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Which speakers should I use with my amp?

Discussion in 'Sound, Music, and Intercom' started by zac850, Oct 28, 2004.

  1. zac850

    zac850 Well-Known Member

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    My school has 2 different sets of speakers. We have 2 Peavey 400 watt speaker cabinets, and 4 JBL 250 watt speakers. None of these are powered. I just got the hardware today to run the JBL speakers, and I'm not sure if I should use them in place of the Peavey's. The reason why I'm wondering is because we only have a 250 watt amp. So, I am not sure if it would be better to run stronger, bigger speakers at 150 watts less then they should have, or run the JBL's at there correct wattage.

    Also, the JBL's are set up so that the the output of one of the speakers can go to another speaker. If I did this, would I need double the wattage of the amp, or would the same 250 watts run down the line to the secound speaker in each chain.

    I'm not sure i'm explaining this very well, so let me attempt to draw a diagram:

    -----
    l.......l ---------l_l-----------------l_l
    l.......l
    l____l----------l_l-----------------l_l

    Amp.........Speaker 1..........Speaker 2

    I hope that diagram helps (and works).
     
  2. mr_sound

    mr_sound Member

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    Technically, you want to overpower a speaker with the amp, not underpower. When you underpower them you run the risk of pushing the amp into clipping just to get the speaker near it's potential, and clipping is a quick way to destroy a speaker.

    Now as for chaining speakers together...this gets a little tricky. You really need to pay close attention to your impedances for this to work right. What impedance are those speakers rated for? If, for example, the speakers are 8 ohms (which is common) and you run them together in parallel, then you're actually running them at 4 ohms. Now if the amp can crank out 250 watts into 8 ohms, then it can do about 500 into 4 ohms. But, by decreasing the impedance, you're running the amp hotter, using up more power, and you need thicker speaker cables so you don't lose all the power your putting into these things.

    Anytime you wire 2 speakers in parallel that are the same impedance, you cut the impedance of one speaker in half. If you wire them in series, you add the 2 impedances together. The output from the JBLs is likely a parallel connection.

    And actually, it may be wiser to go this route..if the amp can handle it. The lower the impedance the more efficient the amp works. Plus using this plan you can either run the amp in bridged mono, or use the second channel for something else like monitors. But, that only works if the speakers are 8 ohms. If they're 4, then you're asking the amp to go down to 2 ohms, which not a lot can do. Some of the higher end ones can do this in bridged mono mode...but you do need to keep in mind this will eat up a lot more electricity and the amp will go thermal a lot sooner than it would if you were running a 8 ohm load.

    Check the manual for the amp and the speakers to figure out what option would be best. And as for using the peavy or the JBLs...I'd go with the JBLs since it is better to overpower a speaker. Also, keep in mind that just because the peavys can handle more power, does not make them bigger or better. You need to read all the specs. Pay close attention to the sensitivity rating. Another thing to look at is coverage, and dynamic range.

    And if all this sounds like gibberish to you...go read the live sound reinforcement handbook, and you'll get what I mean.
     
  3. soundman1024

    soundman1024 Active Member

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    or...

    If your amp is stereo you could parallel the JBLs with the Peavys to get 4 ohms per channel [assuming that both cabnits are 8 ohms], 2 channels. The best solution is another amp, but that isn't always an option. My advice would be to test out both speakers and see what sounds best. If the JBLs sound better use them, if the Peavys sound better use them. Just trying to give you some ideas. I really wouldn't advise trying the 4 speaker method unless you really know what you are doing. It is probably bad advice accually 8O
     
  4. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Mr Sound is correct in saying that you should always have some headroom in your system. Many people think that speakers only blow by over powering them. Whilst this is often the cause, you will hear the distortion (or see the clip lights) and (should) drop the gain.

    So - in saying that, it would seem that your best option is to use a single set of JBL speakers, as they are 250W each and your amp is 250W (I am assuming per side). Also - Is that Nominal or Peak power?

    If you do ever get another amp and what to hook up more speakers, the formulae for working out the impedance when connecting speakers in parallel is:

    1/Rt = 1/R1 + 1/R2 etc

    Therefore, if your speakers are both 8ohms your total resistance (impedance) is equal to 1/8 + 1/8 which is 0.125 + 0.125 which equals 0.25

    1/0.125 equals 4

    As Mr Sound correctly pointed out if the speakers are all the same impedance, then you simply divide the impedance by 2 if you are connecting 2 boxed in parallel. However, it is good to know how to calculate them in case you either have mismatched impedance boxes or are building split systems.

