• 01-07-2011, 05:26 PM
    Luvin Da Blues
    Quote:

    Originally Posted by Feanor
    Maybe, that sort of servo-control is more often called "feedback". :14:

    To use your phrasing in post #11 "Maybe, but I think the more general term for such feedback control, (pretty much only used for subwoofers), is "servo-controlled"." :biggrin5:

    Are we talking in circles here?
  • 01-07-2011, 05:29 PM
    E-Stat
    Quote:

    Originally Posted by Ajani
    So why do we need a discussion of what the correct term is?

    Great question!

    rw
  • 01-07-2011, 06:17 PM
    Feanor
    Quote:

    Originally Posted by Luvin Da Blues
    To use your phrasing in post #11 "Maybe, but I think the more general term for such feedback control, (pretty much only used for subwoofers), is "servo-controlled"." :biggrin5:

    Are we talking in circles here?

    Well, yes, sort of. You likely know that a lot of amp circuits use "negative feedback" to correct what would otherwise be very high distortion; (large amounts of negative feedback are associated with classic solid state designs). However according to some schools of though, negative feedback, while correcting low-order harmonic distortion, creates unpleasant sounding high-order and intermodular distortion. Thus today many designs seek to reduce the amount of negative feedback; however few have eliminated it entirely (because some is good, even though too much is bad).

    So Marsh saying "servo-controlled" really sounds like a cute way to get around saying they use feedback.
  • 01-07-2011, 07:35 PM
    Mash
    No, Mr. Feanor- Totally unrelated!
    You are referring to an amplifiers internal topography. The negative feedback in an amplifier is intended to reduce AMPLIFIER output distortion. [Even at 'low levels' SS distortion is more obnoxious than tube distortion.]..

    I am referring to SPEAKER CONE MOTION ERROR, i.e. a cone motion that is not an exact analog of the preamp input.

    I have been referring to feedback that corrects the SPEAKER CONE's movement so that it more closely matches the motion "requested" by the "preamp input".

    1. You measure the speaker cone movement so as to produce an electrical analog describing that movement;
    2. You SUBTRACT the result of (1) from an equivalent level signal that represents the preamp input;
    3. Then you add that difference (2) to the preamp signal input to the speaker amp...

    Let us say the preamp signal calls for 1" forward cone movement

    But the cone only moves 0.9 inches forward

    Thus: preamp requested 1" of cone forward motion, minus cone actual forward motion of 0.9", equals 0.1" cone forward movement SHORTFALL

    So you ADD a signal representing an additional 0.1" of forward cone motion to the preamp requested cone forward movement input of 1.0" which implies 1.1" requested forward cone movement.

    1.1" requested movement will produce about 1" actual cone movement (actually about 0.99" here).

    The next (almost instantaneous) iteration will move the cone motion much closer to the result of producing 1" cone forward motion when the preamp input is requesting 1" of cone forward motion.

    This is an oversimplified explanation but it is intended to explain the referenced speaker cone feedback control which you can learn about in any number of engineering texts.
  • 01-08-2011, 04:46 AM
    Luvin Da Blues
    Quote:

    Originally Posted by Feanor
    Well, yes, sort of. You likely know that a lot of amp circuits use "negative feedback" to correct what would otherwise be very high distortion; (large amounts of negative feedback are associated with classic solid state designs). However according to some schools of though, negative feedback, while correcting low-order harmonic distortion, creates unpleasant sounding high-order and intermodular distortion. Thus today many designs seek to reduce the amount of negative feedback; however few have eliminated it entirely (because some is good, even though too much is bad).

    So Marsh saying "servo-controlled" really sounds like a cute way to get around saying they use feedback.

    Tru nuff dat, however, given Richard Marsh's reputation in the audio field, I have utmost confidence in his designs. This is one sweet sounding amp, especially when coupled with the p2000b pre.
  • 01-08-2011, 06:22 AM
    Feanor
    Quote:

    Originally Posted by Mash
    You are referring to an amplifiers internal topography. The negative feedback in an amplifier is intended to reduce AMPLIFIER output distortion. [Even at 'low levels' SS distortion is more obnoxious than tube distortion.]..

    I am referring to SPEAKER CONE MOTION ERROR, i.e. a cone motion that is not an exact analog of the preamp input.

    I have been referring to feedback that corrects the SPEAKER CONE's movement so that it more closely matches the motion "requested" by the "preamp input".

    ...

    Trust me, Mash, I understand the distinction.

    However amp and speaker cone feedback are similar in that downstream result is corrected by feeding back the misbehavior to upstream input.

    Speaker cone correction is really only used in case of subwoofers because only at low frequencies is the feedback quick enough to produce the desired result without creating more distortion than it tries to correct. The same applies to internal amplifier feedback: the high-order harmonic distortion is caused by the feedback loop not being in "quick enough". This is why many designs allow "local", (i.e. intra-stage), feedback because it's sufficiently quick, but not "gobal", (i.e. inter-stage and multi-stage), feedback because it's not.

    I really didn't want the discussion to become too serious. Basically I was just getting at the fact that "active" in case of speakers most often means, simply, self-powered, and not necessarily servo-, (or feedback-), controlled. At the same time I observed that "servo-controlled" is most commonly used for subwoofer feedback, though the Marsh amp illustrates that the term isn't reserved for the sort of feedback.
  • 01-08-2011, 11:59 AM
    Mash
    If this worked......
    http://www.mackie.com/products/hrmk2series/design.html

    On right side tabs, Select "Owners Manual: 824mk2"

    Download Adobe file & Open HR824MK2_OM[1].pdf

    Go to Page 18 and
    Look at Lower right corner of schematic (woofer)

    Checkout the "return" circuit.
    What do you think?