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How well do amps control speakers?

Discussion in 'Amps and Cabs [BG]' started by Grooveman1961, Oct 2, 2009.


  1. Grooveman1961

    Grooveman1961

    May 8, 2006
    In another thread, a poster made mention of an amp controlling the speaker.

    What are the factors that contribute to better cone control? The speaker provides no direct feedback to the amp regarding its response to the output from the amp, but there are indirect influences. I know one parameter is damping factor.
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Damping factor is almost irrelevant, as any good quality amp will have adequate damping factor, and there's no particular benefit to a very high figure. For that matter 'controlling the speaker' is also a minor concern. Any amp that didn't adequately do so would sound so bad you couldn't consider using it.
     
  3. Grooveman1961

    Grooveman1961

    May 8, 2006
    Well, that's basically what I think.

    The driver and cabinet design contribute most of what will determine how well the loudspeaker will track to what ever the amp produces.

    What about back emf?
     
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Except for open-back guitar amps, which benefit from a very poor damping factor.

    The DF is indeed virtually irrelevant, since the resistance in the speaker is so large........ how does that last 0.05 ohm affect the circuit when there is still 2.7 ohms (or more) in series?
     
  5. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    A dynamic loudspeaker is a moving diaphragm attached to a voice coil that sits in the field of a magnet.

    A dynamic microphone is also a moving diaphragm attached to a voice coil that sits in the field of a magnet.

    Both are transducers, i.e., converters between electrical energy and acoustical energy.

    When you put a signal voltage on the loudspeaker, the voltage causes current to flow in the coil and produce a magnetic field, which is a force that accelerates the diaphragm (the speaker cone).

    The moving parts have mass, too, and therefore inertia. When the amp puts a voltage into the speaker to make it move, there's some lag. And when the amp tries to make the speaker stop or change direction, there's some lag, too, as the cone assembly tries to keep going in the same direction.

    When these moving parts are doing their own thing--that is, moving or not moving in any way that the amp signal isn't making it do--it acts like a microphone in that it produces a signal voltage. This is called back emf (electromotive force).

    But the typical solid state amplifier is a voltage source, and so the very low output impedance that it's supposed to have and and the very-low-resistance speaker cables that you're supposed to use are a near-short circuit to the back emf.

    If you've ever cranked a generator and then shorted the output, you'll know that the generator suddenly doesn't want to be cranked any more. The same thing happens with the speaker cone's extraneous motion (or non-motion); anything the amp doesn't provide the voltage to do becomes very hard for the speaker. Thus, it is said to be damped. This is how the amp controls the speaker.
     
  6. Viviuos

    Viviuos

    Jul 15, 2004
    Nehawka, Nebraska
    Yes.
     
  7. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I am guilty of a brief monograph on how speakers work, at my little web page.

    The main limiting factor to controlling the cone is the series resistance of the coil, which can be many tens or even hundreds of times higher than the output impedance of the amplifier. In practice, coil resistance is one of the factors that is balanced with other design parameters to achieve a desired response curve.
     
  8. Gearhead43

    Gearhead43

    Nov 25, 2007
    NorCal
    Well slap my butt and call me Sally!
     
  9. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    This is true; the series resistance of the coil is like a high output impedance to the back emf, so it reduces the amount of current that flows to damp the speaker.
     
  10. will33

    will33

    May 22, 2006
    austin,tx
    This is why TB is so cool. An engineer from one of the most respected brands in the industry has just given us weeks worth of googling and reading to truly understand what he just said, but said it in a way that's basically understandable (or at least made it possible to grasp the general concept) on the first read. Thanks, man.
     
  11. Grooveman1961

    Grooveman1961

    May 8, 2006
    Thank you so much for taking the time to respond.

    I'm mired in the fundamentals right now, so I really appreciate the patient responses.

