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How you can tell a servo sub from a slug

Paul's Posts — 06 April 2012

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How you can tell a servo sub from a slug

There are not many subwoofers out there that have servos running them and that’s a shame.

A servo is like a little microphone placed right on the woofer that listens to what the woofer is doing, compares that with what it should be doing and then makes corrections to get it perfect.  Woofers are one of the few loudspeaker categories where you can employ realtime correction to get the output perfect.  They are difficult to design and most subwoofer manufacturers don’t bother.

Servos improve transient response, reduce or eliminate woofer overhang, distortion and flatten frequency response dramatically.  Every subwoofer design I have ever been involved in was servo based and I would never use a subwoofer in my listening room that wasn’t – especially since I need to integrate the bass with my very quick planar loudspeakers.

I am always amused when people tell me they can’t integrate a subwoofer in their system because they always “hear” their subs as separate from their main systems.  Of course that’s true if you’re not using a servo sub!  A conventional subwoofer is like a slow moving slug.

How can you tell?  Simply go to your woofer cone and tap on it.  In a servo system the woofer will be as tight as a drum and resist your tapping – in a non-servo sub it’ll “thud” and give way to your tapping.  When you play the two subs, you get exactly the same results: one is lightning quick with transients, the other “wobbly” and “tubby”.

Your system needs a subwoofer but not if you’re going to add a slug.  Add a race car instead.

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About Author

Paul McGowan

Paul McGowan is the CEO and co-founder of PS Audio Inc. a Boulder Colorado design and manufacturing company of high-end audio products and services. McGowan has been designing and building high-end products for nearly 40 years. Hobbies include skiing, music, hiking, artisan bread baking, kick boxing and cooking. He lives in Boulder Colorado with his wife Terri and his 4 sons.

(6) Readers Comments

  1. hahax

    April 6, 2012 at 6:45 am

    I usually associate fast with quick reaction time, fast starts. But I’ve always thought that bass, by its very nature of being low frequencies, as being slow. So what is a fast woofer? It’s actually one that stops fast rather than starts fast. It’s one that is tightly damped with a low Q factor. It’s kind of like a Formula 1 car. The acceleration is impressive but it’s the braking that is awesome to the 1st time driver. And unfortunately for sales to most people tight damping sounds like there’s bass missing compared to poorly damped woofers and they don’t buy proper damping with the ‘Missing’ bass.

    There’s multiple ways to get good damping. And servo is one of them. But so is plain good design with woofer characteristics and box design properly done. Another one is a big woofer in a small box run below resonance with equalization to give a tight transfer function. They all work well. But can you sell them?

    • April 6, 2012 at 8:21 am

      You’re correct – mostly. The ability to actually use the amplifier’s power to stop the woofer’s momentum and pull it back actively can only be accomplished with a servo – because otherwise there’s no way to know when it’s going to happen.

      • hahax

        April 6, 2012 at 6:50 pm

        I would argue that a passive, closed box woofer with a Q = 0.5 stops awfully quickly with essentially no overhang. And a closed box woofer run below box resonance and equalized to get bass response stops on a dime; that format doesn’t want to move so when the amp stops pushing the driver just stops. But I do agree with you that a well designed servo woofer does just what you say it does.

        I live with an 18″ woofer passively crossed about 85 hz(the inductance of the voice coil is so high it doesn’t want to go any higher) in a box with a Q about 0.57(minimal time delay) with a heavily stuffed port(like a Dynaco A25) and it stops very quickly also. Sometimes you wonder why it makes so little bass and then real bass comes along and you jump out of your seat. I still do occaissionally and I’ve lived with it for years

  2. Bassman23

    April 6, 2012 at 7:47 am

    Bass being slow? Not if it’s a tympani – or Victor Wooten!
    What I like about this post is that there are many aspects of music reproduction that the average enthusiast is not necessarily aware of. When it comes to loudspeaker design, the goal is to pressurize the room in an identical manner as though the original musical event were there. This, of course, is impossible. What is amazing is how much of an illusion of this can be created.
    Explaining different parts of the engineering challenge, and highlighting current solutions, is a very good service. Thanks, Paul.

  3. Adolf

    April 6, 2012 at 12:05 pm

    The best servo system will faul, if the sub has been positioned wrongly. Change of sub position will suppress risk of locating it. Furthermore a powerfull sub with a big membrane and a doubled coil are nice attributes of a quick dry bass. Bass is area. The bigger, the better.
    I prefer a 400 Watt Amp. in the sub, who drives a double coiled Stageline chassis with 38cm. The housing is a 4cm multiplex box. There is no resonance in the box and the sub start at 70 Hz and goes down to 15Hz.
    The sub supports my Dynaudio confidence 5 speakers.

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