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Help me decide the best way to use these amps and speakers!


electrohippy
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Posted (edited)

Hello good people of Harmony Central, I come to you with an question that needs clarity.  Apologies for the TL/DR post but I want to include all the info I think matters!

I got an outside gig with a country band playing for about 150 people next week.  When I pulled my powered 15s out, one has serious buzz after being stored poorly during the pandemic.  I've been looking over my old gear for something I can use.

I have two Peavey Impulse 1015's.  They can be bi-amped or full range.  Per the spec sheet, they are rated at 500W RMS full range, or 500W low , 60W high (RMS) if bi amped.  They put out 100 dB at 1W @ 1M full range.

I have two Crest Audio CKS800-2 Amps.  They spec out at 400W RMS per side into 8 ohms, or 1600W bridged.

My understanding is that amplifier RMS rating is best between 160% and 250% of the RMS rating of the speaker, in this case between 800W and 1250W.  So the problem here is that in stereo mode, one amp would push only 80% per side (too low), and two in bridged mode 320% per side (too high). 

Which is likely to be the safer (for the speakers) option while still providing enough sound reinforcement for 150 people, too high or too low?  This is a family-friendly picnic event, not a rock concert, so I'll probably be shooting for around 85-90 dB about 30 feet out from the band where I'll mix from, but of course that depends on how loud the organizers want it and how cooperative the band is.  

I'm guessing that the 400W per side would be just fine with care, but I'd like to get opinions since I've mostly worked with powered speakers in the past.

I also have a Yamaha P2500S amp, spec'd at 250W RMS per side into 8 and a Carvin FET400 at 100W per side into 8.  Should I throw one of these in the mix, like say bi-amp lows at 400W per side with a Crest and the highs at 100W per side with the Carvin?  I have a Furman TX-3 crossover, too.

How would you guys run this configuration with the amps I have? 

THANKS IN ADVANCE!!!

Edited by electrohippy
I think this is in the wrong section... Mods please move it!
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Posted (edited)

A clean up and re-flow and/or re-cap is no big deal I agree... just no time really.  Show is this Sunday and I'm busy all this week on another project so I can't do it myself.  It's a long haul to a repair shop I trust, and the speakers are old and low quality so not really worth paying someone for the repair. 

I could rent but I'd rather not spend money on this if I don't have to because it's not exactly a high paying gig. 

Since I have all this other gear, I figured I'd just make it work if possible.

Any help and/or opinions about what I actually plan to do would be greatly appreciated.

THANK YOU!!

Edited by electrohippy
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One of the Crest amps unbridged will work just fine. So should the Yamaha by itself. You'd only gain an extra 2 decibels of headroom going from 250W to 400W anyway. I have no idea where the notion of an amp exceeding a speaker's capacity comes from. My background is HiFi gear and in that realm you don't want to exceed what the speaker can handle. No need for more gear IMHO.

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OK so reporting back, the 400W per side amp was not enough, or just barely enough, for the show.  Had to reduce bass guitar and kick levels to prevent clipping at the amp, which was OK, but reduced the quality of the experience.  I didn't have time to make a 4 pole speakon cable to bi-amp with the 400W on lows and 100W on highs, but I'm guessing that would have been just enough to be right for the situation.  Still would have been barely enough I'm guessing, but even without it, it did work out ok (barely).  I would have benefitted from more amp power before clipping for sure!

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21 hours ago, electrohippy said:

OK so reporting back, the 400W per side amp was not enough, or just barely enough, for the show.  Had to reduce bass guitar and kick levels to prevent clipping at the amp, which was OK, but reduced the quality of the experience.  I didn't have time to make a 4 pole speakon cable to bi-amp with the 400W on lows and 100W on highs, but I'm guessing that would have been just enough to be right for the situation.  Still would have been barely enough I'm guessing, but even without it, it did work out ok (barely).  I would have benefitted from more amp power before clipping for sure!

The show must go on.

Next time around you'll know.

