Cómo establecer un limitador para Subwoofers

Los limitadores están destinados a proteger tus altavoces de que se dañen por exceso de potencia o por saturar el amplificador. Lo siguiente es un método para configurar un limitador que debería evitar que los altavoces se dañen, en la mayoría de las circunstancias.

Primero, busca la potencia nominal y la impedancia de tus altavoces. Para este ejemplo, usaré 1000 watts a 4 ohms. Usando una calculadora de Ley de Ohms (que se puede encontrar en línea o descargar como una aplicación) puedes determinar que 1000 W a 4 ohmios requiere 63,25 voltios. Este es el límite de aguante de potencia de tus altavoces en voltios.

Necesitarás el siguiente equipo:

Generador de señal o reproductor de audio con tonos de prueba en CD o MP3
Tu mezcladora/preamplificador
Crossover
Limitador
Amplificador
Voltímetro

El generador de señal puede ser un reproductor de CD o MP3 que utilice un tono de onda sinusoidal pregrabado o puede ser un dispositivo que genera onda sinusoidal. Hay muchas opciones de cualquiera de las dos en internet.

El limitador debe tener un control de umbral, un control de tiempo de ataque, un control de tiempo de relajación y un control de salida.

En este punto, el amplificador técnicamente no está cargado. Cuando conectas la carga de un altavoz, lo más probable es que el amplificador se sature un poco antes de lo que lo haría cuando solo está conectado a un voltímetro. Puede ser necesario reducir la salida del limitador entre 2 y 3 dB para tener en cuenta la corriente adicional requerida para utilizar los altavoces reales. En general, cuanto mejor sea la calidad del amplificador, menos habrá que reducir el nivel.

Si la lectura aún está por encima de tu límite, baja el umbral hasta que la lectura de voltaje permanezca en el límite predeterminado. No tienes que preocuparte tanto por la saturacion si tienes más potencia de la que necesita. Tu limitador se utilizará para evitar el sobrecalentamiento de tus bobinas, en lugar de evitar la saturación de tu amplificador.

Para los subwoofers, el tiempo de ataque debe establecerse entre 4 ms y 10 ms. Los tiempos de ataque de 4 ms se deben usar para amplificadores que estén cerca o en sus límites cuando produzcan el voltaje requerido. Pueden usarse tiempos de ataque más largos cuando un amplificador puede entregar más que la potencia nominal de los altavoces. Esto te permite aprovechar el techo dinámico de potencia del amplificador sin que el calor sature los woofers y derrita las bobinas.

Para los limitadores de pico, se prefiere un tiempo de relajación rápida. Por lo general, el tiempo de relajación se establece como un múltiplo del tiempo de ataque, por lo que un tiempo de ataque de 4 ms requerirá un tiempo de liberación de 8 a 16 ms. En algunos casos, el tiempo de relajación simplemente puede establecerse como 2 x o 4 x veces el tiempo de ataque para obtener el mismo resultado.

Algunos PDS también ofrecen la opción de una velocidad de liberación en dB/s o decibelios sobre segundo. De nuevo, para un limitador de pico, una tiempo rápido es mejor. 100 dB/s es una velocidad de relajación más rápida que 25 dB/s.

Algunas reglas de pulgar:

Una configuración de umbral baja con una configuración de salida alta producirá un sonido más grueso pero puede carecer de impacto. Esto suele ser menos notorio y más útil para la música pregrabada y ayuda a aumentar el nivel promedio de bajos cuando no hay suficiente de este para seguir el ritmo de los tops disponibles.

Una configuración de umbral alta y una configuración de salida baja producirán un sonido más potente y, en general, es mejor para la música en vivo, donde los graves no son tan continuos.

La música en vivo requiere más poder dinámico porque las señales no están tan comprimidas como en la música masterizada y pregrabada. La música pregrabada no requiere tanta potencia dinámica, pero tiende a tener un ciclo de trabajo más alto, lo que significa que la potencia promedio que va hacia los altavoces es mayor durante el mismo período de tiempo. La música en vivo tiende a dañar cosas por saturación de amplificadores y transitorios dinámicos enormes, mientras que la música pregrabada tiende a dañar cosas por saturación de calor. Construir un sistema que pueda manejar cualquiera de estos trabajos requiere el conocimiento de las demandas de ambos, así como la implementación de tecnologías que puedan satisfacer dichas demandas mientras se protege el sistema de los peligros.

