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Buying a power amp

Discussion in 'Pro Audio Equipment' started by The_Red_Fool, May 29, 2005.

  1. The_Red_Fool

    The_Red_Fool Guest

    Alright, i posted here about a month or two ago about power amps. Now i think that i can afford to get one. the specs on the speakers i have are

    RMS Power - 350 watts rms

    Peak - 700 watts

    8ohms

    94db

    20-20,000hz

    Woofer: 50oz. magnet, 2" kapton voice coil, stamp frame basket

    Horn: 5"x15" braced horn lens.
    Tweeters: 4 - 3.5" piezo bullet

    Cabinet: Precision tuned, 5/8" MDF, handsome black carpet, tough steel grills, bar handles, rugged plastic corners, input terminal with 1/4" input + 2 gold plated posts for bare wire leads/banana plugs, dual ports,
    50" tall x 20" wide x 17" deep.
    Weight: 75 LBS Ea.



    The guy at guitar center said that i should get 2 QSC RMX 2450 power amps, one for each speaker. I dunnom what do you guys think? Thanks.
     
  2. Kurt Foster

    Kurt Foster Distinguished Member

    Those sound like Steel Sound speakers .... This is for a PA, right?

    That's an awful lot of power (900 watts a side rms) especially for speakers that use piezos (which do not have a very long throw) for tweets. I would try just one in stereo for 450 watts a side first and see if that does it for you ... if not, you can always pick up the second one

    If the sales guy is confident in what he is saying to you, he should be happy to give you the same price on the two if you need the second amp ... if he's not willing to go the extra mile, he's trying to pressure you into buying more that you need out the gate.

    QSC makes very good power amps. I have owned one for 20 years that still works perfectly and sounds great!

    I also recommend Haffler power amps
     
  3. Groff

    Groff Active Member

    For the PA my favorite is EV Q66
     
  4. The_Red_Fool

    The_Red_Fool Guest

    i was thinking of getting two QSC RMX 850s one for each speaker and then running them both in bridged mode. Would that work?
     
  5. anonymous

    anonymous Guests

    This has to be the best answer to your question.
    once again this has to be the best answer to your question
     
  6. Randyman...

    Randyman... Well-Known Member

    Yeah, and a 2" Kapton Voice Coil rated at 350WRMS Continuous sounds a bit skeptical to me ("Pro" woofers will usually have 3" or 4" coils for high power situations). The Piezos are also a dead giveaway to a "Cheap + Over-Rated" speaker IMO (Piezo horns range from like $0.95 - $10 or so - NOT a high quality part). Most Piezo Equipped loudspeakers usually don't have a true "Crossover" either (just a single 6dB/Octave cap on the piezo - and that is if you are lucky).

    I'd bet you would smoke these with a single RMX850, much less TWO of them in mono!

    Can you post a link to your speakers? We could give better reccomendations to an appropriate amp. Power Ratings are easily fudged in this kind of speaker (w/Piezo horns). PS - The 20-20,000 HZ spec is pure fantasy with this range of speaker, so the power rating specs could very well be inaccurate IMO...

    And that Guitar Center guy was smoking crack if he told you to put TWO RMX2450's on these! (Unless you were looking for a fireworks show). He was after your wallet, not out to help you.

    :cool:
     
  7. The_Red_Fool

    The_Red_Fool Guest

    damn it, i can't find the freaking link, sorry.
     
  8. Randyman...

    Randyman... Well-Known Member

    We'll be here ;) . Post it once you find it, and we will see what advice we can offer you... Just don't trust that GC salesman who told you to buy two of the RMX2450's for these speakers :eek: .

    :cool:
     
  9. Big_D

    Big_D Well-Known Member

    For the cleanest sound you want to get an amp that produces a little more power than your speakers are rated for. The reason for this is that an amp driven to full power has no available headroom but an amp rated for more power can drive the same speakers using less of the amps power and providing more headroom. You just need to keep the volume set lower on the more powerful amp to keep from blowing your speakers. This way you can achieve the same volume level as with the lower powered amp but have plenty of headroom to spare.

    I usually shoot for about 30% more power than the speakers are rated for for mains and monitors. Subs are a different story, I'll go as high as twice the power rating here.

    Here's the specs on the 2450.

    The RMX-2450 delivers 500 watts @ 8 ohms, 750 watts @ 4 ohms, 1200 watts @ 2 ohms and 1500 watts bridge mono (8 ohms).

    That is just about 30% more than your speakers are rated for so in my book 1-2450 would be a perfect match. Just remember don't push the volume up all the way or you could end buying new speakers as well.
     
  10. The_Red_Fool

    The_Red_Fool Guest

    alright that sounds good, now if i got one 2450 would i want to run that in stero mono or in bridged mode?

    Or if i got two RMX 850s and i ran them both in bridged mode would that be dangerous for the speakers because their wattage is not as high as the speakers.

    Thanks
     
  11. axel

    axel Guest

    if you want something really serious sounding for a passive PA than look into either LAB Gruppen (incredible light, for touring rigs!!) or CROWN / AMPCROWN (it's the same! and incredible heavy as f&^k for installation) you get both in tons of power varities, easily the amount your looking for... crowns you can even run down to 2 ohms without a cough!
     
  12. tmcconnell

    tmcconnell Guest

    try this..

    Post your question in http://www.audioasylum.com in the pro audio forum. There's a lot of old foh people in there who know from power amps. I assume just from your post that you are using these for live work, not studio?

    I'd suggest the QSC plx series. Get something that delivers 500 to 700 watts per channel at the rated ohms your speakers will draw and just use one amp.
     
