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Difference between bridged and parallel amp?

Discussion in 'Electronic Design' started by No One Really, Sep 2, 2005.

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  1. Looking at the datasheet for a National LM4780 audio amplifier IC I was
    wondering: what's the difference between a parallel and a bridged amp? They
    sort of seem to be doing the same thing.
  2. Phil Allison

    Phil Allison Guest

    "No One Really"
    ** Only if you think that connecting two identical batteries in series or
    parallel is the same thing.

    ........... Phil
  3. Don Pearce

    Don Pearce Guest

    No. Bridging doubles the output voltage, while parallel doubles the output
    current. You need to choose your speaker impedance to suit either

  4. Arny Krueger

    Arny Krueger Guest

    The pedantic version would be ;-)

    Bridging doubles the maximum available output voltage, which
    may lead to a doubling of maximum output current. The latter
    eventuality needs to be carefully considered because it is a
    common pitfall of bridged operation.

    Parallel operation doubles the output current that is
    available for delivery to the load.
    Or, you need to choose your mode of operation to suit your
    speaker load and needs for dynamic range.
  5. Pooh Bear

    Pooh Bear Guest

    You got your answer but I'll add that I wouldn't recommend that parallel
    configuration. It involves a pair of current sharing resistors that increase
    the output impedance.

  6. Tam/WB2TT

    Tam/WB2TT Guest

    Take a close look to see how the speaker is hooked up in the bridge
  7. I read in that Pooh Bear
    One UK company sold/sells sound system amplifiers with two 60 W MOSFET
    amp modules in parallel. It works, but it worried me a lot when I first
    saw it. There are no current-sharing resistors and no cross-connections
    of any sort. Not happy at full output at 20 kHz, though; smoke happens.
  8. John Fields

    John Fields Guest

    What no one has commented on so far is that for a given load
    impedance, using paralleled outputs will do nothing as far as
    increasing the power into the load goes, while using bridged outputs
    will result in _quadrupling_ the power into the load! That is, of
    course, assuming the power supply can deliver the increased
    (doubled) current.
  9. JohnR66

    JohnR66 Guest

    As stated, output power is quadrupled because of the doubled voltage swing
    and thus, doubled current in the output. However, one must pay attention to
    the maximum current in the output devices. For example, an audio amplifier
    IC may be rated to drive a 4 Ohm minimum load at rated supply voltage. When
    bridged, it can no longer handle the 4 Ohm load. 8 Ohm would be the new
    minimum. So, considering you are trying to get as much power from the amp by
    using the lowest output impedance possible in any case, the maximum you can
    gain by bridging is double the power.

  10. Guest

    Guest Guest

    Use bridging when your amplifier is voltage limited, i.e. when it won't put
    out enough voltage to drive your speaker to full power.

    Don't use parallel amplifiers unless the feedback loop includes both
    amplifiers, which it doesn't unless the amplifiers were specifically
    designed for parallel operation.

    Norm Strong
  11. Tim Martin

    Tim Martin Guest

    The main difference is the input connections. In the parallel amplifier
    echematic, the inputs of the two channels ae the same, but in the bridged
    amplifier schematic, the input of channel B is inverted wrt to the input of
    channel A.

    And so the outputs of the bridged amplifier are also inverted wrt to each
    other; and by connecting the speaker across the outputs for both channels,
    it will receive twice the voltage (one terminal at +V, one at -V, rather
    than one terminal at +V and the other at 0V).

  12. Mac

    Mac Guest

    Isn't this the point where someone usually jumps in and says that MOSFET's
    don't need current sharing resistors because resistance goes up with
    temperature? This person might even cite an excellent and well known
    electronics text.

    And then someone else, possibly even one of the authors of that text jumps
    in and says "No, when used in the linear region, virtually all power
    MOSFET's DO need current sharing resistors?"

    Just wondering.

  13. Pooh Bear

    Pooh Bear Guest

    To answer your question, I've found that the audio specific lateral mosfets made by
    Hitachi and also similar ones from Semelab and Exicon share current very well
    without any ballast Rs..

    Other mosfets types have very variable treshold voltages and can't be used reliably
    in this way.

    But that's only in the application where those devices are in a single amplifier.

    Paralleling entire amplifiers is wholly unwise with current sharing Rs. Mainly
    since they won't have exactly the same voltage gain, given normal component
    tolerances. The feedback loop will win over everything.

  14. <>) about 'Difference between
    bridged and parallel amp?', on Sun, 4 Sep 2005:
    These are not paralleled devices but complete 60 W amplifier cards with
    the outputs paralleled. The output source impedance of each is quite
    low, due to negative feedback, but not audiophool low. Nevertheless,
    each output must look like a near short-circuit to the other amplifier.
  15. Then what about amps using multiple output devices in parallel ? They all
    must look like near short-circuits to each other, don't them ?
  16. Pooh Bear

    Pooh Bear Guest

    John was pointing out correctly the difference between paralleling devices
    inside and outside a closed feedback loop.

  17. Arny Krueger

    Arny Krueger Guest

    Almost always done inside the same feedback loop.

    Almost always done with current-balancing resistors.
    One reason why amps in parallel look more like short
    circuits to each other are the individual feedback loops.
  18. I know. But you can parallel two amp modules, use proper current balancing
    resistors and derive proper feedback (if needed).
  19. Correct. But it's quite easy to properly parallel two amps.
  20. Phil Allison

    Phil Allison Guest

    "François Yves Le Gal"
    Some pommy dickhead

    ** There is at least one proper, engineering type way to do it.

    The two or more power stages are made to have unity gain - ie 100% NFB.

    Then, a single voltage amp drivis them all.

    ........... Phil
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