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6 : 10.6.2. 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 behavior base their operations on a mix based on
threshold, dynamic behavior 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 k Series

power limiters.

TruePower™

: the amplifier’s active output power is es-

timated by measuring the load current. The Truepower
limiter is a powersoft 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.

Empirical observation yields the following equation

where

AES

is the declared aES power and

max

is the maxi-

mum power the speaker can dissipate “in real life”.

if the

AES

is not available, the average or continuous

power, known as

rms

can be used as well; however, it is

important to proceed with caution in evaluating how the

rms

value is obtained. if no other values are declared, this rule of
the thumb can be used: the

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 hap-

pens 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.

TaB. 8

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 deliv-
ered 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 out-
put. Differently from the Truepower limiter, this limiter does
not take into account the real part of the power; however, it
has the advantage of being independent from the number of
cabinets linked together, just as a peak limiter.

Some attention is needed to set the power threshold.

The

AES

can be used if it is available. if no other power rat-

ing is declared, the

rms

can be used as well; however, the

rMS parameter is a value related to the maximum manage-
able power and not the real power. proceed with caution
because the manageable power could be greater than the
real power. Some constructors declare the rMS power at
the minimum impedance point of the speaker; this, again,
may lead to an overestimation of the true power values the
speaker can handle. if no other values are available, the
following rule of the thumb can be used: the

rms

can be

estimated as 6 dB below the peak power (¼ of the peak
power).

in order to preserve the driver in the long term, once the

maximum power limit is decided upon, consider a power
reduction of up to 3 dB of that value.

in order to use this limiter correctly, it is important to re-

calculate the equivalent power at 8 ohm. For example, with
an 4 ohm speaker with 500 W maximum rMS power, the
equivalent power at 8 ohm needs to be calculated as follow:

1. calculate the rMS voltage value needed to generate

the maximum rMS power on the 4 ohm speaker:

Voice coil size

(inches)

Threshold

(W)

Attack time

(ms)

Release

time (ms)

1”

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