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Need help with a subwoofer issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Hollobody, Nov 7, 2005.

  1. Hollobody

    Hollobody Member

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    We were setting up Sat night for a gig at a local watering hole and noticed that one of the subs had a bit of cracle in it. Pulled the bass down a little in the mix and then pressed on. Didin't have any complaints and the gig went real well. On Sunday morning the bass player took the sub apart and noticed that the coil was burned. It's a Peavey SP118. We are running 2 SP118's powered by a Crown XS900 that is rated 900@4ohms & 600@8ohms.

    1> Are we overpowering these subs with this amp?
    2> Should we replace the sub speakers with Black widdows?
    3> Is there some sort of a fuse/circet breaker to prevent overheating?

    Any help would be appreciated. We have another gig this Friday and don't wan this to happen again.

    BTW the last time we played prior to Sat was in early Oct at an outdoor church festival. We had some pre-teen kids pull the power cord on us twice. I'm wondering if that could have caused some damage? (After the 2nd time we packed up and told the church representitive that pulling the power is not what Jesus would do, so we turned our cheeks and split.)
     
  2. bob-i

    bob-i Member

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    It's not likely that you're overpowering the sub, although it's possible. Most likely you've got some clipping going on somewhere causing the coil to overheat and burn. Clipping in an SS power amp or even earlier in the signal chain like at the mixer or eq will burn a voice coil quickly.

    I'm not sure on the sudden removal of power. Depending on the power amp it might send some clipped signal to the speaker. Some power amps shut down quickly when power is removed, others keep operating on the cap discharge causing a clipped waveform to go to the speakers.

    SS Clipping= death to speakers.
     
  3. Hollobody

    Hollobody Member

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    Thanks Bob. One more question. If an individual channel (The bass players bass signal for example) is clipping, but the overall output to the mains are in the green/yellow range, will that still cause the problem?

    Again, thanks for the info.
     
  4. TheAmpNerd

    TheAmpNerd Member

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    This is why some well known audio engineers
    recommend that the amp be more then twice
    the power of the speaker rating.

    This is typically the hi fi and PA guys.

    for the MI guys this is no so common and
    many suggest matching amp and speaker
    or having speakers with at least twice the
    amp power rating.

    Bob Carver was a proponant of over amping
    just for this reason, so the clipped sqare waves
    would not melt down the voice coils. I think
    some of this is addressed in his white papers
    here. Sorry, you'll have to dig it out yourself.

    http://www.sunfire.com/technology.htm

    Somthing to think about

    GOOD LUCK..
     
  5. dave s

    dave s Member

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    Couple of things possibly going on with your subs:

    1) The spec's on the Peavey SP118 say 600w continious, 1200w program and 2400w peak power. I'm going to assume that since the 18" speaker sports a 4" voice coil that the 600w figure is the RMS rating. Since you have TWO subs, the power requirement is 1.5 - 2 times the RMS rating of 1200 watts. If you have fewer than 1800 watts running your two subs, you are UNDERpowered.

    2) You didn't say if you had a cross-over or other other signal processing (like a DriveRack PA) unit to manage both ends of the sub. If you have your subs running full range with no x-over, you're compounding the problem in issue one of not having enough power.

    Two suggestions:

    1) Use a power amp that provides 1800-2400w of power at 4 ohms bridged mono for your subs. Choices include QSC PLX 2402 or 1850HD (great for subs!) or the Crown CE2000.

    2) Look into a signal processing for your system. Subs are meant to handle 40-120 hz, (more like 50-90 hz in practical terms) and have both high- and low-pass filtering to concentrate the power on just the wanted low-end frequencies. Without an x-over or Drive Rack PA signal processing device, most of your power is being burned up below and above those frequencies.

    3) Might want to have the speakers checked and reconed if necessary. I believe those are pretty decent subs.

    all the best,

    dave
     
  6. bob-i

    bob-i Member

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    Yes.
     
  7. Hollobody

    Hollobody Member

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    Thanks for the info Dave.

    I'm going from the mixer (peavey unity 2000) to an Alesis EQ. From there to a DLX223XL crossover. From there the signal goes to a Makie 1400i for the mains (left side to 1 Peavey SP2 and the right to 1 Peavey SP2). And the lows go to the Crown Xs900. Again the left side to a Peavey SP118 and the right side to a Peavey SP118. The Xs900 is rated 1200W per channel @ 2 ohms, 900W @ 4 ohms, 600W @ 8 ohms, 2,100W @ 8 ohms bridged. Are you saying that I should have a Xs900 for each sub? And run them bridged? Or should I run it bridged and then dasey chain the subs together? I got a little confused there (not hard to do)
     
  8. dave s

    dave s Member

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    Ok, here's what I'd do with your setup:

    1) Run the crown amp in bridge mono mode. Connect 1 sub to the amp and run the second sub from the 1st sub.

    2) Your subs are 8 ohm cabinets. By running them in series (jumping the 2nd sub from the 1st one) you are now showing the amp a 4 ohm load. The amp says it will deliver 2500w in the bridged mono 4 ohm load setting. PLENTY of watts on tap to run the two subs very effectively.

    3) If (and I can't see the back of the power amp) the amp has some sort of high pass filter setting, set it to 40 or 50 hz. Some amps have this, some don't. If it does, the amp is controlling how low of frequency being passed to the subs.

    4) Set your crossover setting for the subs somewhere between 90 and 100 hz. This will keep MOST of the higher frequencies out of the subs. I realize that some documentation says 120 hz or even 150 hz, but this is WAY too high for your subs.

    5) Make sure your kick drum and bass guitar channels aren't going more than +3 above zero.

    6) If you have a 'mains EQ' that you use for overall shaping of the system, make sure to drop the sliders at the far left of the EQ to completely off as these just eat power and don't produce any useable or audible sound. These would include the frequency bands that say 25hz, 31.5 hz and probably even the 40hz slider. Leave the 50hz and 63 hz sliders flat, or at "0". If you need more 'thump' from the kick drum, it lives at 80 hz. Maybe bump that slider a bit above the "0" position on the EQ.

    You should now have nice, tight low end coming from your subs. Unless of course, those speakers are indeed fried.

    Best wishes for a smooth operating sound system.

    dave
     
  9. Hollobody

    Hollobody Member

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    Thanks mucho for the advice Dave. We are replacing the speakers to be safe and will institue your advice on the setup for next Fridays gig.
     

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