Powersoft X Series Supplément

APPLICATION NOTES

LIMITERS: BASIC CONCEPTS AND OPERATIONS

DO000274.00 REV.00

LIMITERS: BASIC CONCEPTS AND OPERATIONS

1. Introducon

The main purpose of this applicaon note is to provide beer understanding on how to use the

dierent types of limiters available in Powerso ampliers. This document is applicable to the

following product lines: X series, T series, Oo/Quaro/Due Canali DSP+D series, Mezzo series and

Loto DSP.

Choosing the correct limiter is not an easy task and many aspects need to be considered; e.g.: it can

be dicult to decide thresholds and me constants to obtain the correct balance between protecon

and sound quality, hence every decision should be based on a wider knowledge of the component to

limit.

Liming may be considered an “art” and many dierent approaches and philosophies can be

followed. For this reason, this guideline intends to simply provide a starng point on limiter

parameters and how to handle them with Powerso devices.

NOTE: Powerso is not responsible for misuse of the informaon contained in this document and/or

any potenal loudspeaker or other damages derived from incorrect limiter sengs. This document

serves as a general guideline and is subject to the context in which it is applied.

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2. Liming, speaker damage and power compression

The liming process in sound reinforcement is a way to protect loudspeakers from accidental damage;

therefore, limiters are a safeguard against excessive signal peaks and/or signal long term average

power.

Bear in mind that liming does not only prevent occasional damage, but it rst and foremost

guarantees a long loudspeaker life.

Speaker limiter designers do not usually concentrate on colouring sound, but rather on speaker

protecon. Typical applicaons in the audio chain are:

•Controlling the energy of a signal.

•Controlling the peak levels of a signal.

•Reducing the dynamic range of a signal.

Remember that in sound reinforcement you are dealing with power signal (that means high voltage

and currents), so the target with limiters is to protect the drivers from the two main causes of damage:

•Over-excursion: An impulsive signal can reach the

speakers and cause damage due to over-excursion of

the voice coil that is driven out of the mechanical

limit. This can damage the diaphragm deforming or

breaking it.

Figure 1 – Broken woofer

•Over-heang: Delivering high power for too

much me may lead to overheang of the

voice coil copper, the coil former, the

magnet, the suspensions and the cone

itself. Another eect is power compression,

more noceable in low frequency speakers.

When there is power compression there is

an increase of the impedance and a

decrease of power with the result of less

perceived loudness!

Figure 2 – Burned coil

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3. Limiter type: RMS, Peak and Clip

To avoid mechanical and temperature related damages two kinds of limiters can be used:

•RMS limiter: Protects speakers against thermal damage when excessive power is applied for

extended periods of me, resulng in overheang and eventually burning.

Designers should be aware of the maximum long-term power safely applicable to speakers

(AES power rang). An interesng approach to RMS liming is one that uses coil temperature

control looking at how the peak in the impedance curve changes when the speaker starts

heang.

In gure 3 we can see the relaonship between the temperature and impedance curves: the

green curve is the curve measured when the speaker is at 26°C, meaning it is cold. The orange

curve is when the temperature increases to 38°C. We can noce that when the speaker starts

heang up the impedance peak will become higher in value and will shi slightly in frequency.

Therefore, being able to monitor the impedance changes in real-me will allow us to save our

loudspeaker.

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 avoiding

power compression.

Figure 3 – Live impedance curves

•Peak and limiter: Protect against mechanical damages.

Designers should set this limiter parameters as a funcon of both the maximum displacement

(XMAX) of the diaphragm and the speaker maximum tolerated voltage.

NOTE: Relying on peak liming only for speaker protecon can sll lead to excessive long-term average

power and consequenal damages.

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4. Limiter parameters

4.1 Threshold

When the input signal level exceeds the threshold, the limiter starts to reduce the input gain.

Gain is reduced of an amount equal to the overshoot of the input signal with respect to the threshold.

The threshold value is expressed in Volt because it depends on the power of the loudspeaker and its

impedance.

4.2 Aack me

It is the me the limiter takes to get 70% of the total required reducon aer exceeding the threshold

level. In applicaons where we want to avoid speaker damage, the longer the aack me, the higher

the risk of damaging the equipment. However, sengs with a too fast aack me can generate

distorons or a deep modicaon to the transient of the signal, especially with percussive sounds,

resulng in poor perceived sound quality.