 

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So I'm still wondering, how dangerous would it be to bridge one Crest amp at 1600W into full range on these speakers?  They are actually rated at 2000W peak. 

Am I just asking to blow them up, or will it be relatively safe?

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Power amps are dependent on the load for wattage ratings.  Typically the lowest allowable impedance will give you the maximum wattage.  Simply bridging an amp does not mean you'll get 1600 watts to work with and all you're doing is converting a stereo head into mono so it doesn't actually get much louder (unless the paralleled speaker load drops to the minimum the head can match.  

I suggest you check the manuals. You have to know all your component ratings and check the Math before you know what can work best. You can find on line calculators if you need to mix and match different loads.  Most critical the rule of thumb working with PA heads that have variable impedance is to never go below their minimal impedance rating "Especially" if you're going to pump allot of bass.  You can run the impedance higher and its unlikely you'll overheat it, but running too low you should only run a matched load and use quality cables at 25' max which should be good enough to span a 50' stage. 

I don't recommend using 50 ~100" cables on modern power heads, especially if you're hurting for volume to begin with.  (It is why they use a snake with passive systems. The power amps remain on stage and its the low impedance mic cables that run long distance to the mixer with minimal loss).  The technical reason is due to the way the amps are designed.  SS amps have lower current higher voltage. Long speaker cables consume a huge amounts of power the longer they get and turn power into heat instead of sound.  Powered speakers can be much louder at lower wattages because the head is mounted in the cab and may use only 1' of wire to connect to the speaker.  

As far as fixing your powered speakers.  It may be fairly easy if its only a matter of having set for long periods.  If its speaker cone rubbing due to heat an moisture you can often massage it back to full operation. you can carefully move the cone in and out and see if you can detect ant rubbing. I've had countess occasions where the speaker screws loosen up and the basket gets improperly torqued.  I've even had to reverse warp the basket using washers to shim it up to prevent the cone rubbing.  

you can also try running the speaker alone from another head too. Just be sure to disconnect the internal amp first.  If the speakers blown cone manipulation has no effect. you can also tell by the sound if you have a trained ear.  Cone rubbing increases with volume.  A shorted coil can have both a deformed voice coil that rubs and partial operation that loads the amp down into distortion.  

If the issue is the amp head and there is no apparent reason other then having set for long periods, the first thing you should check is Pots, jacks, and Switches.  If you work the pots rigorously and the distortion diminishes, its simply a case of moisture that's corroded the contacts.  Powered speakers likely use line level attenuators which rate at maybe 1 to 10K ohms and can be cleaned with potentiometer cleaner. Be use to use "Only" pot cleaner. If you don't have some buy it. All audio gear needs it eventually and its typically the leading cause of noise having left gear in storage.  Costs around $10 a can. 

Never use Standard electrical cleaner. It's designed for high voltage switches to remove carbon and contains nearly 100% denatured alcohol. It will leave the pot completely dry making much worse and also decrease the lifespan of a pot.  The stuff used for pots has mild cleaners and water resistant lubricants made of silicone or mineral oil. This prevents arching in the pots and is safe on the kinds of plastics modern components use. 

For the jacks I suggest you spray the plug then work it in and out of the jacks a few times. "Do Not Flood an Open Jack by Spraying into the open hole"  This is the worse thing you can do. Most jacks are PC mount now. When you douse the jack you create an oil stain on the board  which winds up collecting dust like a magnet which eventually shorts the board out.  

Switches, simply work them till they work properly. Most new style switches are designed to work until failure where they get replaced only.  If you attempt to clean them you strip the grease off the rocker inside. This can make a switch freeze up, snap off the plastic inside or simply fail prematurely. I've taken many apart and repaired them but most aren't designed to be repaired and the small parts/springs inside are easily lost. Best option, simply change them when they fail.  