    Concerning cone movement and amp output, my current thinking says that as the amp out put goes from 0 towards its positive peak, the speaker motor causes the cone to move forward. the amount of movment is determined by motor force being greater than the suspension restoring force. as the amp output reaches its peak for the positive half cycle, movement stops, as the change in voltage is 0, and motor strength and suspesion compliance are in equilibrium. As amp output decreases, motor force decreases and the speaker suspension starts to pull the cone back ward against the motor force. As amp output crosses from positive to negative, same thing happens, save for the fact that the cone is moving back from its resting point.

    Mr. Lee stated that the amp at some point is trying to make the speaker stop, and change direction. In my mind, ths means that the amp out put has to change polarity while the speaker is still traveling forward, or is forward of its resting position.

    Am I to take this to mean that the speaker does not track, in phase, to the amps output as a general rule?
     
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Keep in mind that above the speaker's resonance (i.e., nearly all of the audio spectrum), the voltage is an accelerative force as I mentioned in the previous post. It is not a positional force.
     
  13. Grooveman1961

    Grooveman1961

    May 8, 2006
    To any who have read this far, Treat yourself and go to FDeck's site and work through the math. Nicely done. Check out the first installment of speaker physics. Great info for the unwashed like me.
     
  14. Grooveman1961

    Grooveman1961

    May 8, 2006
    :confused: I will have to chew on that for a bit.
     
  15. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A simple explanation is that the force exerted by the amp is so much greater than the inertia presented by the moving mass of the driver that even the relatively low damping factors of tube amps have no difficulty at all controlling the cone. In some circles it's felt that Mms in and of itself is almost inconsequential, and only an electrical force, coil inductance to be precise, has enough effect to be of some concern. Most pro-sound drivers have low Mms and Le, so neither factor presents any particular concern.
     
  16. Grooveman1961

    Grooveman1961

    May 8, 2006
    I made myself a little chart to visualize this relationship between position, voltage and acceleration. The blue is cone position through one cycle with a simple sine as the amps output. The blue line suggests that voltage and cone position are similar in relationship. At 90 deg on the x axis, the cone is extended to is max (for this situation). If the speaker had a sensitivity if you will, that moved the cone 1mm for 1 volt input, max pos voltage occurs at 90 deg.

    The pink, is acceleration. At 90 deg, accel is 0, because the cone has stopped and is preparing to move the opposite direction.

    I guess I'm having a bit of a hard time following the idea of voltage being a function of acceleration and not position. Maybe it's semantics at this point.:meh:
     

    Attached Files:

  17. Grooveman1961

    Grooveman1961

    May 8, 2006
    After considering, answers from other tbers, and materials from G. Randy Sloane and Doug Self, and WIKI:http://en.wikipedia.org/wiki/Damping_factor,

    I'm goiong to conclude at this point that amps really don't control speakers all that much, beyond some damping.

    Speakers respond to an output from an amp. Different speakers will respond differently because of their inherent parameters, and as such have different sound quality.

    Amp design does contribute by affording reasonable damping factor, but more damping factor doesn't yeild more practical control.

    Said another way, amps don't know whats hanging out there on the end of the cable, other than the effect of the impedance that ultimately influences current flow.
     
  18. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    The fun part is when you analyze cone motion with different voltage waveforms and discover how similar they are.
     
  19. Grooveman1961

    Grooveman1961

    May 8, 2006
    Ok, I admitt in advance that this is going to reveal my true bass geek nature, but....that does sound fun. What have you seen used to do this very thing?

    I have a little lightweight kistler transducer (accelerometer) that I was going to glue to the cone of an old 10 incher. I thought that if I differentiated the result, I'd have velocity against a time base. If my data aquisition was quick enough, I could reasonably compare cone response vs amp output.

    Then what?

    I bet I'd just be reinventing the wheel. Sounds like you've already measured. Anything you can share?
     
  20. I have been doing restoration on vintage Pioneer recievers and have been learning just enough to repair them.
    That being said I learned that there is a Positive and Negative voltage at the output transistors and I was under the asumption that is what was responsible for the speaker motor to be driven out by voltage and then returned by opposite polarity.That is why I commented in the other thread that the amp controls the speaker.
    Am I wrong?
     

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