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Loudspeaker protection via limiter

August 19th, 2011 Posted in OMNITRONIC, worth knowing Tags: ,

Omnitronic DXO Digitale Systemcontroller

We are frequently asked how to protect loudspeakers effectively via limiter. Most common loudspeaker controllers (like Omnitronic’s DXO-series) have an integrated limiter which is mostly not used or not used correctly.

In the following tutorial, we would like to show you how to adjust the limiter based on speaker-system and power amplifier.

For doing so, we need the following data:

– Speaker-system RMS-power and impedance

– Power amplifier voltage gain

As an example, we use several Omnitronic PAS-speaker-systems and the Omnitronic SMA-2000 power amplifier.

The popular Omnitronic PAS-212 has an RMS-power of 300 watts and an impedance of 8 ohms. By using the ohm’s law we can calculate which voltage is sent to the speaker-system at full output.

The formula is power P = Voltage U squared / Resistance R. By converting we get: Power P x Resistance R = Voltage U squared.

With values: 300 x 8 = 2400

The root from 2400 is approx. 49. For Omnitronic’s PAS-212 full output, we need an effective voltage of 49 volts. Now we convert this voltage into the voltage level dBu in order to have logarithm with the reference to 0.775 volts. For doing so, we use a dB-calculator like this one available on the internet.

Enter 49 volts into the field effective voltage (Volt eff) we receive a result of 36 dBU voltage level. From these 36 dBu we deduct the power amplifier’s voltage gain. The Omnitronic SMA-2000 is switchable between 26 and 32 dB. Depending on the setting, we get a value of 10 dBu (at 26 dB) or 4 dBu (at 32 dB), which we can use as threshold for our limiter.

In short, two more calculations:

Omnitronic PAS-215

Power: 400 watts RMS at 8 ohms

The voltage level of 57 volts (full power, P = U squared / R) corresponds with 37 dBu

Omnitronic SMA-2000 voltage gain switchable between 26 and 32 dB

Limiter threshold: 11 dBu or 5 dBu

Omnitronic PAS-18

800 watts RMS at 8 ohms

Voltage level 80 volts corresponds with 40 dBu

Limiter threshold: 14 dBu or 8 dBu

When using two pairs of speaker-systems per channel, respectively double power at half impedance, the values do not need to be adjusted. In this way, you can use the same settings for one or two speaker-systems per channel. You must consider not to exceed the maximum power of the amplifier and not to fall below the amplifier’s minimum impedance.

But there’s more to it than that.

You also have to set the limiter’s values for Attack, Hold and Decay. When a limiter starts working, i. e. the sound system is in the limit, the limiter also has a noticible effect on the signal. This is why a compromise of good sound – also at the limit – and loudspeaker protection must be found.

We recommend Attack values between 5 and 15 ms. Smaller values will increase the protection, but will cut every peak making the sound “undynamic”.
The bass should have higher Attack time as mids and highs because of the bass frequencies’ wavelength. The bass is more likely to be distorted by a limiter.

The values for Hold and/or Decay should be higher so that the limiter will not “pump” with every bass beat.
We recommend values for Hold and Decay summing up to approx. 500 ms, e. g. 200 ms Hold and 300 ms Decay or 250 ms each.

We recommend fine tuning by listening to the speaker-system with well-known music.

It is especially important that there is nothing between limiter and power amplifier influencing the output level.

Equalizers or other effectors must be installed before the limiter!

If the system should be protected against manipulation, you should use the limiter function of a digital controller, because this cannot be influenced by rotary controls or faders. Furthermore the settings can be password protected.

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