    Before you connect up two different makes of speakers, always check that the wiring is in fact the same. There are certain manufacturers out there that do not follow the Pin 1, Pin 3 arrangement that is most common in XLR input/thruputs. A multimeter will tell you which 2 pins are used but you may wish to open the box to ensure you have the polarity correct (although the lack of bottom end with give you a heads up if you get it wrong). If you cannot guarantee that no one has been inside the boxes in the past, I would suggest opening them up and having a good look inside. Besides, this is a good opportunity to clean out any crap floating around inside.

    One other thing, the use of passive cross-overs will also cause impedance changes.
     
  5. zac850

    zac850 Well-Known Member

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    Well, our amp is built into an old sound board, so I can't run in bridged mono. I think tomorrow I will bring up 2 of the 4 JBL's and plug them in, see how it sounds.

    I'm trying to remember if the JBL's are 4 ohms or 8 ohms. I think they are 4, but I could be wrong. If they are 8 ohms, I'll think about bringing up the other 2 JBL's. If they are 4 ohms, I guess I'll stick with 2 speakers.

    Does that make sense?
     
  6. mr_sound

    mr_sound Member

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    Keep in mind that your 250 watt rating may be at 8 ohms...or it may be at 4 ohms. You do need to find that out. If you're using 4 ohm speakers, and the amp is rated for 250 watts into 8 ohms, that's a big difference. Although, in my opinion, that's more ideal than underpowering a speaker to begin with.

    But don't necessarily assume that adding a second pair of speakers is going to make everything sound better. If the room isn't big enough to need the power, or the coverage, then don't use it. You'll end up creating more problems, especially if you don't know much about sound.

    Since it sounds like you're using a powered mixer, odds are you're short on features. Especially if you can't even bridge mono. That makes me think this thing can't handle a 2 ohm load....so be careful how you wire these things up.
     
  7. zac850

    zac850 Well-Known Member

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    Well, I wouldn't use the second pair of speakers. I believe that the amp is 250 watts at 8 ohms, and I believe that the speakers are 250 watts at 4 ohms. So, as you said, it would be better to use the speakers. Would using speakers with double the ohm load hurt them at all?

    As I'm sure you can tell, I know almost nothing about sound, and all I know is that the sound is horrible, and we are under-powering our current speakers.
     
  8. zac850

    zac850 Well-Known Member

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    I just checked the speakers, and they are 250 watts at 8 ohms.

    The sound board, if I remember correctly, can output at either 150 watts at 8 ohms, or 250 watts at 4 ohms. Which output should I use for amps requiring 250 watts at 8 ohms?
     
  9. mr_sound

    mr_sound Member

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    Use the parallel out for the second speaker, which will give you a 4 ohm total load, giving you the needed 250 watts. But remember you'll only be using one channel on the amp, and running an amp with a 4 ohm load is going to make it work harder than it would with a 8 ohm load. It should work ok, as long as you're not pushing it too hard. You've also turned your rig into a mono setup....which is actually what most sound people prefer. But don't go thinking you can do any cool stereo effects, or even put a different EQ on each side.

    Just how big is this room anyways? Do you even need the second pair of speakers?

    BTW, as I understand it, an amp's impedance will change according to the impedance on the speaker. Impedance is resistance, measured in ohms. If the speaker is 8 ohms, then the amp will output 8 ohms. A lot of people think that if the amp only has specs for 8 ohm loads then it can't handle a 4 ohm load. I could be wrong about this, but I'm pretty sure that's how it works. The only problem is when you get to too low an impedance, like a 2 ohm load. Only higher end amps can handle that low of an impedance. But you won't ever have 8 ohms going into a 4 ohm speaker, because the speaker is what's causing that resistance. The spec you get on the amp is just telling you that with that much resistance from the speaker, you can output x amount of watts. And generally for every halfing of the impedance, the amp's output will double. That's not 100% exact, but it's a good guideline. When in doubt, read the manual. Even if you know it can put out 250 watts, you need to know how they measured that spec. Is it peak, RMS, program? There's a difference in all 3.

    This is why you should learn how to use the gear before jumping into live sound. I'm not big on people teaching themselves how to use the gear, because you usually end up missing out on a lot of important information. Sure you may know how to use a board, but if something goes wrong, you need to know how to fix it, and fast.

    Yamaha's sound reinforcement handbook. Get it. Read it cover to cover. Keep it with you at all times until you know it. In my opinion, it's the soundman's bible.
     
  10. VipermanGTX

    VipermanGTX Member

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    JBL's all the way man, you want best sounding not loudness.
     

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