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Thread: 1000w Amp and 8 speaker@8ohm, help with ohm matching

  1. #1
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    1000w Amp and 8 speaker@8ohm, help with ohm matching

    I want to setup a PA system (in my new barn/playhouse that will never move) to drive 6-8 speakers, with each speaker rated at 75 watts RMS @8 ohms and 92 db sensitivity. I will use a 750 watt or 1000 watt amp in an 8 Ohm bridged mode (that means a 500 watt@8ohms amp with 2 channels, but will configure it as bridged and get 1000 watt@8ohms). I am new to all of this, so please be patient.

    Please see attached pictures for the specifications and possible configuration. One of my pictures shows one possible “parallel” configuration, but I am just not sure of my ohms understanding.

    I am building a barn/playhouse in Thailand, and it is 15meters x 15 meters inside (50 feet x 50 feet). This system is for the inside. Thai’s like it loud. See attached picture.

    My primary question is getting the resistance/ohms correct with so many speakers, and what is a decent strategy for getting it done. I can see some of the first decisions is wiring parallel or serial. I watched YouTube and read websites to understand this. I focused on Parallel, as the wiring is simpler for me.

    - Do I add resistors to the circuit, so at the end, the amp is presented with something around 8 Ohms? I included a picture of adding a 100 watt@50 Ohm resistor to each speaker, in order to get back to roughly 8 ohms. If I add a 50 ohm resistor to 8 ohm speaker, I get 58 ohm speaker. If I have 8 speakers, I assume I take 58 ohms/8 speakers to get around 7 ohms total resistance?
    - For a system with 75 watt speakers, driven by a 1000 watt@8Ohms bridge mono amp, so I use 100 watt resistors? I hope so, as they seem easy to get. Bigger ones are harder to get and get expensive? I have attached a picture from China on the type of resistors I can easily get.
    - What happens if my overall ohm presentation to the amp is much lower, like 4 or even 1 ohm? Seems to me if I had 2 speakers in parallel @8 ohms, it would present to the amp only 4 ohms. Is this bad? Is there a range of allowable ohm tolerance?
    - Or am I thinking about this all wrong?

    I will add a separate amp for some subwoofers. I was thinking 4 subwoofers that are 150 watt RMS @8 Ohms, and use a similar amp and use the low cut filter option on the amp. So your advice on my 8 speaker main system on how to get the correct impedance, will also apply to this separate subwoofer system.

    I bought a pair of these speakers already, and hooked them to an existing stereo so hear how they sounded. To me, they are perfect. I also bought 1 sub-woofer for testing – and I love it too. I just need to get the resistance and wiring correct.

    If you come to Thailand, please visit. I like beer.

    Thanks - Pat

    PS: I tried to upload more than 1 picture, but it only allows 1 tiny picture, so I chose the one with the parallel wiring diagram. Maybe I can add more later, but this one picture does some up what I am trying to achieve.
    Attached Images Attached Images

  2. #2
    You can't use resistors. They will burn off your amp power as heat. Using a lot of low power low sensitivity speakers isn't a good idea, two or four high power high sensitivity speakers will work much better.
    Bill Fitzmaurice
    Bill Fitzmaurice Loudspeaker Design
    www.billfitzmaurice.info/forum

  3. #3
    You don't need bridge mode for this.. if you can get an amp that's capable of 600W/ch @ 2 ohms (pretty easy to find). Put four of your 8 ohm speakers in parallel on each channel.. that presents a load of 2 ohms on each channel.

    That's not the only way to do it, but it's the easiest.

    Don't use resistors.

  4. #4
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    Thanks for the resistor feedback. I won't do it.

    Is this another way to do it? See attached picture. The amp has 2 x 500 watt@8 ohm channels (or 2 x 750 watt @4ohms channels).

    Can I mix serial and parallel to get my 8 ohm balance? the numbers work out well, with 8 speakers. For the right channel, I would put 2 speakers in serial to get 16 ohms, and put 2 more in serial, for another 16 ohms, and then connect these 2 serial loops in parallel so it goes back down to 8 ohms. Repeat the same thing for the left channel. See attached picture.



    Is there any downside to this approach?

    Sorry for my amateur questions.

  5. #5
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    Good idea, thanks.

    From Light-o-matic: "You don't need bridge mode for this.. if you can get an amp that's capable of 600W/ch @ 2 ohms (pretty easy to find). Put four of your 8 ohm speakers in parallel on each channel.. that presents a load of 2 ohms on each channel."

  6. #6
    Quote Originally Posted by Uprightfan View Post
    Thanks for the resistor feedback. I won't do it.

    Is this another way to do it? See attached picture. The amp has 2 x 500 watt@8 ohm channels (or 2 x 750 watt @4ohms channels).

    Can I mix serial and parallel to get my 8 ohm balance? the numbers work out well, with 8 speakers. For the right channel, I would put 2 speakers in serial to get 16 ohms, and put 2 more in serial, for another 16 ohms, and then connect these 2 serial loops in parallel so it goes back down to 8 ohms. Repeat the same thing for the left channel. See attached picture.



    Is there any downside to this approach?

    Sorry for my amateur questions.
    Yes that would work as well and is the best choice with the amp you have.
    I would say tho that I agree with the advice already given that spending the same money on four better quality speakers (each costing double what each of the 8 speakers were going to cost you) will give you much better sound than 8 lesser quality speakers. And.. depending on your budget you might be better off putting all your money into just two speakers where each one cost 4x as much as the ones you were going to get.

    If you want it to thump, you'll need a subwooofer.

  7. #7
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    Quote Originally Posted by light-o-matic View Post
    Yes that would work as well and is the best choice with the amp you have.
    I would say tho that I agree with the advice already given that spending the same money on four better quality speakers (each costing double what each of the 8 speakers were going to cost you) will give you much better sound than 8 lesser quality speakers. And.. depending on your budget you might be better off putting all your money into just two speakers where each one cost 4x as much as the ones you were going to get.

    If you want it to thump, you'll need a subwooofer.

    The problem I have is that I have a large room with an L-shape, and with 2 giant speakers up front, that area will get blasted with sound in one area, and a lot less sound in another area.

    I agree that 2 high quality and larger speakers would be great, I can even get stereo separation, wiring is easy, and much more. But there will be so much sound pressure in just one area, it may be pretty uncomfortable.

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