Los DJ tienden a dañar los woofers de los sistemas en vivo. Si tienes un sistema para música en vivo que tiene mucha potencia dinámica, y no es un sistema BASSBOSS, es probable que tengas más potencia de la que sus woofers pueden tomar a largo plazo. Para sonido de DJ o de música pregrabada, se sugiere que ajuste sus limitadores a un umbral inferior. El DJ puede terminar montando los limitadores pero tus woofers tendrán una mejor experiencia. Puedes reducir el umbral dramáticamente y compensar un poco con la ganancia de salida. Esto dará la ilusión de más graves mientras se espera que evite problemas a los altavoces. También puedes reducir el nivel de salida de los tops en el crossover en un porcentaje significativo del corte que realizaste en el umbral del subwoofer. Las grabaciones masterizadas son mucho más densas que la música en vivo, por lo que el nivel percibido no será tan diferente.

La música en vivo tiende a volar los altos en los sistemas para DJ. Si planeas usar un sistema para DJ o club para música en vivo, probablemente encontrarás los amplificadores saturándose en los picos porque la música en vivo es más dinámica. Además de comprar amplificadores más potentes, no hay mucho que puedas hacer aparte de reducir los niveles o los umbrales del limitador. Bajar el nivel general mantendrá la calidad del sonido y mantendrá los altavoces funcionando pero puede que no mantenga a todos contentos. Bajar los umbrales del limitador sacrificará un poco la calidad del sonido, pero ese es un precio menor por pagar que tener tweeters o medios quemados.

28 Comentarios

  1. Sam Alonge

    David, Is it preferred to have release times be the same for Low, Mid, and Hi , limiter output settings ? SAM / S.A. Concert Sound

    • morethanbass

      Hi Sam,

      The short answer is no. The release times should be appropriate to the frequency bands. You’re better off with shorter attack and release times for higher frequencies. This helps make the limiting action less perceptible.

  2. Mando

    Hello, i used your guide and is very useful. My required voltage was about 63 volts, and my amp deliver about 70 volts (clipping amp), i lowered the threshold until my reading was about 59 volts (no more clipping) I used 50hz test tone. But sometimes when i play music (recorded music, iTunes music) the amp clipping starts. Should i pay attention to this? i measured the voltage with speakers connected and the volts don’t go over my speaker limit (63 volts) they stay around 30-40. Why my am is still clipping?

    Thanks!

    • morethanbass

      Hi Mando,

      The amp may be clipping because the transient peaks are getting through the limiter before it clamps down. In other words, because the attack time on the limiter is set long enough to allow brief peaks that will trigger the clip light on your amp to come on before the limiter reduces the level. If the sound quality is not compromised then you can allow this to happen.

      In many cases, amplifier manufacturers will have the clip light illuminate for longer than the time the amp is clipping in order to allow you a better chance to see the light and do something about it. This helps you to avoid blowing tweeters but is less useful for protecting woofers. To put it another way, the attack time on your full-range amp’s clip light is shorter than the attack time on your set-for-subwoofers limiter.

      If the limiter is doing its job, the clipping will be very brief and the woofers will not be at risk. You can try reducing the attack time on the limiter but, depending on the limiter, the result may sound worse than a tiny bit of clipping. A 10ms attack time will provide a more dynamic sound than a 5ms attack time but the 10ms attack time may allow the transients to briefly clip the amp and trigger the clip lights even though your woofers are very likely able to survive a 10ms peak. Ultimately your woofers can probably live with the clip light coming on occasionally and you can think of it as an indicator that you shouldn’t turn up any more once you start seeing it. Of course, if you hear audible distortion, turn the levels down.

      • Mando

        Glad to hear this!

        My woofer is working now on 44hz – 200hz (settings from crossover) i set limiter attack to be on 16ms, hold on 100 and release on 260ms. Do you think should i change this values to a betters ones? my sub is 15″ 500w (RMS) 8ohm / 2000w (PEAK).

        My amp is 590w 8ohm.

        Thank you for helping me out!

        • morethanbass

          You might try setting the attack time at 10ms, the hold to 50ms or less and the release to something very near the attack time. I want the limiter to «let go» just a bit slower than it «grabbed on» so between 10 and 20ms would be about right. If your limiter specifies a release time in milliseconds then it should be much shorter than 260.

          When a limiter has a «hold» setting then the release setting is often a speed setting, not a time setting. The release may be specified in dB/s which means Decibels per Second, which is a way of describing release speed, or how fast the limiter lets go of the signal and allows it to rise back to full level. If that’s the case with your limiter, a higher number would be faster.

          With long hold and release times you kill dynamics, or worse, you hear the limiter «breathing» because its action is out of time with the dynamics of the music.