Typically, a good compromise is to set an aack me no longer than the lowest frequency you need

to protect (i.e. 1 ms for an HF driver with X-over at 1kHz octave band).

Figure 4 – Limiter acng on an audio signal

4.3 Release me

It is the me that the gain takes to go from the maximum reducon to 30% of reducon. In general,

the release me, has to be adequate to avoid a pumping eect and protect the speaker. The release

me can be set between 1 to 32 mes the aack me.

4.4 So knee

A so knee slowly increases the compression rao as the level increases, this way the changing from

un-compressed to compressed sound is less audible. This value in Powerso limiters is expressed in

dB below the threshold.

DO000274.00 REV.00

5. How Powerso limiters work

5.1 RMS Limiter

The RMS limiter is intended to prevent burning the driver’s voice coil, while at the same me

exploing their maximum performance. The RMS limiter should not be engaged at normal working

levels since a common musical signal has very high peaks, but a rather small average level.

5.2 Peak limiter

The peak limiter avoids potenally dangerous displacement of the cone (larger excursion with respect

to the allowed values), and it is based only on the quanty of signal that is sent through that channel.

Peak limiters, as well as RMS and Clip limiters, are independent from the number of parallel wired

speakers: you can use the power and impedance of a single speaker or the power and impedance of

the parallel circuit, the given result will be the same.

Pushing into a peak limiter can reduce the crest factor, thus “squashing” the dynamics of the original

signal. For most applicaons, 3 to 9 dB of gain reducon is usually suggested.

5.3 Clip limiter

The Powerso clip limiter is a “brickwall” limiter that blocks the driving signal, not allowing it to exceed

the threshold. This limiter has a xed aack me of 0.3 ms with a lookahead of 0.3 ms that allows to

ancipate peaks before they occur and therefore with the benet of less distoron by not having a

hard clip. Clip limiters are very fast and are the last safety tool that can be used to avoid loudspeaker

damage. To achieve the best result in quality and performance, it is suggested a gain reducon

between 0 and 3 dB and to engage also Peak and Rms limiters.

Figure 5 – Limiters (basic) in ArmonìaPlus.

DO000274.00 REV.00

6. How to calculate limiter values

To calculate Voltage thresholds, aack, and release me for the three basic limiters available in

ArmonìaPlus, Powerso has developed a tool called the Power Sharing Tool.

In this tool there is a dedicated secon called “Suggested Limiters threshold” that can be used as a

safety starng point while working with limiters.

Simply add the loudspeaker’s AES power, the nominal impedance and select its prole (or the way of

the loudspeaker). Once these values are specied the tool will calculate and suggest a possible limiters

conguraon.

The tool provides calculaons for both Low-Z loudspeakers and Hi-Z lines (100V, 70V and 25V)

Remember that this is only the starng point, especially for a mul-way loudspeaker where it is always

suggested to analyse the behaviour of the ways with a musical or pseudo-musical signal.

In this way, tonality unbalances due to wide dierences in gain reducon between the transducers

can be avoided.

Figure 5 – Powerso Power Sharing tool limiters secon.

DO000274.00 REV.00

7. Conclusions

Aer this basic analysis of Powerso limiters we can focus on two key points: the rst one is that all

Powerso limiters are made to protect the loudspeaker, so it is best pracce to have them always

enabled.

The second one concerns the sound quality and good system design: the best performances in audio

quality are achieved when, as menoned previously, RMS limiters are able to do the heaviest part of

the job in terms of gain reducon, then Peak limiters to control transients and Clip limiters as a nal

safety point.

By following this simple workow, you will achieve the best audio quality from your system, ensuring

its long lifespan.

DO000274.00 REV.00

Document Title: LIMITERS: BASIC CONCEPTS AND OPERATIONS

Reference: DO000274.00 REV.00

Powerso S.p.A

Via E. Con, 5 - Scandicci (Fi) 50018 - Italy

TELEPHONE: +39 055 7350230

General Enquires: info@powerso.it

Sales: sales@powerso.it

Applicaon & Technical Support: support.audio@powerso.com

Service & Maintenance: service@powerso.it

www.powerso.com

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