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Question about ohms and bridged amp

Discussion in 'Sound, Music, and Intercom' started by KProductions, Mar 31, 2009.

  1. KProductions

    KProductions Member

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    Hello,...

    If I'm running an amp that specs 1000 watts at 8 ohms bridged, - into (2) 8 ohm speakers (output of amp to speaker "1", parallel output of speaker to speaker "2")... am I running 1000 watts to both speakers or 500 to each...??

    Thanks,
     
  2. TimmyP1955

    TimmyP1955 Active Member

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    Each box will be getting 1/2 of whatever power it is that the amp can put out into the 4 Ohm load that results from two paralleled 8 Ohm loads. This may be more than, less than, or the same as the power the amp puts out into an 8 Ohm load.
     
  3. museav

    museav CBMod CB Mods Departed Member

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    Like Timmy said, neither. If the speakers are 8 Ohms each and are wired in parallel then the amp is seeing a 4 Ohm load and each speaker is potentially getting 1/2 of the 4 Ohm bridge rated power of the amp, which is likely somewhere between 500W and 1,000W per speaker.

    You might also want to verify how it is wired on the amp. Bridge mode usually requires wiring to the two positive connections rather than to + and - for one channel, many people set amps for bridge mode and don't realize this.
     
  4. Chris15

    Chris15 CBMod CB Mods Premium Member

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    When running bridge mode, you need to pay particular attention to impedance. To run the load you are currently talking about, you need an amp that is normally happy running a 2 ohm load on a channel. Many are not.

    Now for why:
    When you run an amp in bridge mode, the amp (or your external adapter) is taking your incoming signal feeding it to channel A and at the same time, inverting it and feeding that to channel B. You then get double the voltage swing to what you would on a single channel which is why you need to connect the speaker coil(s) to the 2 + terminals. But what it also does is effectively put half the load on channel A and half on channel B in terms of the impedance...
     
  5. dafunkmonster

    dafunkmonster Member

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    Yes, if the amp is rated at 1000W under a 4ohm load, and you're running two 8ohm cabs in parallel, the amp will see a 4ohm load, and each cab will see 500W of power.

    In this configuration, you'll want to make sure that each speaker cab is rated for at least 500W RMS power. Don't confuse this with Peak power handling. The peak power rating indicates how much instantaneous power the speaker can handle for a split second. The RMS power rating indicates how much average power the speaker can handle over an extended period of time (this is based on the thermal dissipation of the cab/coils and physical/mechanical constraints, etc). If you exceed this, you'll likely end up burning your coils eventually. This is obviously bad. The power rating on the amp is essentially how much power it can put out for an indefinite amount of time, so if you have your amps setup powering your speakers at more than their RMS handling, you're just slow-cooking them, and they'll eventually die (assuming you're running signal through them).

    So make sure your speakers are rated for at least 500W RMS.
     
  6. mixmaster

    mixmaster Active Member

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    ....and when he cranks his amp to max to get his 500 watts output (assuming it will actually get 500 watts) and drives it into speakers rated for 500 watts, the amp is going to go into clipping and still fry the speakers. As impedance goes down power is going to go up. At this point we are out of spec anyway (8 ohm amp, 4 ohm load) so ratings just don't matter. In an ideal world one would like to see an amp delivering peak power that's 1 and 1/2 to 2 times the rms power that the speaker is rated for. This is to preserve headroom on the amp. Especially being out of balance, I wouldn't try to drive anything bigger than 250 to 300 watts per each of two drivers. Ideally if he has an amp rated 1000 peak into 8 ohms, an appropriate driver would be a 5 or 600 watt 8 ohm driver.
     
  7. museav

    museav CBMod CB Mods Departed Member

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    That is not really accurate for several reasons. The simple one is that it ignores the application. In an ideal world one would like to see the power (actually voltage and current) that is appropriate for the application. The 1.5 to 2 times the continuous rating comes from an EAW white paper that has actually been updated to clarify that it is for a generic situation but in an installed system with a known application, it is much better to work back from the desired results to determine the power required rather than assuming the power based on the speaker rating. An extreme example but if 1W provided all the power you would ever need for the application, would you still need to use a 1,000W amp because the speaker is rated at 500W continuous?

    Another reason is that headroom exists only if you let it. If you increase the power available but also increase the average level, then the headroom may not change at all. So one can't address headroom without also addressing the average level. It also typically makes little sense to have a bunch more headroom at the amp than in the rest of the system, once again looking at the system and application and not just the speaker rating.

    Also, it is important to understand that what is clipping is the amp input. Two amps with different power ratings may clip at the same level, the difference is in how that input signal level relates to the voltage and current out of the amplifier. An amplifier is basically a voltage multiplier and an amp with the same maximum input level but a higher rated output power allows you to get the same output from a lower input signal level, it is not allowing a greater signal level as many people seem to think. So any increase in headroom is due to either using an amp with a greater maximum input level or being able to lower the input signal level, it is not a result of the greater rated output, that increases level but not headroom.

    This is always a balancing act. Too little power for the application and you can drive the system too hard, clipping the amp input and potentially damaging the speakers. Too much power and you run the risk of damaging the speakers via either a short term peak in the signal or excessive long term average levels, typically via overexcursion or thermal failures, respectively. So the decision is what is the right balance for the application, which includes the abilities of the operators. The 1.5 to 2 times the continuous power rating of the speaker is a good rule of thumb, but it is just that and more a way of determining a viable 'ballpark' figure than it is any design criteria.

    It should probably also be noted that 1.5 to 2 times the rated continuous power would be a crest factor (the difference between the peak and average signals) of 2-3dB. That might be achieved with very heavily compressed music or a signal with very limited dynamics, however most system designers plan for 10-20dB of headroom in the system to accommodate more dynamic sources, so the 2-3dB provided by the amp sizing is typically much less than the overall headroom required to be accommodated by the system. The point is that there are major differences between the approach used to 'match' a speaker and amplifier amplifier without considering the specific application versus the approach used to verify that a system works for the application.
     
    Last edited: Apr 14, 2009
  8. mixmaster

    mixmaster Active Member

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    In a perfect world, I would agree. Ideally, I would like to have a speaker that can provide the necessary volume for an application without coming close to it's limits, driven by an amp with enough headroom so that is capable of powering the speaker cleanly to it's full rated power. Determined in that order.
    Unfortunately, the OP gave us little info on the application so the best we can do to is provide a "rule of thumb" response. ASSuming one can safely drive 500 watts (no one noted weather this is peak or RMS) each per driver from a 1000 Watt (peak or RMS???) amp seems a bit dangerous, especially given that we know the amp's output impedance is not correctly balanced by the load. If anything trying to squeeze all 500 watts out of an amp loaded below it's rating could possibly cause the amp to fail sooner. I had several object lessons in this back in my car stereo days before I got to know Mr. Ohm.
    Thus my response.

    Matt
     

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