Lastly, make sure you use good cables. Don't trust anything you may have used before, especially your signal cables.  If you use high impedance cables for connections (Guitar style phone cables) these often get used as speaker cables in a pinch or by accident.  These cables are designed to pass maybe 1V maximum. Speaker signals are hundreds of times stronger and will literally fry the cable from the inside out. Once they overheat the copper separates from the insulation. This allows oxygen and moisture into the cable and the copper strands in the cable tarnish/oxidize. This makes a signal cable super noisy. Not only microphonic and crackly but it can also quit intermittently. It can be tough to troubleshoot too. 

By the way contact cleaner can get sucked up into the cable wire cause the same issues. It turns the copper brown and causes the same kinds of failure as oxidation.  

If the connections and pots are clean and the noise is still there, feed the amp into a different speaker to see if its a problem with the head.  If it is the head,  you'll likely need to contact the manufacture for a replacement.  Dot trust you're local tech to try and repair it.  Most self powered speakers use Class D amplifiers which are not only highly complex to repair and troubleshoot.  Manufactures do not provide schematics or parts any more.  Most simply swap the entire unit which is cheaper then paying a tech to repair it.  If its the speaker you may be able to find a close replacement.  Best recommendations for  DIY is to try Parts Express. They have both speakers and amps for cabs.  Maybe you could get what you need there if the manufacturers cost is too high.   Parts Express: Speakers, Amplifiers, Audio Parts and Solutions (parts-express.com)

 

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Posted (edited)

Thanks so much for the great reply!  Lots of extremely useful info in there!!!

In my first post, I quoted several specs from the manuals of both the amplifiers and the speakers.  It was probably TL/DR, so to restate:

  • Amp = 400W RMS per side into 8 ohms
  • Amp manual doesn't give numbers, but states the obvious, that bridging results in "...effectively quadrupling the power..." 
  • Speakers = 8 ohms, 500W RMS, 1000W program, 2000W peak
  • Plan under consideration was to use one bridged amp per speaker, so 1600W RMS into 500W RMS 8 ohm speaker
  • QUESTION:  Just how dangerous it is to run a speaker rated at 500W RMS with an amp rated at 1600W RMS?  Or is it, considering the peak handling of the speaker is 2000W?
  • Related question: What steps can be taken to mitigate the danger if it is in fact dangerous?

The powered speaker is a cheap American Audio from the mid 2000s (I think 2006).  The buzz is continual even without signal, likely relating to a bad cap on the power supply.  I plan to use the speaker to practice a recap and board reflow, and if I fail, it's no big deal, speakers are old and cheap.

I have Deoxit, but thanks for clearly explaining why that matters and how to use it!

I make my own cables mostly, have never made a speaker cable smaller than 12 gauge, and use only Neutrik Speakon connectors.  I would never use a guitar cord for speakers, but thanks for drawing attention to that detail in case someone else is reading this.

THANK YOU!!

Edited by electrohippy
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On 8/26/2021 at 3:15 PM, electrohippy said:
  • Amp manual doesn't give numbers, but states the obvious, that bridging results in "...effectively quadrupling the power..." 
  • Speakers = 8 ohms, 500W RMS, 1000W program, 2000W peak
  • Plan under consideration was to use one bridged amp per speaker, so 1600W RMS into 500W RMS 8 ohm speaker
  • QUESTION:  Just how dangerous it is to run a speaker rated at 500W RMS with an amp rated at 1600W RMS?  Or is it, considering the peak handling of the speaker is 2000W?
  • Related question: What steps can be taken to mitigate the danger if it is in fact dangerous?

Every amp manual I've ever seen said bridging doubles the power but into twice the impedance, i.e., an amp that puts out 400 Watts x 2 into 4 Ohms in stereo can put out 800 Watts bridged but into 8 Ohms and a 4 Ohm load is no longer safe.

Personally, I wouldn't run a speaker beyond rated power but if you must, keep the volume as low as you can get away with. Bear in mind that 400 Watts RMS (continuous) is a lot and in most circumstances very loud. You'll hit peak power more-or-less frequently depending on the style of music and the venue but it sounds like the speakers are designed pretty ruggedly.