  13. RickShepherd

    RickShepherd Guest

    I have heard great things about Carver zr1600 amps!
     
  14. Kurt Foster

    Kurt Foster Distinguished Member

    The discription you gave is remarkably similar to some speakers I have seen on E Bay ... from a company called "Steelsound" http://www.steelsound.com who make some remarkably affordable speakers that actually have metal diagphragms on the horns ... yes the do use piezos for the tweets too ...

    I've been shopping for some light duty PA gear myself recently. That's why I noticed that ... and again, just one of those QSC 2450 amps should be fine. This gives you a hundred more rms watts and 200 more peak watts than the speakers are rated for. That really should be plenty.

    Run it in stereo so it can handle the 8 ohm load from each of the speakers, unless the QSC can drive a 4 ohm load bridged. When you run 2 - 8 ohm loads on the same amp, it presents a 4 ohm load. Most amps don't drive loads lower than 8 ohms, when bridged.
     
  15. tmcconnell

    tmcconnell Guest

    ... and 100 watts is...

    nada.

    Since doubling of watts only results in 3 more db of output or so, 100 watts in a 400 watt system is like 1 db of output improvement. This does not provide enough headroom to keep the average speaker from blowing when the average band decides to crank it up. You are safer with DJ since the material in compressed. My foh buddies usually assume the amp should be double the power rating of the speakers as a good number. 50% would be nice. Remember this: if you blow your horns it will cost you as much as to fix as the incremetal watts would have - and you won't get that money back when you sell the amp. The other thing to remember is that really quality power amps (Crest, QSC PLX series etc.) retain their resale value better - and you can buy them second hand with confidence. If your amp blows up, that's the end of your gig unless you carry backup - and power amps take a beating. They collect floor dust, they get very hot. .. and quality amps sound better all the while. Oh yeah - and they can kill you - as all high voltage equipment can. Why go cheap?

    As far as the Carver, they sound good and perform well, but they used to make a switching type amp that would blow a fuse upon a relatively minor voltage sag. This happens in bars all the time in the summer when the beer coolers and AC are running full blast.
    I had a sound guy who knew this so well he could put foil in a carver fuse with his eyes closed while the amp was powered on. Gutsy or stupid, you be the judge - but the PA would always come back to life.

    (If you'll be in churches, never mind) :).
     
  16. Kurt Foster

    Kurt Foster Distinguished Member

    I think double power is excessive and will lead to speakers and tweets being fried.

    Again, one of those QSC 2450's will provide one hundred more rms watts and 200 more peak continuous watts than the speakers are rated for ..... and if he gets the one power amp and it still is not loud enough ... then he can get another.

    He can spend the extra cash on a couple of decent DBX comps to help protect the mains. That actually will do more to get the extra added average volume than the extra power amp.

    I tend to want to error on the conservative side. No sense spending more, to lug more weight, if you don't need it. Remember that even though a doubling of watts results in only 3 more db of output that the dB scale is not linear ... so a 3 dB increase in output is perceived as twice as loud!
     
  17. axel

    axel Guest

    ugghhh... guys, do we talk good sound here? QSC? Carver? ugghhh...
     
  18. Randyman...

    Randyman... Well-Known Member

    Don't get me wrong - I am a HEADROOM FREAK!!! (You should see my Car Stereo - Headroom for days!!!). I still want to see the spec sheet on these "20Hz-20,000Hz" PA speakers. I'm still saying a 2450 is WAY too much power for speakers like these (I'm thinking these are a very cheap "entry level" speakers with fairly poor components and no crossover).

    With a pure fantasy spec like "20-20K" (as posted on the first post), why would the other specs (power handling) be any more accurate? Can you see a 2" voice-coil woofer handling 350 WRMS all night long and down to 20Hz? I seriously doubt it... Maybe 200 WRMS all night if you are lucky IMO (and rolling off at 50-60Hz :eek: ).

    I guess my objection is getting a $600 - $800 amp (or TWO $600 amps) for a ~ $400 pair of speakers? Should be the other way 'round if you are going to skimp on something (get good speakers - as speakers are the weak link in any system).

    Did I mention I just Can't Stand PIEZO's? :wink: . Kiss those Piezos goodbye on a 2450 if you even think about getting any screeching feedback (unavoidable IMO)...

    Axel, Let's not forget the Quality of Speakers we are referring here (Piezo equipped mains - Ewww!). I think QSC is probably an overkill for these level speakers :) . Think "Pyle Power" amps as an equivalent (lol) . These are the kind of speakers I was picturing when I made my power recommendations FWIW.

    Later :cool:
     
  19. axel

    axel Guest

    oi, sorry, randyman apologies here.
    i have overlooked the specs of the speakers, i am just spoiled with PA's as i did a lot of stage work in my early days and i was always very spoiled :D le accoustics and the like, you now what i am talking about... so apologies again, and yupp agreed it's pointless having an overkill amp for a crap cabinet, it's enough if it matches.
     
  20. Randyman...

    Randyman... Well-Known Member

    No apologies needed! Specs are easy to accept as "accurate" - but we all know to be skeptical...

    The Red Fool - See if you can find that link. I hope you didn't take any negative connotations from my previous post (I just hate to see people mislead by "embellished specs" if that is the case here).

    Kurt - I'm sure you know, but 3dB is twice the power, not perceived twice as loud :wink: . Roughly 6-10dB is perceived twice as loud (~20x the power). I'm just sayin' is all :wink: .

    :cool:
     

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