  3. Mando

    My limiter is in milliseconds.
    Those are the specs

    Threshold: –20 … +15 dB
    Attack time: 1 … 100 ms
    Holding time: 0 … 100 ms
    Decay time: 10 … 1000 ms

    • morethanbass

      Limiter settings are based on time, amplitude and speed of action. All frequencies relate to time. Higher frequencies have shorter rise times and require faster attack times. Lower frequencies require longer attack times because if your attack time is too short, the lower frequency doesn’t have time to get through before the limiter reduces the level, so the limiter ends up squashing the low-end out of your system. It’s a balancing game.

      44Hz requires just about 23ms for a full wavelength cycle to get through the limiter. 200Hz, the other end of your operating range, requires only 5ms for a full cycle to get through the limiter. The 200Hz may clip your amp but the 44Hz will stress your woofers more. 10ms is nicely between the two so that’s about where I’d put the attack time.
      Hold time is something I like to keep to a minimum for a limiter. Since you can go as low as 1ms, I’d try that first.
      Decay time is how fast the limiter allows the signal to return to an un-attenuated state. I’d go with 10 to 20ms. If the limiter holds too long or takes a long time to release then the signal may still be reduced when it’s un-necessary so the limiter will not be tracking and reacting to the dynamics of the music.

      You want a limiter to attenuate the signal just enough to prevent audible clipping and damaging drivers but you don’t want it reducing the levels when those peak signals aren’t present because then you miss the details in the signals that aren’t the biggest ones.

  4. Mando

    Hi again,

    Another question, my amp can deliver about 910w in 4 ohms (per channel), and i have 2 pairs of subs, each pair has 2x500w rms (1000w total in 4ohms per pair).

    I know my amp is underpowered for each of my pair, but i can use it if i use the limiter correctly and i don’t put my amp in clip? can i drive my subs safe?

    What is your advice for protecting my amp and also my subs.

    Big thanks!

    • morethanbass

      Yes, you can run your subs absolutely safely if you follow the instructions above. Set the limiter so the amp doesn’t clip and you’l be fine.

      The reason under-powering is considered dangerous is because most of the time people will tend to clip their amps trying to get greater output and the clipping tends to burn voice coils. The only consequence of under-powering your subs, provided you are not clipping your amp, is that they will not reach their absolute maximum output. In your case you’re only slightly under-powered so you’ll still be able to get very nearly as loud as they can go.

      And your amp will be fine, too. Almost any amp on the market is capable of running at 4-ohms per channel so it doesn’t sound like you have anything to worry about there. Your amp is not at risk because it can’t produce as much power as your speakers can handle. Clipping the amp is likely to damage speakers but not likely to damage the amp. What damages amps are things like shorted speaker wires, too many speakers on a channel, voltage spikes and sags or overheating. If you avoid those, your amp should be fine.

  5. Adrian Prado

    Hi, I have an amp and it is 800w a 8ohms and a have a pair of Wharfedale Delta 15 speakers (500w RMS each one) here is a link with the description of it.

    My digital processor is a STS 214b and it has a limiter. Now, how do you think do I have to configurate it, I mean the Threshold, Attack, Hold and Release. Thanks for your time.

    • morethanbass

      Hi Adrian,

      If you go through the steps for setting the limiter outlined above you will have most of the work done. Because these are full-range speakers, not subwoofers, you will need to configure a little differently. When limiting a full-range signal, the limiter will respond to the highest signal peak, which is often the bass, and so it may cause the highs to be attenuated when the bass hits.

      Because of this you will want to minimize the engagement of the limiter, so keep the threshold a little higher and reduce the gain to prevent clipping. The idea is to use the limiter to catch the peaks and prevent clipping but have it engage as little as possible so as to retain as much of the dynamics as possible.

      Try setting your threshold at 0dB and output at 0dB with 4ms attack, 10ms hold and 10ms release. When setting the limiter according to the instructions outlined above, if the amp exceeds your voltage limit or clips with these settings then reduce the output to -1dB followed by reducing the threshold to -1dB. Continue to reduce both the output and threshold equal amounts until your amp isn’t clipping or exceeding your voltage limit. (You may have to go above 0dB with some amps but keep the two levels equal.) After that, try listening to music at high levels to evaluate whether the limiter is affecting the audio quality. Ultimately the limiter should improve sound quality at high levels by preventing overdrive, distortion and potential damage. If it prevents your speakers from getting quite as loud as they would without the limiter, but doesn’t sound bad, then it’s doing its job.

      • Adrian Prado

        Thank you very much for your advice, I’ll put it into practice and I will be commenting later.

