threshold value, the display
shows the gain reduction (GR) in dB enforced by the limiter.
This information, together with the limiting voltage referred
to the signal in the input amplifier stage (I) expressed in dBu,
is displayed in real time to allow monitoring of the limiting
actions as they are performed.
9.11.6.1 Peak Limiter
The peak limiter avoids potentially dangerous displacements
of the cone (an excursion larger that allowed). It acts by re-
ducing the amplifier gain in order to reduce the measured
output peak voltage. To limit the dangers of dangerous very
fast transient signals, all limiters implement a look ahead
time of 0.5 ms.
As a rule of thumb, use the declared peak power or twice the
program power as a loudspeaker safe-zone output power.
The peak limiter’s setting do not change with the number of
parallel speakers connected to the amplifier, this is because
the same voltage is applied to all the components in a paral-
lel circuit. When deciding parameters for a peak limiter of an
amplifier with many loudspeakers connected to it in parallel,
the peak power to be taken into consideration is that reach-
ing only a single speaker.
You can refer to the following formulas:
In order to prevent the mentioned phenomena two kinds of
limiters are provided:
•
Peak limiter: protects against mechanical damages.
The peak limiter may also be used to control amplifier
clipping. Designers should set this limiter’s parameters
as a function of both the maximum displacement (Xmax)
of the diaphragm as well as the speaker’s maximum
tolerated voltage.
•
Power limiter: protects speakers against thermal
damage when excessive power is applied for extended
periods of time, resulting in overheating and, eventually,
burning. Designers should be aware of the maximum
long term power safely applicable to speakers (AES
power rating). An interesting approach to RMS limiting
is one that uses coil temperature control. A complete
knowledge of the driver’s limits allows to keep the
temperature level in a safe interval not only to avoid
damage but to maintain the speaker in a “linear” zone
that avoids power compression.
as to limit potentially harmful phenomena which persist
for no more than one or two periods of the related signal
bandwidth.
TABLE 8 gives a few examples of attack and release times
with respect to the frequency range of the signal to be
limited.
The peak limiter menu allows the user to define the following
Active:ON
back
sel
Thresh.(Vpk):169
Attack(ms):10
CH1
FIGURE 29: Peak limiter settings.
P
peak
=
Re
V
2
peak
V
peak
= √
Re
∙
P
peak
Bias Series User Guide V1.0
Page 20
