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Thread: servo sub?

  1. #1
    Forum Regular hmmmm's Avatar
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    Jul 2003

    servo sub?

    A newbie question. What is the difference between a "servo controlled sub" and any other sub? Thanks!

  2. #2
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    Feb 2002
    Quote Originally Posted by hmmmm
    A newbie question. What is the difference between a "servo controlled sub" and any other sub? Thanks!
    In a servo sub, a sensor, possibly an accelerometer produces an electrical signal which corresponds to the speaker cone motion being physically attached at some point to the moving voice coil or cone. This signal is combined out of phase with the input signal to a dedicated amplifier which drives the speaker. The amplifier input signal is therefore related to the difference between what the speaker is doing and what it is supposed to be doing. This is the principle of servo control which was developed around the 1940s and is one of the most important principles in electrical control theory. It is the same principle BTW as negative feedback in amplifiers. How well it works depends on how well the design is thought out and executed. Properly done, it will extend frequency response, reduce resonances, and reduce distortion. Poorly executed, it can result in greater resonances and in some extreme case, spontaneous oscillation. The equations are very complicated-senior level undergraduate and graduate level electrical engineering. It is not for beginners, wanabees, and amatures. Velodyne makes some good ones.

  3. #3
    Forum Regular hmmmm's Avatar
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    Jul 2003
    Thanks for the reply. My friend just purchased two of them and I was curious to know the difference. I didn't want to ask him.

  4. #4
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    Nov 2003
    A servo controlled sub has a feedback mechanism to lower distortion, perhaps involving a piezo-electrict sensor on the woofer cone. This has two basic effects:

    1) It lowers the distortion by electronic correction. But a few percent distortion is not that audible in the deep bass, and some assume it becomes really significant only if higher than 10% or so.

    2) It limits the maximum output to keep the distortion from rising beyond a certain level, be it 1% or 5-6%.Even servo mechanisms can only do so much! This does not mean that it won't play loud enough for you.

    Most powered subs have limiters to keep the driver within its excursion limits and prevent damage, but this tends to be at a level where distortion is fairly high. But anything has limits, and you have to determine what limits are acceptable for you.

    Servo subs should always sound clean, but a comparable non-servo design might have a higher maximum output. Servo subs tend to cost more.

    Many of us don't think servo feedback is necessary in a sub. My 15" non-servo Paradigm PW-2500 (no longer made) I have does me just fine. I am sure I would be very happy with a Paradigm Servo-15 or an 18" Velodyne, which have greater output capability, but then I could also be happy with non-servo subs like the big Hsu or SVS models.

    Whether this feature is important for you is something you will have to decide for yourself.
    "Opposition brings concord. Out of discord comes the fairest harmony."
    ------Heraclitus of Ephesis (fl. 504-500 BC), trans. Wheelwright.

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