  6. Adrian Prado

    Oh man I forgot some, in the limiter, the attack button is expressed in db/s…

  7. morethanbass

    When using dB/s, the larger the number of dB/s, the faster the attack time. To convert to ms, take 3250 and divide by the time in ms. Thus 10ms attack time ~= 325dB/s. 4ms attack time ~= 800dB/s

  8. etone

    Great article I set my pa up today to set my limiters but ran in to 2 problems the first one I had to set my limiters to their lowest setting -24db! but the amplifier is still putting out WAY to much power before the limiter even gets enough signal to limit, I wanted to have my amplifiers set full up (so no one can go and turn them up more) and let the limiters do the work, So this leaves me 2 choices reduce the input gain within the dsp unit or reduce the output gain in the dsp unit or both (wont this effect my dynamic range? Also before I go putting my multimeter in to my amp unloaded is this safe for class D and all over power amplifiers ? I was always told never to run a power amp flat out with out a load. Thank you

    • etone

      I should also mention my inputs to the dsp (signal coming in) was no where near 0 so lowering the input gain would actually stop the limiters getting enough signal even more

      • morethanbass

        Hi etone,

        What kind of DSP do you have? Knowing that I might be able to offer more detailed advice. If your input is at unity, threshold is at -24, your outputs are at unity, and you still have too much signal, the limiter function may be bypassed or the attack time may be too long.

        What you were told about running amps without a load probably dates back to earlier days of audio when all amps had tubes. As a rule, tube amps should not be run into infinite impedance, aka an open load. I’ve never encountered a solid state amp that couldn’t be run into an open load. They might exist but I wouldn’t want to own one. An amp like that would self-destruct if a speaker came unplugged or blew up.

        The one word of caution I would offer is, don’t touch the terminals or bare wires of an amp that is running flat out because it will shock you. Bigger amps will give you a bigger shock, so be careful. Don’t hold the wires onto your voltmeter probes with your fingers! Put the probes into the terminals or use alligator clips or something like them to make the connection solid.

        Regarding the limiter action and dynamic range, I understand your concern. In reality your overall dynamic range is limited by the weakest component in the signal chain. If your amp clips before your speakers distort, your amp is the weakest link. If your speakers distort before your amp clips, the speakers are the weak link. A limiter is deployed to deliberately reduce the dynamic range of the signal in order to keep the signal within the dynamic range of all the components downstream of the limiter. One might say that the term «limiter» is an abbreviation for the term «dynamic range limiter».

        That said, you still want to keep the signal gain structure operating with the greatest dynamic range possible within those limits. Raising input gain of the DSP will boost signal AND noise coming in from the device preceding the limiter. (In the case of DSPs, that’s usually the mixer, and everything putting unwanted noise into it.) Lowering input gain on the DSP will demand more output from the mixer to reach the same level but also lowers the incoming noise level. Provided the output of the mixer isn’t clipping or distorting when you achieve the necessary level, lowering the input gain of the DSP is appropriate.

        I understand your desire to run your amp with the taps wide open so that no-one can turn the system up beyond the limit. Unfortunately running the amp with the input attenuators wide open has the same effect as running the DSP inputs wide open. It amplifies all incoming signal AND all noise. Setting your system up this way is safer if unauthorized hands can get at the knobs but because of what you have to do upstream to keep the levels down, this method does sacrifice a little of your noise floor and consequently a little of the dynamic range.

        Most fixed-architecture DSPs have the crossover stage directly preceding the limiter in the signal flow. The output gain of the crossover is the level that drives into the limiter threshold. Raising and lowering this gain, which is within the digital domain of the DSP, can usually be done without sacrificing sound quality or dynamic range. Some DSPs are better than others in this regard. Some floating point DSPs have virtually no limits on their dynamic range within the digital domain.

        It’s a combination of the limiter threshold and the limiter output gain that determine the maximum signal being sent to the amplifier. Lowering the output gain and keeping the threshold higher preserves more of the dynamic range. Lowering the threshold and leaving the output gain higher compromises the dynamic range in order to achieve a higher signal density and a higher average sound level… Both methods also prevent distortion and damage as much as possible.

        Enjoy!

        • etone

          Hi morethanbass. Thank you very much for your detailed reply.

          I have a Behringer dcx2496 dsp. (3 input model)

          I have also realised one of my mixes I use with this setup has aes/ebu output so I might also give that a try at a later date.

          Since reading your info I have done some experimenting, It seems the limiter section of this dsp is the last part of processing in this unit SO If I turn the output gain down on the dsp the limiter «sees» less signal and limits less, This same with the input gain. Weird I would have thought the output gain was a final thing (like an attenuator) but its not, Its all starting to make sense now.

