negative feedback...what's the big deal?

Discussion in 'Archived Threads 2001-2004' started by John Doran, Feb 24, 2002.

  1. John Doran

    John Doran Screenwriter

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    why do the people who think badly of amplifiers that use (global) negative feedback, think badly of it?

    how audible are the effects, and what ARE the effects?

    can any of the amplification engineering types around here shed a little light on this?

    thanks.
     
  2. John Doran

    John Doran Screenwriter

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    .....c'mon, guys.
    someone's got to know something about this.
    please?
     
  3. Jagan Seshadri

    Jagan Seshadri Supporting Actor

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    John,
    Check out this link. It mentions Transient Intermodulation Distortion. The faster the components used (i.e. the greater the slew rate), the less of this there is.
    Part of the reason for using negative feedback (I like to think of it as corrective feedback) is that is greatly reduces the electromechanical loading effects of the loudspeaker on the amplifier circuit (i.e. increases the damping factor). This makes it easier to have a nice flat (linear) frequency response over the audible band.
    To me, negative feedback and linear amplification is the way to go (as in low THD measurements). But some prefer the sound of even-order harmonics that are produced by tubes, and the rolled-off high end (and further non-linear distortion) that results from not using negative feedback.
    Sorry for not being more detailed (it's quite late as I'm writing this), but read the above link for more detail, or do a search online for transient intermodulation distortion.
    Addendum: This is the link I remember reading a few months back. I think it more clearly explains the drawbacks of negative feedback.
    As for T.I.M. distortion being audible, I have a hunch that the speaker drivers themselves would filter out such troubles. But that's just a hunch. If anyone else would like to weigh in on this, be my guest [​IMG].
    -JNS
     
  4. John Doran

    John Doran Screenwriter

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    thanks, jagan.

    would anyone else care to throw their hat into the ring on this?

    someone? anyone?
     
  5. John Doran

    John Doran Screenwriter

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    ok. for real - 187 looks and no answers. that's not right...
    i know there are people around here who both know about this and have an opinion about it...
     
  6. John Kotches

    John Kotches Cinematographer

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    John,

    The catch with negative feedback is it has to be utilized properly. All too many people feedback globally in large doses. While it does reduce the measured THD, it exacts a price. That price is that the amount of odd-order harmonic distortion, which our ears perceive as edgy, is increased. Look at some of the receiver specs (0.007% THD) and you'll realize this is acheived with heavy global feedback. It does look great on paper though, doesn't it?

    Many of the great amplifier designs have used small amounts of local feedback to improve performance. The key is in the intelligent application of that feedback.

    Jagan's thoughts are fairly accurate in that it does help remove distortion (ie as a corrective aid) but like any other adjustment must be applied carefully.

    Regards,
     
  7. John Doran

    John Doran Screenwriter

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    john,

    man. you're everywhere today...

    i, too, have read that global feedback is the real enemy.

    but, if that's true, do amplifier designers advertise that they use it? assuming they don't, how does one find out if they do?

    i suppose there's the argument that if you can't hear a difference when you audition the piece, then what difference does it make?

    however, if i'm not mistaken, the sonic detriments of (global) negative feedback (often) take effect only after prolonged exposure to them, perhaps as a general dissatisfaction with the sound. or something like that.

    if that's true, i'd like to avoid coming to the conclusion, after 6 months or so, that i'm just not satisfied with my expensive amps because the odd-ordered harmonics caused by their use of global negative feedback makes everything ever-so-subtly off.

    anyway. i'd like to get a lot of these issues sorted out before i drop a lot of cash on what i hope will be my reference system for the next 10 or 20 years.

    thanks...
     
  8. Kevin C Brown

    Kevin C Brown Producer

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    John- The only 2 cents that I can throw in are this: I read a review once of an American made amp. The reviewer commented that it didn't have much negative feedback, unlike a lot of Japanese amps at that time (this is mid-90's). He went on to say that the reason why negative feedback was used, is that amps made like that give really good measurement numbers. But he said they didn't sound as good as amps without it.

    But I can't remember the review or product.

    Probably like anything though, good amps can be made with negative feedback, but maybe it's just harder to design them.
     

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