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18 hours ago, DeepEnd said:

Every amp manual I've ever seen said bridging doubles the power but into twice the impedance, i.e., an amp that puts out 400 Watts x 2 into 4 Ohms in stereo can put out 800 Watts bridged but into 8 Ohms and a 4 Ohm load is no longer safe.

Thanks for the reply! 

@DeepEnd, So let me aks you, maybe I'm reading this incorrectly.  Besides the quote about "...effectively quadrupling..." here's more quoted from the section on bridging from the manual (the user manual was written to cover several related amps, and these amps are CKS 800-2 amps):

"For CKS Series amplifiers, the recommended minimum nominal load impedance in bridged mono mode is 8 ohms, the equivalent to driving both channels at 4 ohms. [For models CKS 800-2, 1200-2 and 1600-2, the recommended minimum nominal load impedance in the bridged mono mode is 4 ohms, equivalent to driving both channels at 2 ohms].

So since my load is an 8 ohm load, then this should be, per their example, " the equivalent to driving both channels at 4 ohms." 

So is it true then that, since the specs sheet actually show the amp produces 600W into 4 ohms both channels driven, that it will in fact be 1200W into 8 ohms if bridged??  Or is it just double the 8 ohm rating like you said, at 800W?  Or is it actually just 600W bridged into 8 ohms???  AAACK!

Really wish they had given the actual bridged specs in the literature, but it's in neither the user manual nor the spec sheet I've found online.  Tried contacting Peavey and they were basically like "amp is too old...  bye."

Thanks again for all the help folks!

Edited by electrohippy
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OK so i tried out a couple of things and was surprised what I learned.

Re: bridging:  I set up a test system as follows:

  • First Crest amp set up in stereo mode delivering one side to one of my speakers
  • Second Crest amp set up bridged delivering both sides to one of my speakers
  • Set speakers at equal distance and position from a dB meter
  • Connected one side of my Yamaha mixing board to each amp, and set output and inputs identical all around, and both amps set to just below clipping
  • played the same mono source out through each amp, looped a 15 second section
  • watched the dB meter and noted the highest level through the loop
  • went back and forth between them several times and checked, also swapped the channels and even which amp was bridged, so there's no way it's false!

CONCLUSION:  The bridged amp setup was actually QUIETER than the dual stereo setup! 

This seems counterintuitive to me, what gives? 

Anyone want to chime in?

Edited by electrohippy
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Re: bi-amping, I set the system up as follows:

  • I didn't have enough speaker cable to run both a standard stereo to one speaker and a bi-amp setup to the other, so I went off my previous test results of one side pushed from a dual stereo setup (board settings were not changed between tests)
  • I went from the left out on the board to a vintage Furman TX-3 active crossover.  I set the input on the crossover to unity.  I set the XO point at 1500 Hz.  I started with the output levels for high and low at 5 just to be on the safe side
  • I then went highs out to one side of my Carvin FET-400 amp that's supposed to do 100W per side into 8 ohms, and speaker out to the high side on the speaker cabinet
  • Lows went into one side of a Crest amp, which is supposed to be 400W per side into 8 ohms, and speaker out to the low side of the speaker cabinet.
  • I followed the same test setup as before for the speaker and meters (speakers and meter were not moved between tests)

CONCLUSION:  About 10-15 dB louder, and could have been more so, but the Crest amp was underpowered for the situation.  I had to turn the output on the XO lows to 10, and the highs to 7 to get a good balance.  I turned the highs all the way up just as a test, and it was much louder, no distortion or clipping, but a poor high/low mix.

I then did a hybrid bi-amp and bridged thing, set up as above but with the Crest bridged (properly, per the manual, I should add).  That tripped the safety on the crest amp after a few loops of the reference track (I use Golden Brown by The Stranglers as my ref track), even with the clip light staying off.  It was quieter than stereo mode anyway, and I guess... at least the Crest protected itself...?

FINAL CONCLUSION:  I need to get a bigger amp than the Crest for larger outdoor shows, but I will be bi-amping the system with the Carvin in there on the highs in the future no matter what.  The change was phenomenal! 

Edited by electrohippy
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