          I have managed to fix it up pretty good now and my amplifier is not clipping at full on regardless how much signal is put in to the dsp even If someone comes along and pushes the desk in to clipping.

          Thanks again for all your help 🙂

    • morethanbass

      Hi There,

      I don’t have specific settings to recommend but as far as suggestions go I would try to get the 15″ in the KW153 to align with the 18″ in the KW181 so they can both contribute to the low end of your system. If the KW153 and KW181 were set up to work together by QSC then you should not need any additional processing. The KW181 is specified to have response to 38Hz and the KW153 to 35Hz. From the specs it appears they have similar high-pass filters so I would suggest using them both without additional crossover filters in the dbx. This would give you the best low-end response from the combination. Adding a KW181 to a KW153 in this way should give you a flat response to 125 Hz with a 6dB boost below that. More than 6dB if the gain on the sub is set higher than the top.

      What I’d do with the dbx is put a parametric filter on the inputs at about 40 to 45Hz with a Q of 3 or 4 and boost that by a few more dB. Then I’d set the output limiters for the two boxes. You can set the limiter to fast attack, fast release and threshold to where the speakers don’t clip. (Don’t use overeasy or soft knee.) To this point the signal path should be the same for both the top and sub except perhaps for the limiter settings. If you then wanted to EQ the tops you could do that with their channels’ output EQs.

      Conventional wisdom would suggest rolling the bass out of the tops in order to allow them to get louder but since these tops are specified to go lower than those subs it seems to me you’d be giving up on a large part of why you have such big tops. You will be giving up a bit of level from the tops but the limit is the limit and if it sounds much better at a slightly lower level, in my opinion, it would be worth it. It’s the best thing I can think of short of adding more or better subs, which is always a good idea!

      Let me know how it works out..
      Best,
      David Lee

  9. Kabir

    Hi David,

    Rest post and very helpful! I think I’ve got it but before I start I’ve got a couple of questions…

    My amps are the Crown Macrotech amps and don’t have an obvious clip light on them. I think the ODEP lights start to dim on them when they are clipping but I could be wrong. If I know the voltage my speakers can handle should I follow all the instructions above and reduce the threshold on my DSP limiter until the voltage required is met on the multimeter?

    By using this method I’m assuming the speakers will be protected if I set the voltage to the correct level by reducing the threshold on my limiter but how do I know my amp is not clipping?

    Lastly I’m using my amps for pre-recorded music so should I set my gain to maximum and threshold to around +3db (this is because my mixer shows a yellow light for +3db). Then slowly reduce the gain level until the multimeter gives the correct reading?

    Regards
    Kabir

    • Kabir

      Just a quick update…

      I meant to say Great Post. Autocorrect :/

      I’ve also been in touch with Crown regarding clipping lights on the Macro-Tech amps and they have told me the following:
      «The signal/IOC lights are the indicators. The green lights will react with input signal. If there is significant distortion or clipping, the lights will go a solid bright green.»

      • morethanbass

        Hi Kabir,

        Sorry I couldn’t get back to you sooner but I would have told you the same thing about the crown clip lights. If you use a sine wave signal you can see when the green LEDs go bright and that will tell you the amp is clipping. Since you’ll be doing this without a speaker connected it’s OK to turn the signal up until the lights go bright so you know what that looks like.

        Follow the procedure for setting the limiter and watch for the signal lights to go bright. If they don’t go bright, you’re not clipping.

        You can correlate the mixer’s lights to the level of the amp but it isn’t necessarily numerically correlated. In other words +3dB on the limiter threshold isn’t automatically the number to choose because it’s the point that the LEDs go yellow on the mixer. I would estimate that a much lower threshold may be necessary. The + (plus) range of a mixer is plus relative to the input level. Amps are different from mixers in that they don’t really have a + (plus) range. Everything below clipping is – (minus) and clipping is the + range.

        The way to correlate the mixer indicator to the amp’s output is to put the test signal through the mixer and adjust it so that when the red LEDs come on on the mixer, the amp’s clip indicators come on. First set the test signal output from the mixer so it’s lighting the first red output LED. Then set the threshold and gain to prevent clipping, (as described above.) With the signal on the mixer still showing the red LED, raise the output gain of the limiter until the amp clips, then back the output of the limiter down slightly so the clipping stops. This will tell you that when the red lights are showing on your mixer that you’re pushing the limiter to the very edge. Then, if you keep the mixer out of the red you’ll be keeping your amp out of trouble.

        Best,
        David Lee

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