4 quadrant amplifier

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  • 10-23-2004, 11:34 AM
    4 quadrant amplifier
    Do you know what is 4 quadrant amplifier ?

    "...What many people often forget is that the speaker is bouncing back electrical energy to the amplifier. The typical dynamical speaker with a moving coil in a static magnetical field is also acting as a current generator.

    To explain this look at this example: when the speaker gets a single sided
    pulse the diaphragm is following that pulse as best as it can. When it
    bounces back to the original center position the electrical energy from the
    pulse is transformed into movement energy of the diaphragm and a few side
    effects like heat energy (copper loss in the coil etc.).

    The movement energy of the diaphragm is partially transformed into movement
    energy of the air (which we hear as the sound we want), partially into heat
    energy, and the rest of the energy is kept as movement energy of the
    diaphragm. The remaining movement energy (which is still stored in the
    diaphragm) will force the diaphragm to swing to the opposide direction of the
    original pulse. The altitude of this swing back maneuver will depend on
    several factors, mainly the efficiency of the speaker (which is sometimes
    pretty bad and may be in the range of 20 to 50%).

    What happens if a coil is moved into a (statical) magnetical field? A current
    is induced! This current is fed back into the amplifier in the reverse

    This means that a relevant portion of the original electrical pulse from the
    amplifier is fed back into the amplifier with a small time difference. The
    amplifier should be able to handle this "reverse current". Unfortunately a
    lot of amplifiers DON'T!

    Most amplifiers are build as 2-quadrant amplifiers, that means, they can
    supply positive and negative current, but they only act a current source, not
    as a current sink. Only 4-quadrant amplifiers are able to supply positive and
    negative current (acting as a source), AND also can "eat" positive and
    negative current (act as sink).

    So what happens when the amplifier is able to "eat" the reverse current?

    Ideally the amplifier has an input impedance for the reverse current which is
    close to zero. Then the electrical energy bouncing back from the coil is
    immediately consumed and transformed into heat. This means that the unwanted
    movement of the coil is immediately damped.

    The lower the impedance is the better is the damping of unwanted movements.
    This of course includes the speaker cable. It makes no sense to build an
    amplifier with 0,1 Ohm impedance (at the output interface) if the speaker
    cable adds 1 Ohm.

    The lower the impedance of the amplifier is, and the lower the impedance of
    the speaker cable is, the better the diaphragm can follow the electrical
    signal and therefore reproduce more precisely the original signal. ...."

    Do you know whetehr there are in the market these amps ?
  • 10-23-2004, 03:57 PM
    dunno, but it is the first time i ever heard of sth like that... sounds like a new marketing strategy to me. however i though that the signal given to a speaker was continous? As in the cone moves forwards or backwards according to the strength of the magnetic field generated in the coils. This means that the cone is pushed-pulled, not just pushed and allowed to bounce back. If it was just pushed, there is no way u could reproduce they higher frequencies. Someone correct me if i'm wrong.
  • 10-24-2004, 08:39 PM
    Hmmm I always thought this is why we bi-wire, to keep any back EMF (Electro Motive Force) generated by the woofer from feeding back into the tweeter circiut provided the speakers have seperate woofer and tweeter crossovers. (which all properly designed bi-wireable speakers have). This is supposed to keep the tweeter circuit free from back EMF and allow it to sound cleaner.

    As far as this back EMF reaching the amp....all well designed amplifiers should have no problem with this and if i'm not mistaken the damping factor rating is a measure of the amplifiers control over the speaker. So a amp with a reasonable damping factor should be able to suppress any EMF created by a woofer.

    Yes Kaboom your right when you say the woofer is pushed and pulled. Music is generated with alternating current with a positive and negative waveform meaning the cone is pulled just as far in from the center position as it is pushed out from the center position. But when the signal stops suddenly the cone will tend vibrate very slightly and this is what causes that back EMF, the voice coil moving through the magnetic field. Hope this makes sence to anyone who reads it.

    I wouldnt concern myself with trying to find an amp with this "4 quadrant" stuff attached to its feature list and just listen to reputable manufacturers offerings.
  • 10-25-2004, 09:04 AM
    yeah, I fully agree with the "But when the signal stops suddenly the cone will tend vibrate very slightly and this is what causes that back EMF, the voice coil moving through the magnetic field" since a speaker is basically the same design as a microphone, but when does the signal actually STOP suddenly? i would guess that would only be at very abrupt stops in the music, which do not happen that often, which is was the seller is trying to make us believe. the moderately high-end audio and upwards is not so full of marketing crap as other products are, you should see some of the stuff they try to sell us cyclists... including 400$ saddles.
  • 10-25-2004, 10:09 AM
    Ever hear of a "damping factor"?
    It was a popular term in the 70's. This is the ratio of the speaker's impedance to the output resistance (yes, resistance) of the amp's output stages. The higher the number the better. The higher the damping factor, the more control the amp has over the speakers (unwanted) movements.